Neuroanatomy

Autonomic Nervous System

The autonomic nervous system controls and coordinates the internal environment of the body. This system has two divisions:

1. Sympathetic
2. Parasympathetic

Both these divisions are complementary to each other. They function in coordination and adjust the body unconsciously to maintain the internal environment.

Most organs of our body receive innervations from both sympathetic and parasympathetic fibers. In general, a nerve impulse from one division stimulates the organ to increase the activity while the other will have the opposite action, i.e. decreases the activity.

For example, stimulation of sympathetic nerves increases the heart rate while stimulation of parasympathetic innervations decreases the heart rate.

The ANS consists of both afferent (sensory) and efferent (Motor) components and an integration center.

Autonomic Sensory (Visceral Afferent) Component The sensations of the autonomic system are transmitted from the viscera to the CNS through somatosensory fibers.

The cell bodies of sensory neurons are located in the dorsal root ganglion of spinal nerves and the sensory ganglia of some cranial nerves.

Somatosensory neurons are also responsible for visceromotor reflex activities. Though these sensations hardly reach the level of consciousness, the sensations of nausea and pain in
the heart is perceived.

Integration and Control of the Autonomic Nervous System. The autonomic activities are integrated at higher levels. These centers are situated in the reticular formation of the brainstem, hypothalamus, limbic cortex, and prefrontal cortex.

The hypothalamus is considered the higher center (control and integration) of ANS. It controls both sympathetic and parasympathetic divisions of ANS.

The neurons of sympathetic and parasympathetic nuclei in the brainstem and spinal cord are connected with the nuclei of the hypothalamus through the reticular formation.

Autonomic Motor (Visceral Efferent) Component. The autonomic activities regulate body temperature, heart rate, respiration, blood pressure, gastrointestinal motility, and secretion from glands.

Thus, ANS consists of motor (efferent) fibers that innervate smooth muscle, cardiac muscle, and glands.

The effector cells of different organ systems are not under voluntary control; they work mostly at an unconscious level.

Both sympathetic and parasympathetic motor pathways consist of two motor neurons that conduct the impulses from the CNS to the effector organ.

The cell bodies of the first motor neurons are located in the grey matter of the CNS (brain and spinal cord). These are called preganglionic or presynaptic neurons.

The preganglionic neurons send their myelinated axonal processes to synapse with the second motor neurons in the pathway.

The axons of presynaptic neurons come out of the CNS as part of either the spinal or the cranial nerve.

The cell bodies of second motor neurons are located in the autonomic ganglia outside the CNS.

These are called post-synaptic or post-ganglionic neurons because presynaptic neurons make synaptic contact with them.

The axons of post-synaptic neurons are unmyelinated and terminate on the effector organs (smooth muscle, glands, or cardiac muscle).

Sympathetic Division Of Autonomic Nervous System

The sympathetic division of the autonomic nervous system is also called thoracolumbar outflow as the preganglionic nerve cells are situated in the thoracic and upper lumbar segments of the spinal cord.

The cell bodies of presynaptic neurons of the sympathetic division of ANS are located in the lateral grey column (horn) of the spinal cord between the T1 and L2 spinal segments.

The cell bodies of postsynaptic neurons are located in two groups:

1. Paravertebral ganglion (sympathetic trunk ganglia) and
2. Pacvcrtebral ganglion.
3. Sympathetic trunk ganglia lie in a vertical row on either side of the vertebral column extending from the base of the skull to the coccyx.
4. On the other hand, prevertebral ganglia are situated anterior to the vertebrae and close to the origin of the main branches of the abdominal aorta.
5. Examples of prevertebral ganglia are celiac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion close to the beginning of arteries of the same name.
6. The axons of presynaptic neurons leave the spinal cord through ventral roots and enter the ventral rami of spinal nerves (T1 to L2).
7. These axons are myelinated fibers. They leave the ventral ramus as a slender branch called white ramus communicantes and enter the sympathetic trunk.
8. The presynaptic fibers of white ramus communicantes, after entering the sympathetic trunk, may synapse with the postsynaptic neurons present in the sympathetic ganglion at the same level.
9. These fibers may also ascend or descend in the sympathetic trunk for a few levels before forming synaptic connections.
10. It may also happen that these fibers pass through the sympathetic trunk without synapsing and join the splanchnic nerve to reach the prevertebral ganglia and synapse with the post-ganglionic neurons there.

The splanchnic nerves of the thorax are as follows

1. Greater (T5 to T9)
2. Lesser (T10 and Til)
3. Least (T12)

The splanchnic nerves of the thorax are greater (T5 to T9), lesser (T10 and Til), and least (T12) while lumbar splanchnic nerves take origin from LI to L4 sympathetic ganglia.

The post-synaptic neurons of sympathetic ganglia send their axons (post-synaptic sympathetic fibers) to the adjacent ventral rami of spinal nerves.

These axons are non-myelinated and are called gray rami communicantes. From the ventral rami, they go to all branches of the spinal nerve including the dorsal rami.

The post-ganglionic fibers from paravertebral sympathetic ganglia (sympathetic trunk ganglia) supply the blood vessels (vasomotor), sweat glands, and smooth muscle of hair follicles (erector pili) in the skin of limbs and body wall.

The post-ganglionic fibers from T1 to T5 supply thoracic viscera (heart, lung, trachea, and esophagus) via cardiac and pulmonary plexus.

The post-ganglionic fibers from prevertebral sympathetic ganglia (coeliac, superior and inferior mesenteric, and hypogastric ganglia, and plexuses) supply the abdominal and pelvic viscera (liver, kidney, stomach, intestine, rectum, colon, urinary bladder, genital organ, etc.)

The post-ganglionic fibers from the prevertebral sympathetic ganglia follow the course of various arteries.

(Students should note that the prevertebral ganglia consist of post-ganglionic sympathetic neurons only. However; these plexuses consist of both sympathetic and parasympathetic fibres.)

The sympathetic nervous activities are widely diffused activities that affect the whole body. The sympathetic reaction deals with emergencies or emotional stress.

The sympathetic reaction leads to the dilatation of the pupil, pale face (due to vasoconstriction in the skin), dry mouth, increased heart rate, and raised blood pressure.

The peristaltic movements of the intestine are suppressed and sphincters are closed. The blood vessels of skeletal muscles, heart, and brain dilate to supply more blood to these vital organs.

Parasympathetic Division Of Autonomic Nervous System

The parasympathetic division of ANS is also known as craniosacral outflow as preganglionic neurons are situated in the brain and sacral segment of the spinal cord.

Cranial outflow: The cell bodies of the cranial part of parasympathetic preganglionic neurons are situated in the brain and axons come out along with cranial nerves 3, 7, 9, and 10.

Sacral outflow: The sacral parts of preganglionic neurons are situated in the lateral grey column (horn) of spinal cord segments S2 to S4.

Their axons come out of the spinal cord through the ventral rami. The fibers from ventral rami then travel into pelvic splanchnic nerves.

The axons of pre-ganglionic neurons are very long, as they reach up to the effector organs. Close to the organs or within the substance of the organ, there is the presence of ganglia called terminal ganglia.

The terminal ganglia consist of post-synaptic neurons. Pre-synaptic axons form synaptic contacts with these neurons.

The terminal ganglia consist of post-synaptic neurons. Pre-synaptic axons form synaptic contacts with these post-synaptic neurons.

As the terminal ganglia are close to the organ supplied by the parasympathetic nerve, the axons of post-synaptic neurons are very short. They innervate the smooth muscles and glands in the wall of an organ.

The terminal ganglia associated with cranial outflow are ciliary (3 cranial nerve), pterygopalatine ganglia, submandibular ganglia (4 cranial nerve), and otic ganglia (9 cranial nerve).

The post-ganglionic fibers from these ganglia supply the eye, salivary glands, and other structures of the head and neck.

The pre-ganglionic parasympathetic fibers in the vagus nerve extend to many terminal ganglia in the thorax and abdomen. The post-ganglionic fibers supply the heart and lungs in the thorax (through cardiac and pulmonary plexuses). They supply the liver, gall bladder, stomach pancreas, small intestine, and part of the large intestine pancreas, small intestine, and part of the large intestine in the abdomen (through coeliac and superior mesenteric plexuses).

The postganglionic fibers from sacral outflow supply smooth muscle and glands in the wall of the colon, ureter, urinary bladder, and reproductive organs (through the hypogastric plexus).

Sympathetic And Parasympathetic Systems A Comparison

Most of the organs in the body receive both types of motor innervations (sympathetic and parasympathetic). These two types of motor innervations usually have opposite ctions (antagonistic); that is, if one type of innervation increases the activity (excitation) of viscera, then the other type will decrease (inhibition) the activity.

For example, sympathetic stimulation leads to decreased mobility of the intestine and contraction of sphincters. On the other hand, parasympathetic stimulation leads to increased motility of the intestine and relaxation of sphincters.

Sympathetic overactivity causes dilatation of the pupil while parasympathetic overactivity leads to its constriction.

In parasympathetic nerves, the neurotransmitter at pre- and post-synaptic nerve terminals is acetylcholine.

Acetylcholine is liberated at the sympathetic preganglionic nerve terminals while norepinephrine is liberated at postganglionic nerve terminals.

The autonomic innervations of some important organs of the body are described in brief in the following text

1. Eyeball

• Sympathetic innervation—from the Tl spinal cord segment
• Parasympathetic innervation—from the Edinger- Westphal nucleus
• Stimulation of sympathetic innervation causes dilatation of the pupil and relaxation of ciliary muscles.
• Stimulation of parasympathetic innervation causes constriction of pupil and ciliary muscle.
• Horner’s syndrome (which occurs due to deinnervations of sympathetic nerves of the head and neck) has the following characteristics: Constriction of a pupil, partial ptosis, absence of secretion on the face, flushing of the face, and enophthalmos.

2. Submandibular and sublingual salivary glands

• Sympathetic innervation—from Tl and T2 spinal segments
• Parasympathetic innervation—from the superior salivatory nucleus of the facial nerve

3. Lacrimal gland

• Sympathetic innervation—from Tl and T2 spinal segments
• Parasympathetic innervation—from the lacrimatory nucleus of the facial nerve.

4. Parotid gland

• Sympathetic innervation—from T1 and T2 spinal segments
• Parasympathetic innervation—from the inferior salivatory nucleus of the glossopharyngeal nerve

5. Heart

• Sympathetic innervation—from T1 to T5 spinal segments
• Parasympathetic innervation—from the dorsal nucleus of the vagus nerve.

The pain of a heart attack is felt over the middle of the sternum, left shoulder, jaw, and medial aspect of the left arm.

This is because these areas are supplied by the same segments of the spinal coral as the heart (T1 to T5).

6. Lungs

• Sympathetic innervation—from T2 to T5 spinal segments
• The pain of a heart attack is felt over the middle of the sternum, left shoulder, jaw, and medial aspect of the left arm.
• This is because these areas are supplied by the same segments of the spinal coral as the heart (T1 to T5).

7. Gastrointestinal tract

• Sympathetic innervation—from T5 to L2 spinal segments
• Parasympathetic innervation—from the dorsal nucleus of the vagus and from the S2 to S4 spinal segment

8. Urinary bladder

• Sympathetic innervation—from T10 to L2 spinal segments
• Parasympathetic innervation—from S2 to S4 spinal segments
• Sensory innervation—from T10 to L2 and S2 to S4

9. Arteries of the upper limb

• Sympathetic innervation—from T2 to T8
• Arteries of the lower limb
• Sympathetic innervation-from T10 to L2 spinal cord segments
• Autonomic innervations for the erection of the penis
• Parasympathetic innervation—S2 to S4 spinal segments
• Autonomic innervations for ejaculation from the penis
• Sympathetic innervations from the LI segment.

Localisation Of Visceral Pain

Viscera are usually insensitive to touch, heat, and cutting (cutting of the intestine in a conscious person does not elicit visceral pain).

But if receptors are stimulated in large areas (diffuse stimulation) due to inadequate blood supply, collection of metabolites, distension (stretch), and spasm, then visceral pain can be very severe. When the kidney stone obstructs and distends the ureter, it causes severe pain.

The visceral pain is poorly localized and dull because receptors are stimulated in a large area. The pain may be felt in the viscera itself or it may be felt just deep in the skin that overlies the viscera. However, in many cases, pain cord segments may also be felt in a surface area of the skin far from the stimulated organ. This phenomenon is called refereed pain.

Pain originating from a particular viscus is usually felt at a distance from the site of the visceral organ involved. This may be because afferent fibers from the skin (dermatome) and viscera enter the same segment of the spinal cord.

The first-order neurons of the afferent fibers of both visceral and somatic are situated in the dorsal root ganglia of the spinal nerve.

Probably, both the fibers synapse with the common (same) second-order neuron of the dorsal grey horn of the spinal cord.

The axons of the second-order neuron reach higher centers (via the thalamus) which probably fail to recognize the source of pain (skin or viscera).

Functions Of Autonomic Nervous System

• Both sympathetic and parasympathetic systems of ANS innervate almost all the organs of the body. Thus, their action on one particular organ is antagonistic to each other.
• If one system stimulates (excites) the organ, the other system depresses (inhibits) it. This is because their postganglionic neurons secrete different neurotransmitters
• (Ach for parasympathetic and noradrenaline in case of sympathetic) and their effector receptors for parasympathetic and adrenergic receptors for sympathetic).
• As the hypothalamus is the higher center of ANS it controls and balances the sympathetic and parasympathetic activities.
• However, students should also note that organs such as sweat glands, arrector pili muscles, kidneys, adrenal medulla, and many blood vessels are innervated only by the sympathetic system of the autonomic nerves.
• As there is no parasympathetic innervation in these organs, they do not face opposition. Thus, sympathetic stimulation leads to action while its inhibition leads to cessation of action.
• For example, when sympathetic fibers release norepinephrine into the smooth muscles of blood vessels, blood pressure rises due to the contraction of smooth muscles by mediation of the al receptor.
• Meanwhile, the epinephrine of circulating blood acts on the same smooth muscles by mediation of P2 receptors in the cell membrane leading to vasodilatation and a fall in blood pressure. Parasympathetic fibers do not terminate on many blood vessels.

Enteric Nervous System

The enteric nervous system (ENS) is defined as the system of neurons that is found within the wall of the GIT, gallbladder, and pancreas.

The entire length of GIT is supplied by sympathetic and parasympathetic parts of ANS.

The ANS forms the following two different nerve plexuses in the gut wall:

Submucosal plexus (plexus of Meissner): Situated in the subamucosa

Myenteric plexus (plexus of Auerbach): Situated in between circular and longitudinal muscle coats

These plexuses consist of the following:

• Parasympathetic preganglionic fibers terminate on postganglionic parasympathetic neurons situated in the myenteric plexus.
• Postganglionic sympathetic and parasympathetic fibers forming a network in Meissner and myenteric plexuses
• Sensory and motor enteric neurons and their processes consist of 100 million neurons in the plexuses of GIT.

All these innervate the mucosa, submucosa, muscle coats, and blood vessels. These regulate the secretion from the mucosal gland and motility of GIT.

The gallbladder and pancreas also have ganglia and nerve plexus. The ENS was previously considered a part of ANS.

However, now it is considered a system separate from the ANS. The neurons of the ENS arise from neural crest; thus, they differ from the origin of sympathetic and parasympathetic neurons.

Functions of the Enteric Nervous System

• The functions of ENS include both sensory and motor.
• The sensory neurons of ENS monitor the stretching of the walls of the intestine and chemical changes within the gastrointestinal tract.
• The motor neurons of ENS control the contraction of the smooth muscle, secretion of the gastrointestinal gland, and endocrine cells associated with GIT.

Autonomic Nervous System Summary

• The autonomic nervous system (ANS) innervates the cardiac muscle, smooth muscle (present in the wall of viscera and blood vessels), and glands.
• The ANS works by forming the reflex arc which is organized in afferent (sensory) pathways, integrating higher centers and efferent (motor) pathways.
• Visceral sensation hardly reaches the level of consciousness. The cell bodies of sensory neurons are situated in the sensory ganglia of cranial nerves and the dorsal root ganglia of spinal nerves.
• The visceral activities are integrated at higher levels, especially in the hypothalamus.
• Efferent pathways convey motor impulses from the CNS to the cardiac muscle, smooth muscles, and glands.

Two neurons are involved in this pathway:

• Preganglionic and
• Postganglionic neurons.

The visceral motor signals reach various organs through two major subdivisions of the ANS:

• Sympathetic and
• Parasympathetic nervous systems.

Sympathetic nervous system

• The preganglionic motor neurons are situated in the lateral horn of the grey matter of the spinal cord from T1 to L2 segments.
• The postganglionic motor neurons of this division are situated in the sympathetic trunk ganglia and prevertebral ganglia.
• The preganglionic fibers reach the sympathetic trunk ganglion through white rami communicans while the postganglionic fibers after coming out through the sympathetic ganglia join the ventral ramus through grey rami communicans.
• Some preganglionic sympathetic fibers, which do not relay in the sympathetic chain (paravertebral), may relay in the prevertebral ganglion through splanchnic nerves.

Parasympathetic nervous system

• The parasympathetic nervous system consists of cranial and ‘sacral’ outflow.
• The preganglionic parasympathetic fibers of both cranial and sacral outflow end in ‘terminal ganglia’ containing postganglionic parasympathetic neurons.
• The terminal ganglia associated with cranial outflow are the ciliary, pterygopalatine, submandibular, and otic ganglia. The terminal ganglia of the vagus and sacral outflow are situated in the wall of the viscera and are unnamed.
• The actions of sympathetic and parasympathetic parts of ANS on any particular organ are antagonistic to each other.

Enteric nervous system

• The enteric nervous system (ENS) consists of motor, sensory, and interneurons, which are found within the walls of GIT.
• The functions of ENS include both sensory and motor.

Autonomic Nervous System Multiple Choice Questions

Question 1. The following structures are innervated by ANS except

1. Smooth muscles of viscera
2. Smooth muscles of blood vessels
3. Cardiac muscles
4. Glands
5. Articular capsule of joints

Answer: 5. Articular capsule of joints

Question 2. Which of the following facts about visceral sensations are true?

1. They hardly reach the level of consciousness
2. Sensations of nausea and retrosternal pain are perceived
3. The sensation of distension of the bladder and bowel reaches the level of consciousness
4. Pain sensation from the viscera is perceived as secondary to a lack of oxygen supply
5. All of the above

Answer: 3. Sensation of distension of bladder and bowel reach the level of consciousness

Question 3. Which of the following facts about the autonomic motor pathway is false?

1. Visceral motor signals reach organs through the sympathetic and parasympathetic nervous systems
2. Both sympathetic and parasympathetic pathways consist of two neurons—preganglionic and postganglionic
3. Preganglionic neurons are situated in the CNS while postganglionic in the peripheral ganglion
4. The axons of preganglionic neurons are myelinated and synapse with postganglionic neurons
5. None of the above

Answer: 5. None of the above

Question 4. Which ofthe following statements about the sympathetic nervous system is false?

1. Preganglionic sympathetic neurons are located in the lateral horn of the spinal cord from T1 to L2 and from S2 to S4 spinal segments
2. Postganglionic sympathetic neurons are located in the sympathetic trunk (sympathetic chain) ganglia
3. They are also located in prevertebral ganglia
4. Prevertebral sympathetic ganglia are situated in front of the vertebral column

Answer: 1. Preganglionic sympathetic neurons are located in the lateral horn of the spinal cord from T1 to L2 and from S2 to S4 spinal segments

Question 5. Prevertebral sympathetic ganglia consist of the following except

1. Pulmonary ganglion
2. Superior mesenteric ganglio
3. Inferior mesenteric ganglion
4. Celiac ganglion

Answer: 1. Pulmonary ganglion

Question 6. Which of the following statements about splanchnic nerves is false?

1. They arise from the thoracic sympathetic chain
2. They are medically directed branches of the sympathetic trunk
3. They contain postganglionic myelinated nerve fibers
4. There are three splanchnic nerves—greater, lesser, and least splanchnic nerves
5. These splanchnic nerves reach the prevertebral ganglion

Answer: 3. There are three splanchnic nerves—greater, lesser, and least splanchnic nerves

Question 7. The following facts about the distribution of sympathetic fibres are true except

1. Preganglionic sympathetic fibers travel in the ventral spinal nerve root
2. They then travel in the ventral ramus of spinal nerves
3. They soon leave the ventral ramus to join the sympathetic trunk through the white ramus communicans to synapse with the postganglionic neurons
4. The axons of postganglionic neurons join the ventral ramus again through grey ramus communicans
5. Grey ramus communicans contain myelinated fibers of postganglionic neurons

Answer: 5. Grey ramus communicans contain myelinated fibers of postganglionic neurons

Question 8. Following is the location of preganglionic motor neurons of the parasympathetic nervous system except

1. Few cranial nerve nuclei in the brainstem
2. Lateral horn of spinal cord between S2 and S4 spinal segments
3. Terminal ganglia of the parasympathetic nervous system
4. Pterygopalatine ganglion

Answer: 3. Terminal ganglia of the parasympathetic nervous system

Question 9. Which of the following statements is false?

1. Acetylcholine (ACh) is liberated at the terminals of preganglionic parasympathetic neurons
2. ACh is liberated at the terminals of preganglionic sympathetic neurons
3. Adrenaline is liberated at the terminal of postganglionic parasympathetic neurons
4. ACh/norepinephrine is liberated at the terminals of
   postganglionic sympathetic neurons

Answer: 3. Adrenaline is liberated at the terminal of postganglionic parasympathetic neurons Part of the thalamus, subthalamus, middle and posterior parts of the hypothalamus, part of the midbrain

Blood Supply Of The Brain And Spinal Cord

The brain is supplied by a pair of vertebral and a pair of internal carotid arteries. In the cranial cavity, both pairs of arteries divide extensively into many branches, some of which are anastomose with each other.

The spinal cord is about a 45-cm long structure; hence, many arteries supply it. The spinal cord is supplied by branches of vertebral arteries and also by segmental arteries, i.e. intercostal and lumbar segmental arteries.

Vertebral Arteries. Each vertebral artery enters the cranial cavity through the foramen magnum after piercing the dura and arachnoid mater.

It curves around the ventrolateral aspect of the medulla and at the lower border of pons it joins with its fellow, from the other side, to form a median basilar artery. The branches of the vertebral artery are given in the following text.

• Posterior spinal artery: The posterior spinal artery on either side passes inferiorly along the spinal cord, among the dorsal rootlets of spinal nerves.
• Posterior inferior cerebellar artery: The posterior inferior cerebellar artery is the largest branch of the vertebral artery and arises near the lower end of the olive. Had) artery runs posteriorly on either side of the medulla. 1 lore, it supplies the choroid plexus of the fourth ventricle and reaches the cerebellum to supply the posterior part of its inferior surface
• Anterior spinal artery: The anterior spinal artery arises near the inferior border of the pons. It descends anteromedially in front of the medulla and joins its fellow, from the opposite side, to form a single trunk. It is the single trunk of the anterior spinal artery that runs in the anterior median fissure throughout the length of the spinal cord and supplies it

Basilar Artery and Its Branches: The basilar artery runs cranially in the median groove on the pons.

At the superior border of the pons, it divides into two terminal branches:

1. Right and
2. left posterior cerebral arteries.

The branches of the basilar artery are as follows:

Arteries of the labyrinth: On each side, these arteries arise close to the inferior border of pons and run along with the vestibulocochlear nerve towards the internal ear.

Anterior inferior cerebellar arteries: These arteries arise from the lower part of the basilar artery, on each side, and supply the anterior part of the inferior surface of the cerebellum.

Pontine arteries: These are many small branches that supply pons and the adjacent parts of the brain.

Superior cerebellar artery: On each side, the superior cerebellar artery arises near the upper border of the pons. It supplies the upper pons, lower midbrain and superior surface of the cerebellum.

Posterior cerebral arteries: These arteries are two large terminal branches of the basilar artery, which arise at the superior border of the pons.

Each artery breaks up into branches to supply the temporal and occipital lobes.

The branches of the posterior cerebral arteries are as follows:

Central branches: These are numerous small branches which enter the cerebrum through the posterior perforated substance between two crue

Internal Carotid Artery (ICA). After passing through the cavernous sinus, the ICA pierces the dura mater medial to the anterior clinoid process.

Here, the artery lies immediately inferior to the anterior perforated substance and lateral to optic chiasma.

The ICA has two major branches—anterior and middle cerebral arteries which are its terminal branches and middle cerebral arteries which are its terminal branches ICA also gives smaller branches—ophthalmic, posterior communicating and anterior choroidal arteries.

Anterior cerebral artery: The anterior cerebral artery is a smaller terminal branch of the ICA. After its origin, the right and left anterior cerebral arteries come close to each other and are joined by the anterior communicating artery.

The anterior cerebral artery now ascends in the longitudinal fissure and runs posteriorly along the corpus callosum. The artery gives central and cortical branches.

Central branches: They form the anteromedial group which enters deep inside the cerebral hemisphere by passing through the anterior perforated substance and lamina terminalis. These branches supply the rostrum, septum pellucidum and corpus.

Anterior cerebral artery: The anterior cerebral artery is a smaller terminal branch of the ICA. After its origin, the right and left anterior cerebral arteries come close to each other and are joined by the anterior communicating artery.

The anterior cerebral artery now ascends in the longitudinal fissure and runs posteriorly along the corpus callosum. The artery gives central and cortical branches.

Central branches: They form the anteromedial group which enters deep inside the cerebral hemisphere by passing through the anterior perforated substance and lamina terminalis. These branches supply the rostrum, septum pellucidum and corpus.

Middle cerebral artery: The middle cerebral artery is a larger terminal branch and a more direct continuation of the ICA. It runs in the stem of the lateral sulcus, between the frontal and temporal lobes, and then turns laterally in the posterior ramus of the lateral sulcus. It gives central and cortical branches.

Central branches: They form the anterolateral group and enter the deeper part of the cerebral hemisphere through the anterior perforated substance. These central branches are grouped into medial and lateral striate arteries.

The medial striate arteries ascend through the lentiform nucleus and supply the lentiform nucleus, internal capsule and caudate nucleus The lateral striate arteries ascend through the external capsule.

Then, they turn medially to supply the lentiform and caudate nuclei along with the internal capsule.

One of the larger lateral striate arteries is more susceptible to rupture; hence, it is called the artery of cerebral haemorrhage (Charcot’s artery of cerebral haemorrhage).

Cortical branches: They emerge in the superolateral surface of the cerebral hemisphere between the lips of the lateral sulcus.

These cortical branches supply most of the superolateral surface and the adjacent part of the orbital surface, tentorial surface and temporal pole.

This artery is responsible for supplying the motor and premotor areas, primary sensory and auditory areas in both hemispheres. The left cerebral hemisphere supplies Wernicke’s language area and Broca’s area of speech.

Occlusion of Cortical Branches of the Middle Cerebral Artery

• The thrombosis or occlusion of the cortical branches of the middle cerebral artery will lead to motor paralysis and sensory loss of the opposite half of the body above the leg.
• Involvement of motor and sensory speech areas of the left cerebral hemisphere (in right-handed persons) will result in aphasia (loss of speech).
• The thrombosis of the middle cerebral artery may also involve the auditory area with little hearing loss, as auditory impulses are projected to both the cerebral hemispheres.
• Occlusion of central branches may lead to hemiplegia
  because of infarction of motor fibres in the internal capsule.

Circulus Arteriosus Circle Of Willis

Circulus arteriosus is a polygonal circle of arteries, present in the interpeduncular fossa. It is formed by posterior cerebral, posterior communicating, internal carotid, anterior cerebral and anterior communicating arteries.

Formation: The circulus arteriosus is bounded posteriorly by two posterior cerebral arteries that communicate with the middle cerebral artery on each side by the posterior communicating branch.

Anteriorly, two anterior cerebral arteries communicate with each other through the anterior communicating artery.

Branches: The circle of Willis gives origin to many slender central branches.

These arteries arise in four groups:

Anteromedial, Anterolateral, Posteromedial and Posterolateral.

These branches immediately pierce the surface of the brain (mostly through anterior and posterior perforated substances) to supply its internal parts such as the corpus striatum, thalamus, internal capsule and midbrain.

These arteries do not anastomose significantly within the brain substance. As these arteries act as end arteries, the damage to the branches entering the substance of the brain leads to the destruction of the brain tissue.

Significance of Circle of Willis

This arterial anastomosis acts as a route through which blood entering the ICA or the basilar artery may be distributed to any part of the cerebral hemisphere.

If one of the major arteries forming the circle of Willis is blocked, then this arterial anastomosis provides an alternative route through which blood can be supplied to the area of the blocked artery.

Arterial Supply Of The Individual Parts Of The Brain

After studying the major arteries and their branches supplying the brain, we shall now study the arterial supply Of the individual parts of the brain as follows

• Arterial supply of the cerebral cortex

• Arterial supply of the internal structures of the cerebral hemisphere
• Arterial supply of brainstem
• Arterial supply of the cerebellum

Arterial Supply Of The Cerebral Cortex

• The cortical branches of three cerebral arteries (anterior, middle and posterior) ramify on the medial, superolateral and inferior surfaces of the cerebral cortex and supply it
• Arterial Supply of Motor and Sensory Areas supply most of the superolateral surface and supply most of the superolateral surface and the temporal pole.
• The middle cerebral artery is responsible for supplying the motor and premotor areas, primary sensory and auditory areas, in both hemispheres. In the left cerebral hemisphere, it supplies Wernickes’ language area and Broca’s area of speech.
• The cortical branches of the anterior cerebral artery supply the motor and sensory areas for the leg and perineum of the opposite side. These areas are present on the medial surface of the cerebral hemisphere.
• The cortical branches of the posterior cerebral arteries supply the visual cortex (area 17) in the occipital lobe.

Arterial Supply Of The Internal Structures Of The Cerebral Hemisphere

The central branches enter the cerebral hemisphere and supply the thalamus, hypothalamus, corpus striatum and internal capsule.

Arterial Supply of Brainstem

The medulla is supplied by the anterior and posterior spinal arteries and the posterior inferior cerebellar arteries.

The pons are supplied by the basilar artery, anterior inferior cerebellar arteries and superior cerebellar arteries. The midbrain is supplied by the basilar, superior cerebellar and posterior cerebral arteries.

Arterial Supply Of Cerebellum

The superior and inferior surfaces of the cerebellum are supplied by the superior cerebellar and inferior cerebellar arteries.

Both of these arteries are branches of the basilar artery. The inferior surface is also supplied by the posterior inferior cerebellar artery which is a branch of the vertebral artery.

Venous Drainage of the Brain

The veins draining the brain are thin-walled because their walls are devoid of muscles. They are also without valves as functionally they have to drain towards the direction of gravity.

These veins drain their blood into dural venous sinuses after piercing through the arachnoid mater and inner layer of the dura mater.

The cerebral veins are classified into external and internal cerebral veins.

The external cerebral veins lie in the subarachnoid space on the surface of the cerebral hemisphere. These are superior, middle and inferior cerebral veins.

The internal cerebral veins, on the other hand, drain the internal parts of the cerebrum and ultimately pour their blood into the great cerebral vein.

The internal cerebral vein is formed near the interventricular foramen by the union of the thalamostriate vein and choroid veins.

Internal Cerebral Veins

The internal cerebral vein is formed near the interventricular foramen by the union of the thalamostriate vein and choroid veins.

The right and left internal cerebral veins run posteriorly in the transverse fissure where they unite beneath the splenium to form the great cerebral vein.

This vein receives the right and left basal veins and then opens into the straight sinus.

Blood-Brain Barrier

The endothelial lining of brain capillaries does not permit some substances to pass from blood to brain tissue.

The barrier protects the delicate brain tissues from harmful (toxic) substances, lie blood-brain barrier is highly permeable to water, glucose, lipid-soluble substances, O2, C02 and drugs such as alcohol, coffee, nicotine and anaesthetics.

This barrier is, however, an obstacle to delivering drugs such as antibiotics and cancer drugs.

The following structures form the blood-brain barrier:

• The endothelial lining of capillary: There is the presence of tight junctions between endothelial cells.
• The basal lamina of endothelium
• The perivascular end feet of astrocytes on the basal lamina

The barrier is mainly formed by the endothelial cells resting on the basal lamina and not by the astrocytes as they do not fully surround the capillary. However, the foot processes stimulate the formation of tight junctions between endothelial cells.

Blood Supply Of The Spinal Cord

The spinal cord is a long structure present in the vertebral canal. Because of its length, the spinal cord is supplied by many arteries. It is drained by longitudinally arranged veins which further drain into radicular veins.

Arterial Supply

The arteries supplying the spinal cord can be grouped into the following:

• Branches of the vertebral arteries, i.e. one anterior and two posterior spinal arteries.
• Multiple spinal branches of segmental arteries
• Radicular arteries

Branches of the vertebral arteries: As the right and left vertebral arteries lie on the anterolateral surface of the medulla, each artery gives two branches that supply the spinal cord. These are the anterior and posterior spinal arteries.

The anterior two-thirds of the spinal cord is supplied by the anterior spinal artery while the posterior one-third is by the posterior spinal arteries.

Segmental arteries: As the spinal cord is a long structure, many segmental arteries supply it.

The spinal branches of the segmental arteries (vertebral, deep cervical, posterior intercostals and lumbar arteries) enter the vertebral canal through the corresponding intervertebral foramen.

Radicular arteries: Each spinal artery divides into anterior and posterior radicular arteries, which run towards the spinal cord along the ventral and dorsal nerve roots, respectively.

The total number of anterior radicular arteries varies from 12 to 17 (mostly six in cervical, two to four in thoracic and two to three in lumbar segments).

Out of these radicular arteries, a single anterior radicular artery is very large and is called the arteria radicularis magna (artery of Adamkiewicz) which usually arises from the lower thoracic or upper lumbar level.

Venous Drainage

Six longitudinally running veins that drain the spinal cord are as follows:

• One along the anterior median fissure
• One along the posterior median sulcus
• Two along the line of right and left ventral nerve rootlets

Two along the line of right and left dorsal nerve rootlets All these veins freely communicate with each other and are drained by anterior and posterior radicular veins.

Blood Supply Of The Brain And Spinal Cord Summary

• The brain has a rich blood supply. It is supplied by a pair of vertebral arteries and a pair of internal carotid arteries.
• The branches of vertebral arteries in the cranial cavity are posterior spinal, posterior inferior cerebellar and anterior spinal arteries.
• Two vertebral arteries join each other to form a median basilar artery at the lower border of the pons.
• The branches of the basilar artery are the artery of the labyrinth, anterior inferior cerebellar artery, pontine arteries and superior cerebellar arteries. It terminates in two posterior cerebral arteries.
• The circle of Willis is formed by the posterior cerebral, posterior communicating, anterior cerebral and anterior communicating arteries.
• The circle of Willis gives origin to many central branches. These arteries arise in four groups:
• Anteromedial,
• Anterolateral,
• Posteromedial and
• Posterolateral.
• Various surfaces of the cerebral cortex are supplied by the cortical branches of the anterior, middle and posterior cerebral arteries.
• The superolateral, medial and inferior surfaces are supplied by all three cerebral arteries:
• Anterior, Middle and Posterior cerebral arteries.
• The corpus striatum and internal capsule are supplied by central branches of the anterior and middle cerebral arteries.
• The thalamus is supplied by the central branches of the posterior cerebral artery and basilar arteries.
• The veins of the brain are grouped as external cerebral veins and internal cerebral veins.
• The veins of the brain drain into intracranial dural venous sinuses.
• The blood-brain barrier is formed by capillary endothelial cells and their basement membrane.
• The spinal cord is supplied by branches of vertebral arteries, namely the anterior spinal artery and two posterior spinal arteries.
• The spinal cord is also supplied by multiple spinal branches of segmental arteries.
• Six longitudinally running veins drain the venous blood from the spinal cord.

Blood Supply Of The Brain And Spinal Cord Multiple Choice Questions

Question 1. Which of the following is not a branch of the vertebra artery?

1. Posterior spinal arteries
2. Anterior spinal artery
3. Superior cerebellar artery
4. Posterior inferior cerebellar artery
5. Anterior inferior cerebellar artery

Answer: 3. Posterior inferior cerebellar artery

Question 2. Which of the following are the branches of the basilar artery?

1. Anterior inferior cerebellar artery
2. Pontine artery
3. Superior cerebellar artery
4. Posterior cerebral artery
5. All of the above

Answer: 5. All of the above

Question 3. The Circle of Willis is formed by the following arteries except

1. Posterior cerebellar artery
2. Post communicating artery
3. Anterior cerebral artery
4. Middle cerebral artery
5. Anterior communicating artery

Answer: 4. Anterior communicating artery

Question 4. Which of the following arteries supply the internal capsule?

1. Anterolateral central arteries
2. Posterolateral central arteries
3. Anterior choroidal artery
4. Artery of Heubner
5. All of the above

Answer: 5. All of the above

Question 5. Which of the following are the group of central arteries?

1. Anteromedial
2. Anterolateral
3. Posteromedial
4. Posterolateral
5. All of the above

Answer: 5. All of the above

Question 6. Which of the following is not a tributary of the internal cerebral vein?

1. Septal vein
2. Thalamostriate vein
3. Choroidal vain
4. Great cerebral vein

Answer: 4. Great cerebral vein

Meninges And Cerebrospinal Fluid

• The delicate nervous tissue of the central nervous system is protected by structures such as bones, meninges, and cerebrospinal fluid (CSF).
• Meninges are connective tissue membranes, which cover the brain and the spinal cord. The CSF surrounding the brain and the spinal cord acts as a cushion.
• The meninges surrounding the brain are called cranial meninges and those surrounding the spinal cord are called spinal meninges.
• Meninges are of three different types: From without inwards they are named as Dura mater, Arachnoid mater, and Pia mater.

Cranial Meninges

Dura Mater

The dura covering the brain is known as cerebral dura. The cerebral dura is made up of an outer endosteal layer and an inner meningeal layer. The endosteal layer of the dura mater is nothing but the endocranium or the inner periosteum.

The meningeal layer of the dura mater is the membranous layer. It covers the brain and then becomes continuous with the dura mater covering the spinal cord.

The two layers of dura mater are tightly fused except in a few places. At these places, the meningeal layer separates from the endosteal layer to form a double-layered fold or partition.

These folds of dura mater extend between the major parts of the brain. As the meningeal layer separates from the endosteal layer, a triangular space is formed.

This space encloses the dural venous sinus. Here, the internal surface of the dura mater is smooth, shining, and lined by endothelial cells.

Folds of Dura Mater

The folds of the dura mater play an important role in supporting the brain tissue.

The following folds or septa of the dura mater are formed in the cranial cavity due to duplication of the meningeal layer of the dura.

Dural Venous Sinuses

• As mentioned earlier, the dural venous sinuses are formed due to the separation of meningeal and endosteal layers.
• The dural venous sinuses formed between the layers of the dura mater can be paired or unpaired.
• The dural venous sinuses drain the blood from some cranial bones, meninges, and brain. They ultimately pour the blood into the internal jugular veins.

Superior Sagittal Sinus

• The superior sagittal sinus runs at the superior border of the falx cerebri. It begins at the crista galli and is formed by venous blood that is drained from the frontal sinus and veins of the nose.
• This sinus drains superior cerebral veins and becomes continuous with the right transverse sinus at the internal occipital protuberance.
• The inferior sagittal sinus is present at the inferior border of the falx cerebri and drains the falx and medial surface of the cerebral hemisphere. Posteriorly, it becomes continuous with the straight sinus.

Straight Sinus

• The straight sinus is present at the junction of the falx and tentorium.
• The union of the inferior sagittal sinus and the great cerebral vein forms the straight sinus. At the internal occipital protuberance, it becomes continuous with the left transverse sinus.

Transverse Sinus

• The transverse sinus is present on each side of the attached margin of tentorium cerebelli. This sinus extends from the internal occipital protuberance to the base of the petrous temporal bone.
• The right transverse sinus is formed by the continuation of the superior sagittal sinus while the left transverse sinus is formed by the continuation of the straight sinus.
• The transverse sinuses receive blood from the veins of the occipital lobe of the cerebrum and cerebellum and ultimately drain into the right and left sigmoid sinuses.

 

The sigmoid sinus is situated behind the base of the petrous temporal bone.

This S-shaped sinus is the continuation of the transverse sinus. It passes through the jugular foramen to form the internal jugular vein.

Cavernous Sinus

The right and left cavernous sinuses are situated on either side of the body of the sphenoid. These are formed due to the separation of the endosteal and meningeal layers of the dura mater. These layers are lined by endothelium.

Extension: From the superior orbital fissure anteriorly to the apex of the petrous temporal bone posteriorly.

Tributaries: Superior and inferior ophthalmic veins, cerebral veins, sphenoparietal sinus and frontal trunk of the middle meningeal vein.

Drainage: Into superior and inferior petrosal sinuses and to the basilar plexus of veins.

Communication: With the facial vein through the superior ophthalmic vein, with the pterygoid plexus through the emissary’s veins and with the internal vertebral plexus through the basilar venous plexus

Thrombosis of Cavernous Sinus

• Sometimes, infection may reach the cavernous sinus from the dangerous area of the face and scalp through deep facial and ophthalmic veins.
• This septic thrombosis of the cavernous sinus compresses nerves passing through its lateral wall, and this produces the corresponding symptoms.
• Cavernous thrombosis also causes pain in the eye and swelling of the eyelids.

Nerve Supply of Dura

• The branches of the trigeminal nerve supply the dura mater of the anterior and middle cranial fossae.
• The dura of the posterior cranial fossa is supplied by the vagus nerve and the meningeal branches of Cl to C3 spinal nerves.

Arterial Supply of Dura

Several branches of the following arteries supply the dura mater:

External carotid: Branches of the middle meningeal, ascending pharyngeal and occipital arteries.

Internal carotid

Subclavian: Vertebral branch of the subclavian artery

Extradural (Epidural) Haemorrhage

• Epidural haemorrhage occurs due to the rupture of meningeal vessels running between the endosteal and meningeal layers of the dura mater.
• The tear in the meningeal vessels is usually secondary to the fracture of the skull.
• The most common artery affected is the anterior branch of the middle meningeal artery and vein, which lies in the area of the pterion.

Subdural Haemorrhage

• The superior cerebral veins open into the superior sagittal sinus. Just before their opening in the sinus, they run for a short distance in the subdural space.
• Trauma to the head (forceful movement of the brain within the cranial cavity) may tear the superior cerebral vein(s). This results in the collection of blood in subdural space.

Arachnoid Mater

• Deep to dura mater, there lies a delicate, thin and almost transparent membrane known as arachnoid mater.
• It is separated from the dura mater by a capillary space which is called subdural space- it contains a thin film of fluid.

The subarachnoid space lies beneath the arachnoid mater, between the arachnoid and pia mater.

It is filled with CSF. The CSF acts as a buffer, which distributes and equalises the pressure on the surface of the brain.

A meshwork or filaments (trabeculae) extends through the fluid-filled subarachnoid space between the arachnoid and pia mater.

The arteries, veins and cranial nerves, while entering or leaving the brain, lie in the subarachnoid space.

Subarachnoid Cisterns

The arachnoid mater forms bridges over sulci and other irregularities on the surface of the brain. In some situations, on the surface of the brain, the pia and arachnoid mater are widely separated from each other to form subarachnoid cisterns. Large cisterns are formed around the brainstem and cerebellum.

Cisterna Magna

Cisterna magna is the largest cistern and is also known as the cerebellomedullary cistern. It lies in the angle between the cerebellum and the medulla oblongata.

This cistern is continuous above the fourth ventricle through its median aperture and below the subarachnoid space of the spinal cord.

Cisterna Pontis

Cisterna pots lie anterior to the pons and medulla and contain vertebral and basilar arteries.

Cistnrna Intetrpeduncularis

Cisterna interpeduncularis lies between two cerebral
peduncles. It contains the circle of Willis.

Arachnoid Villi and Granulations

Arachnoid villi arc minute finger-like elevations of arachnoid mater that project into the dural venous sinuses (especially the superior sagittal sinus) through apertures in dura mater. These act as channels of communication between the subarachnoid space and the dural venous sinus.

However, these capillaries (tubules) act as a valve or a one-way communicating channel for the escape of CSF into the venous blood.

The CSF from the subarachnoid space passes into the bloodstream of dural venous sinuses through these villi.

As the age advances, the arachnoid villi become large and globular in shape. These arachnoid villi are now known as arachnoid granulation.

Pia Mater

The pia mater is a delicate, thin membrane adherent to the surface of the brain, i.e. covering the gyri and sulci. It is made up of flattened mesothelial cells. There exists a microscopic subpial space between the pia and the brain.

The blood vessels present in the subarachnoid space anastomose with the neighbouring vessels on the surface of the pia before penetrating the pia mater. Thereafter, the blood vessels pass into the substance of the brain as end arteries.

At certain places, the wall of the ventricles of the brain is thin and only lined by ependyma. In this situation, the fold of the pia mater along with the blood capillaries invaginates into the ventricular cavities.

This invaginating vascular tuft is known as the choroid plexus of the ventricles. It is made up of (from outside to inside) ependyma, pia mater and blood vessels.

The pia mater of the choroid plexus is called tela choroidea. In the ventricles, the choroid plexus forms the CSF.

Meninges of the Spinal Cord

• Similar to the cranial meninges, the spinal meninges also consist of the spinal dura mater, arachnoid mater and pia mater.
• These tubular membranes cover the spinal cord and extend into the vertebral canal from the foramen magnum to the level of the second sacral vertebra.

Spinal Dura Mater

• The spinal dura mater is a tough fibrous membrane, which is continuous with the cranial dura at the level of the foramen magnum.
• Below, it narrows at the lower border of the S2 vertebra and covers the thin filum terminale. A the level of the coccyx, it blends with the periosteum covering the posterior aspect of the coccyx.

Arachnoid Mater

The arachnoid mater is the continuation of the cranial arachnoid mater. Similar to the dura, it also extends from the foramen magnum to the S2 vertebra where it blends with
the filum terminale.

Pia Mater

• The pia mater closely covers the spinal cord. Above, it is in continuation with the pia mater covering the brain. At the lower end of the spinal cord (conus medullaris), it covers the filum terminale.
• On each side of the spinal cord, the pia mater is present in the form of a fold. This fold is attached between the origin of dorsal and ventral spinal roots. It is known as ligamentum denticulatum.

Cerebrospinal Fluid

• The CSF is a clear, colourless liquid containing a small amount of protein, glucose and potassium and a large amount of sodium chloride.
• The CSF is present in the ventricles and subarachnoid space surrounding the brain and the spinal cord.
• It protects the brain and the spinal cord from physical injuries and carries oxygen and nutrients from blood to neurons and neuroglia.

Formation of the Cerebrospinal Fluid

The CSF is produced by choroid plexuses of the lateral, third and fourth ventricles. The net production of CSF is about 400-500 mL/day.

Blood-CSF Barrier

The choroid plexues are made up of a single layer of cuboidal epithelium (modified ependyma) enclosing an extensive capillary network embedded in the connective tissue stroma. Thus, the blood-CSF barrier is formed by the following structures:

• Endothelial cells which are fenestrated
• Basement membranes of endothelial cells
• Layer of pale cells and their processes (derived from pia mater)
• The basement membrane of the choroidal epithelium
• Choroidal epithelium (modified ependymal cells) with tight junctions

The CSF is formed due to the passage of materials through these barriers and also by active secretions from the choroidal epithelium.

The barrier protects the brain and spinal cord from potentially harmful blood-borne substances.

Circulation and Absorption of the Cerebrospinal Fluid

• After being produced by the choroid plexus, the CSF circulates through the ventricles ofthe brain, subarachnoid space and central canal of the spinal cord.
• The movement of the vertebral column and the pulsation of arteries present in the subarachnoid space assist the movement of CSF.
• The CSF is reabsorbed into the venous blood through arachnoid villi and granulation. Ependyma, arachnoid capillaries and lymphatics of meninges also absorb some CSF.

Functions of the Cerebrospinal Fluid

• The CSF surrounds the brain and the spinal cord; therefore, it serves as a cushion between the delicate nervous tissue and the surrounding cranial and vertebral bones.
• The CSF provides a medium for the exchange of nutrients and waste products between the nervous tissue and blood.
• By providing a medium for the exchange of nutrients and removal of waste products, the CSF maintains intracranial pressure.
• Certain hormones are transported by the CSF; for example, the secretion of the pineal gland is carried by the CSF to the pituitary gland.

Hydrocephalus

Hydrocephalus is defined as excessive collection of CSF leading to an increase in CSF pressure. High CSF pressure causes atrophic changes in the brain substance.

Hydrocephalus may be caused due to the following reasons:

Blockage in the normal circulation of CSF

In rare cases, it may also be due to excessive production of CSF as in the case of a tumour of choroid plexus Lumbar Puncture (Spinal Tap)

The CSF can be obtained for biochemical analyses by a procedure called lumbar puncture.

Lumbar puncture is sometimes also used to inject drugs into the subarachnoid space, for example, spinal anaesthesia.

The CSF is obtained by inserting a long needle into the lumbar subarachnoid space in the midline between the third and fourth lumbar spine.

Apart from lumbar puncture, CSF can also be obtained from cisternal puncture.

Summary

• The three meninges—dura mater, arachnoid mater and pia mater—are connective tissue membranes, which cover the brain and the spinal cord.
• Arachnoid and pia are collectively called leptomeninges.
• The cerebral dura mater consists of two layers:
• Endosteal and Meningeal.
• At places, the meningeal layer separates from the endosteal to form a double-layered fold known as ‘dural folds’. These folds or septa lie between the major parts of the brain, for example, falx cerebri and tentorium cerebelli.
• At places, the separation of meningeal and endosteal layers of dura encloses a triangular space. This space encloses
  the ‘dural venous sinus’.
• The extradural haemorrhage occurs between the endosteal and meningeal layers of the dura mater.
• Superior sagittal, cavernous and sigmoid sinuses are common sites of thrombosis.
• The arachnoid mater is a delicate, thin and transparent membrane.
• The cerebellomedullary cistern is the largest cistern, which lies at the angle between the cerebellum and the medulla oblongata.
• Arachnoid villi and granulations are minute projections of arachnoid mater into dural venous sinuses.
• The spinal cord is also covered by three meninges:
• Dura, Arachnoid and Pia mater
• The dura and arachnoid mater extend from the foramen magnum to the S2 vertebral level where they blend with the filum terminale.
• The pia mater closely covers the spinal cord, rootlets and filum terminale.
• CSF is produced in choroid plexuses, which are present in the ventricles of the brain.
• The fold of the pia mater along with blood capillaries invaginates into ventricular cavities. This invaginating vascular tuft is known as the choroid plexus of the ventricles.
• A blood-CSF barrier is formed by choroidal epithelium, a layer of pale cells and fenestrated capillary epithelium.
• The movement of CSF, in the spinal subarachnoid space, is assisted by the pulsation of the arteries which are situated around the spinal cord. The movement of CSF is also assisted by the movement of the vertebral column.

Multiple Choice Questions

Question 1. The delicate nervous tissue of the CNS is protected by the following structures except

1. Bones
2. Meninges
3. Ligamentum denticulatum
4. Cerebrospinal fluid

Answer: 3. Ligamentum denticulatum

Question 2. Which of the following is known as leptomeninges?

1. Dura mater
2. Arachnoid mater
3. Dura and arachnoid mater
4. Arachnoid and pia mater
5. Pia mater

Answer: 4. Arachnoid and pia mater

Question 3. Which are the structures drained by dural venous sinuses?

1. Cranial bones
2. Brain
3. Meninges
4. All of the above
5. Only b and c

Answer: 4. Only b and c

Question 4. The following are the paired dural venous sinuses except

1. Cavernous
2. Sigmoid
3. Occipital
4. Sphenoparietal
5. Inferior petrosal

Answer: 3. Sphenoparietal

Question 5. The following facts about extradural haemorrhage are correct except

• Most commonly, it occurs due to rupture of the middle meningeal artery
• It is usually secondary to a fracture of the skull in the area of the pterion
• Blood collects between the endosteal and meningeal layers of dura mater
• The shape of the blood clot, in the CT scan, is biconcave
• The blood clot presses the lateral surface of the cerebrum

Answer: 4. The shape of the blood clot, in the CT scan, is biconcave

Question 6. Which are the subarachnoid cisterns surrounding the brainstem and cerebellum?

1. Cerebellomedullary cistern
2. Pontine cistern
3. Medullary cistern
4. Interpeduncular cistern
5. All of the above

Answer: 5. All of the above

Auditory And Vestibular Systems

• Hearing and equilibrium are two special somatic senses. The receptors for these two special senses are housed in a complex sensory organ, the membranous labyrinth.
• Encased in a bony labyrinth, the membranous labyrinth is situated in the internal ear.
• The cochlear part of the membranous labyrinth is concerned with auditory impulses. The vestibular part of the membranous labyrinth, on the other hand, has receptors for equilibrium.

Auditory System

The auditory impulses travel through the external ear, middle ear, and cochlear part ofthe internal ear. Thereafter, impulses travel through the cochlear nerve before finally reaching the cerebral cortex.

Cochlear Nerve

The cochlear nerve is predominantly a special somatic sensory nerve. It also contains a small motor (somatic efferent) component. Thus, it is a mixed nerve.

Sensory Component

• The cochlear nerve arises as the central processes (axons) of bipolar neurons of the spiral ganglion. Most of these fibers are myelinated.
• The nerve passes through the internal acoustic meatus along with the vestibular nerve.
• After coming out through the internal acoustic meatus, the nerve reaches the pontomedullary junction where it bifurcates to enter the brainstem.
• One branch of the cochlear nerve ends in the dorsal cochlear nucleus while the other ends in the ventral cochlear nucleus.
• The fibers of the nerve ending in both the nuclei in an orderly sequence (i.e. fibers responding to high frequencies terminate in dorsal regions and those of low frequencies in ventral regions).

Motor Component

• The outer and inner hair cells of the cochlea are innervated by cholinergic neurons of the superior olivary nuclei of both sides.
• The fibers after arising from the superior olivary nucleus of both sides form an olivocochlear bundle and travel through the cochlear nerve to the hair cells of the organ of Corti.
• The stimulation of efferent fibers inhibits auditory nerve response to acoustic stimuli (reduces the sensitivity of the ear).
• Central inhibition is necessary to suppress the background noise when attention is being paid to a particular sound.

Location and Parts of Cochlear Nerve Nuclei

The cochlear nerve nucleus consists of two parts:

1. Dorsal and
2. Ventral cochlear nuclei.

These nuclei are situated on the dorsal and ventral aspects of the inferior cerebellar peduncle, respectively. These nuclei are located at the level of the pontomedullary junction.

Auditory Pathway

The impulses in the auditory pathway travel between the receptors and the auditory area of the cerebral cortex. The receptors in this case are the hair cells in the organ of Corti.

Sensory Neurons in the Auditory Pathway

The following sensory neurons are involved in the auditory pathway:

Bipolar neurons: The bipolar cells of spiral ganglia are first-order sensory neurons.

The central processes of bipolar neurons form a cochlear nerve, which bifurcates to terminate in the dorsal and ventral cochlear nuclei on the same side.

Cochlear nuclei: The dorsal and ventral cochlear nuclei are the second-order sensory neurons in the auditory pathway.

Superior olivary nucleus: The neurons of the superior olivary nucleus consist of third-order sensory neurons.

This nucleus is present in the lower pons at the level of the motor nucleus of the facial nerve.

The nucleus of the trapezoid body and the nucleus of the lateral lemniscus are considered a part of the superior olivary nucleus and represent the third-order sensory neurons in the auditory pathway.

Inferior colliculus: The neurons of the inferior colliculus constitute fourth-order sensory neurons in the auditory pathway. The inferior colliculus is concerned with the integration of acoustic impulses.

Medial geniculate body: The neurons of the medial geniculate body (MGB) constitute the first-order sensory neurons in the auditory pathway.

Fibers of the Auditory Pathway

The nuclei in the auditory pathway are interconnected by fibers that form different bundles or tracts. These are as follows:

Trapezoid body: The axons from the ventral cochlear nucleus run towards the ipsilateral superior olivary nucleus.

Some of these fibers terminate here while others cross the midline to terminate in the opposite superior olivary nucleus.

The crossing fibers of two sides form a prominent band called the trapezoid body.

Lateral lemniscus: It is the ascending tract formed mainly by the axons of the superior olivary nucleus. This lemniscus contains both crossed and uncrossed fibers.

The fibers of the lateral lemniscus make synaptic contact with the neurons of the inferior colliculus.

Inferior brachium: The axons of the inferior colliculus travel in the inferior brachium to terminate in the MGB.

Auditory radiation: The axons of the MGB form the auditory radiation, which travels in the sublentiform part of the internal capsule to reach the primary auditory cortex of the temporal lobe.

Cortical Area For The Auditory Pathway

• The primary auditory area (areas 41 and 42) is located on the floor of the lateral sulcus on the dorsal surface of the superior temporal gyrus.
• The recognition and interpretation of sound based on experience occur in the auditory association cortex. It is located posterior to the primary auditory cortex (areas 41 and 42).

Auditory Reflexes

The important auditor)’ reflexes are given in the following text.

Reflexes Turning of the Head and Conjugate Movement of the Eyes

• This occurs in response to a sudden loud sound. As indicated earlier, some fibers from the inferior colliculus connected to the superior colliculus.
• The superior colliculus through the tectospinal tract is connected with motor neurons innervating the neck muscles.
• Similarly, the collateral branches of the lateral lemniscus are connected with the nuclei of extraocular muscles via medial longitudinal fasciculus (MLF).
• These pathways are responsible for turning the head and conjugating the movement of eyes toward the source ofthe sudden loud sound.

Reflexes Reduction in Vibration of the Tympanic Membrane

• This occurs following a loud sound. The fibers of the superior olivary nucleus are in synaptic contact with the motor nuclei of 5 and 7 cranial nerves.
• The motor nuclei of these nerves innervate the tensor tympani and stapedius muscles, respectively.
• Following the loud sound, reflex contraction of tensor tympani and stapedius muscles occurs, which finally results in the reduction of vibration of the tympanic membrane and stapes (stapedius reflex).
• This reflex protects the delicate structures of the cochlea.

Vestibular System

• The vestibular system is concerned with the maintenance of equilibrium of the body and the fixity of gaze.
• Apart from the vestibular apparatus, the cerebellum plays an important role in the maintenance of the body equilibrium.

Vestibular Nuclei: Central Connections

The vestibular nuclei are present in the lower pons and upper medulla beneath the vestibular area of the floor of the ventricle.

The vestibular area consists offour vestibular nuclei:

Superior, Lateral, Medial, and Inferior.

Afferent Connections of Vestibular Nuclei

• The vestibular nuclei receive afferents from vestibular receptors through the vestibular nerve, cerebellum, and vestibular nuclei of the opposite side.
• The vestibular pathway consists of first-order neurons (bipolar neurons), second-order neurons (vestibular nuclei), and third-order neurons (in the thalamus).
• The third-order neurons of this pathway project to the postcentral gyrus, the cortical area for vestibular sensation.

Efferent Connections of Vestibular Nuclei

The efferents from vestibular nuclei project to the cerebellum, brainstem (motor nuclei of cranial nerves), spinal cord, and cerebral cortex.

Vestibulospinal Tract

• The vestibulospinal tract originates from the cells of the lateral vestibular nucleus.
• The fibers of this tract are uncrossed and descend in the medulla dorsal to the inferior olivary nucleus and continue throughout the spinal cord in the ventral funiculus.
• The vestibulospinal fibers terminate in the anterior horn cells (motor neurons) that supply skeletal muscles.
• This tract is concerned with the maintenance of balance by regulating the tone of the muscles involved in posture.

Medial Longitudinal Fasciculus

• The axons of medial and inferior vestibular nuclei descend in the MLF (medial vestibulospinal tract) of both sides.
• These fibers travel through the floor ofthe fourth ventricle and medulla to terminate in the cervical part of the spinal cord.
• This tract influences the cervical motor neurons which move the head in such a way that equilibrium and fixation of gaze are maintained.

Medial Longitudinal Fasciculus Functions

• The vestibular nerve is concerned with conveying impulses associated with equilibrium The hair cells in the ampulla of the semicircular canal are sensors of kinetic balance (rotation of the head in any plane).
• The hair cells of the utricle are sensors of changes in gravitational forces, linear acceleration in the long axis of the body, and position of the head in space (i.e. static balance).
• The hair cells of the saccule are sensors of linear acceleration in the ventrodorsal axis of the body.

Stimulation of Labyrinth

• Vertigo is a sensation of movement in which the surrounding environment seems to revolve.
• It is a common symptom of the disease of the vestibular system. Probably, the cortical projections are responsible for the sense of vertigo.

Motion sickness: It is characterized by many symptoms such as nausea, headache, dizziness, and vomiting.

• This disease is caused due to motion during travel by road, sea, or air.
• Motion sickness occurs due to different messages received by the vestibular apparatus and eyes.
• For example, while traveling in a vehicle, the vestibular apparatus senses the motion but the eyes looking at the interior of the vehicle perceive it as still. These conflicting messages give the feeling of nausea.

Auditory And Vestibular Systems Summary

Hearing and equilibrium are two special somatic senses. The cochlear part of the membranous labyrinth is concerned with the reception of sound waves while the vestibular part of the labyrinth contains receptors for equilibrium.

Auditory system

• The cochlear nerve ends in dorsal and ventral cochlear nuclei.
• The axons of cochlear nuclei terminate on the superior olivary nucleus.
• The axons of the superior olivary nucleus form the lateral lemniscus.
• The lateral lemniscus ends on the neurons of the inferior colliculus whose fibers terminate on the medial geniculate body.
• The fibers of the medial geniculate body form auditory radiation which terminates on the auditory cortex.

Vestibular system

• The vestibular nuclei are present in the lower pons and upper medulla beneath the ‘vestibular area’ of the floor of the fourth ventricle.
• The vestibular part of the vestibulocochlear nerve ends in these nuclei.
• The afferent connectors of vestibular nuclei are from the vestibular nerve, from the cerebellum, and the opposite vestibular nuclei.
• The efferent connections of vestibular nuclei go to the cerebellum, brainstem, spinal cord, and cerebrum.

Auditory And Vestibular Systems Multiple Choice Questions

Question 1. Which of the following statements is false?

1. Hearing and equilibrium are two special visceral senses
2. The receptors for these two special senses are located in the membranous labyrinth
3. The membranous labyrinth is located in the internal ear
4. The cochlea is concerned with hearing
5. The vestibular part of the membranous labyrinth has receptors for equilibrium

Answer: 1. Hearing and equilibrium are two special visceral senses

Question 2. Which of the following steps is false in the transmission of sound from the tympanic membrane to the cochlea?

1. Vibration of the tympanic membrane
2. Vibration of malleus, incus and stapes
3. Vibration of the membrane covering the round window
4. Vibration of perilymph of scala vestibule and scala tympani
5. Vibration of the basilar membrane

Answer: 3. Vibration of perilymph of scala vestibule and scala tympani

Question 4. Which are the parts of a membranous labyrinth?

1. Utricle
2. Saccule
3. Semicircular canals
4. Cochlea
5. All of the above

Answer: 1. Utricle

Question 5. Which of the following functional components are present in the cochlear nerve?

1. General somatic efferent
2. Special visceral efferent
3. General somatic efferent
4. Special somatic efferent

Answer: 3. General somatic efferent

Question 5. Which of the following ganglia/nuclei is not involved in the auditory pathway?

1. Bipolar cells of the spiral ganglion
2. Cochlear nuclei
3. Inferior olivary nucleus
4. Inferior colliculus
5. Medial geniculate body

Answer: 3. Inferior colliculus

Question 6. Which of the following is not a part of the vestibular apparatus?

1. Scala vestibule
2. Utricle
3. Saccule
4. Semicircular ducts

Answer: 1. Scala vestibule

Question 7. The vestibular area consists ofthe following nuclei except

1. Superior
2. Inferior
3. Medial
4. Lateral
5. Dorsal

Answer: 5. Dorsal

Visual System

The visual system is concerned with the special sense of vision. The visual system or the optic pathway begins from the retina in the eyeball and ends in the visual cortex of the occipital lobe

Optic Chiasma

The optic nerves of two sides cross and form optic chiasma. In optic chiasma, the fibres of the optic nerve arising from the nasal half cross to the opposite side while fibres from the temporal half do not.

Optic Tract

Each optic tract consists of crossed and uncrossed axons that project from optic chiasma to the lateral geniculate body (LGB) of that side. The optic tracts curve around the midbrain before they terminate in the LGB.

Each optic tract consists of fibres from the temporal half of the retina from the same eye and the nasal half of the retina from the opposite eye.

Retina

The retina is the innermost coat of the eyeball. The retina consists of a layer of pigmented epithelium, a layer of rods and cones, a layer of bipolar cells and a layer of ganglion cells.

Optic Nerve

The axons of ganglion cells form the optic nerve, and these fibres exit the eyeball at the optic disc’. As soon as optic nerve fibres come out of the sclera, they acquire a myelin sheath.

The optic nerve which has about 1 million fibres is surrounded by the meningeal layers, i.e. pia, arachnoid and dura.

The central artery and central vein of the retina are present in the anterior part of the optic nerve.

Lateral Geniculate Body

• The LGB is the main terminus for input to the primary visual cortex (striate cortex, area 17).
• The LGB is a small projection from the pulvinar part of the thalamus retina of the opposite side (crossed fibres) and terminates in layers 1, 4 and 6 while those from the ipsilateral retina (uncrossed fibres) end in layers 2, 3 and 5.
• The fibres from the temporal half of each retina terminate in the LGB of the same side while those from the nasal half terminate in the LGB of the opposite sides.

Geniculocalcarine Tract (Optic Radiation]

• The fibres arising from the LGB first traverse the sub lentiform and then the retrolentiform parts of the internal capsule.
• Thereafter, these fibres terminate in the visual cortex (area 17) ofthe same side. These fibres constitute the geniculocalcarine tract or the optic radiation.

Visual Cortex

The visual cortex consists of a primary area and an association area. The association area or the association cortex is involved in the recognition of objects and perception of colours, depth and motion.

Primary Visual Cortex

• The primary area or the primary visual cortex is an area where optic impulses reach the level of consciousness The visual cortex has a representation of the retina.
• The central part of the retina is represented on the occipital pole while its peripheral part is represented at the anterior part of the visual cortex.

Visual Association Cortex

The association cortex (areas 18 and 19) is involved in the recognition of objects and perception of colours, depth and motion. This is achieved by relating the present to past visual experience.

Visual Field

• The area seen by one eye, when one looks ahead with the eyes fixed, is the visual field of that eye. The visual field of each eye is divided into the nasal half and the temporal half.
• The light rays from an object situated in the temporal half of the visual field fall on the nasal half of the retina and the light rays from an object situated in the nasal half of the visual field fall on the temporal half of the retina.
• The upper half of the visual field projects onto the inferior half of the retina while the lower half of the visual field projects to the superior half of the retina.
• Therefore, damage to the upper retina will produce a deficit in the lower visual field. It should be noted that due to the presence of a convex lens in the eye, the visual image that is formed on the retina is inverted.

Visual Field Defects

• Visual field defects are characterised by the loss of a part of the normal area of vision in one or both eyes.
• The visual field defects may range from loss of area at the outer edges of vision (peripheral vision), or from a small blind spot or from a large area to complete blindness.
• Hemianopia means loss of vision in one half of the visual field. The visual defects may be caused by damage to any part of the visual pathway.

Reflexes Associated With Vision

Reflexes associated with vision include pupillary light reflex and accommodation reflex.

Pupillary Light Reflex

The pupillary light reflex includes both a direct response and a consensual response.

Direct Light Reflex

• When light is thrown on one eye with the help of a torch, it causes constriction of the pupil (iris) in that eye. This is known as direct response or direct light reflex.
• The pupillary constriction or the constriction of the constrictor muscles of the iris occurs due to stimulation of the Edinger-Westphal nucleus.

Consensual Light Reflex

Simultaneously with the direct light reflex, the pupil of the other eye also constricts. This is known as consensual response or consensual light reflex.

This reflex is seen because of the following two facts:

1. Each retina sends afferent signals to the optic tracts of both sides. Fibres cross in the optic chiasma; these fibres later terminate in the pretectal nuclei of both sides.
2. The pretectal nucleus of each side sends connections to the Edinger-Westphal nuclei of both sides.

Accommodation Reflex

After looking at a distance for some time and then looking at a near object, the visual responses that are observed are given in the following text.

Ocular Convergence

When we look at a close object, our eyes must rotate medially to focus the light rays on the same corresponding points on both the retinas. This is achieved by the contraction of the medial recti muscles on both sides.

Accommodation

• As the eye focuses on close objects, the lens becomes more curved. This increase in curvature of the lens for the near vision is called accommodation.
• The accommodation is achieved through the contraction of the ciliary muscles.
• As a result of the contraction of ciliary muscles, the tension in the lens and suspensory ligaments is released and the lens therefore becomes more convex.

Constriction of Pupil

• Simultaneously with the accommodation and convergence, the pupil also constricts due to the constriction of circular muscle fibres of the iris.
• This helps to prevent the light rays from entering the eye through the periphery of the lens. This is needed to prevent blurring of vision and to achieve sharpness of image on the retina.
• The pathway of these responses observed during the accommodation reflex is illustrated.

Argyll Robertson Pupil

• In certain central nervous system lesions (e.g. syphilis), the pupillary light reflex is abolished without affecting the accommodation reflex.
• This means that the constriction of the pupil after exposure to torch light gets abolished; however, constriction of the pupil occurs when the eyes are focused on a near object. The condition is known as Argyll.
• Robertson pupil. This indicates that the pathways of the light reflex and those of the accommodation reflex are different.
• In the case of light reflex, the pretectal nucleus and its axons are involved which are probably destroyed in syphilis (tabes dorsalis) due to dilatation of the cerebral aqueduct. On the other hand, the accommodation reflex involves the visual cortex and not the pretectal nucleus.

Visual System Summary

• The visual system is concerned with the special sense of vision.
• The retina contains rod and cone cells that convert stimuli of light into various electrical Impulses.

The retina consists of four layers:

• Pigment epithelial,
• Rods and cones,
• Bipolar cells and
• Ganglion.
• Rods are more sensitive to dim light while bright light stimulates cones.
• Bipolar cells are neurons connecting rods and cones with ganglion cells.
• The axons of ganglion cells form the optic nerve. The optic nerves of two sides now cross at optic chiasma to form optic tracts.
• The optic tracts curve around the midbrain and terminate in lateral geniculate bodies.
• The fibres arising from the lateral geniculate body terminate in the visual cortex (area 17) on the same side. There, fibres constitute optic radiation.
• The optical impulses reach the level of consciousness in the primary visual cortex (area 17).
• The association visual cortex (areas 18 and 1 9) is involved in the recognition of objects and perception of colours, depth and motion.

Visual System Multiple Choice Questions

Question 1. Which of the following cells are not present in the retina?

1. Pigment epithelial cells
2. Rods and cones
3. Bipolar cells
4. Ganglion cells
5. Stellate cells

Answer: 5. Stellate cells

Question 2. Which of the following statements about rod cells is false?

1. Each retina contains about 120 million rod cells
2. Rods are absent in the central part of the fovea
3. Rods are more sensitive to dim light
4. Rods are involved in colour vision
5. Rods are specialised for night vision

Answer: 4. Rods are involved in colour vision

Question 3. Which of the following facts about the macula lutea is false?

1. It lies lateral to the optic nerve
2. It lies along the visual axis of the eye
3. Its central part is known as the fovea
4. It contains only cone cells
5. None of the above

Answer: 3. It contains only cone cells

Question 4. Which of the following statements about the optic nerve is false?

1. The optic nerve is formed by the axons of ganglion cells of the retina
2. The fibres of the optic nerve are myelinated
3. These fibres come out of the sclera through the macula lute
4. The optic nerve is surrounded by dura, arachnoid and pia mater
5. The optic nerve has about 1 million fibres

Answer: 3. The optic nerve is surrounded by dura, arachnoid and pia mater

Question 5. Which of the following is not a part of the optic pathway?

1. Optic nerve
2. Optic chiasma
3. Optic tract
4. Medial geniculate body
5. Geniculocalcarine tract

Answer: 3. Medial geniculate body

Question 6. Name the fibres received by the right geniculate body.

1. From the temporal half of the right retina
2. From the nasal half of the right retina
3. From the temporal half of the left retina
4. From the nasal half of the left retina

Answer: 1. From the temporal half of the right retina

Question 7. The visual association cortex (areas 18 and 19) is involved in which of the following functions?

1. Recognition of object
2. Perception of colour
3. Depth of vision
4. Motion
5. All of the above

Answer: 3. Motion

Question 8. Which of the following responses is not associated with the accommodation reflex?

1. Ocular convergence
2. Accommodation
3. Constriction of pupil
4. Saccadic eye movements

Answer: 4. Saccadic eye movements

Ventricles Of The Brain

• Ventricles of the brain are cavities that are present inside the brain. They are lined with ependyma and filled with cerebrospinal fluid (CSF).
• The ventricles are four in number and are named lateral ventricles (a pair) third and fourth.
• These CSFs flow from the lateral ventricles to the third ventricle and from the third to the fourth ventricle.
• The fourth ventricle is continuous with the subarachnoid space through openings in its roof (refer to Chapter 23 for circulation of CSF).
• The choroid plexus, which is responsible for the production of CSF, is present in all the cavities (ventricles).

Lateral Ventricle

• Each cerebral hemisphere has a cavity called the lateral ventricle, which consists of the anterior horn, body, posterior horn, and inferior horn.
• The central part or the body of the lateral ventricle is situated in the parietal lobe.
• The central part of the right and left lateral ventricles lies close to the median plane separated from each other by septum pellucidum.
• The anterior horn extends into the frontal lobe, the posterior horn extends into the occipital lobe and the inferior horn extends into the temporal lobe.
• Each lateral ventricle communicates with the third ventricle through the interventricular foramen.
  • Central part
  • Anterior horn
  • Posterior horn
  • Inferior horn

Central Part

The central part of the lateral ventricle extends J anteroposteriorly between the interventricular foramen and the splenium of the corpus callosum. The central part becomes continuous anteriorly with the anterior horn while posteriorly with the posterior and inferior horns. Moreover, the central part of the ventricle is triangular in cross-section and thus has three walls.

1. Roof: It is formed by the undersurface of the corpus callosum.
2. Floor: It is formed from the lateral to the medial side by the caudate nucleus, thalamostriate vein and stria terminalis, upper surface of the thalamus, choroid plexus and fornix.
3. Medial wall: It is formed by the septum pellucidum.

Most medially, there is a slit-like space between the upper surface of the thalamus and fornix. ‘Ibis space is called a choroid fissure through which the choroid plexus invaginates the lateral ventricle.

Anterior Horn

The anterior horn is present anterior to the interventricular foramen and extends forwards, laterally and downwards in the frontal lobe of the brain. It is triangular in section and has

• Roof: Undersurface of corpus callosum
• Floor: Upper surface of the rostrum of the corpus callosum
• Medial wall: Septum pellucidum
• Lateral wall: Head of the caudate nucleus
• Anterior wall: Genu of corpus callosum.

Posterior Horn

The posterior horn extends backwards and medially into the occipital lobe

Roof and lateral wall: Tapetum of corpus callosum

Medial wall: Formed by two elevations, i.e. bulb of the posterior horn and calcar avis.

Inferior Horn

The inferior horn begins at the posterior end of the body ofthe lateral ventricle at the level ofthe splenium of the corpus callosum.

The inferior horn at first passes backwards and laterally and then passes downwards behind the thalamus into the temporal lobe.

In the temporal lobe, it takes a turn forward and ends in uncus. The inferior horn has a roof and a floor.

Roof: Above by stria terminalis and tail of caudate nucleus; below and laterally by tapetum. The amygdaloid nucleus lies in its anteriormost part of the roof.

Floor: Medially, it is formed by the hippocampus and laterally by collateral eminence.

Third Ventricle

The third ventricle is a slit-like narrow space of diencephalon. It is situated between two thalami.

The cavity of the third ventricle communicates with the right and left lateral ventricles through the interventricular foramen. Posteriorly, the cavity communicates with the fourth ventricle through the cerebral aqueduct.

As the cavity is situated in the midline, it has two lateral walls. It also consists of an anterior wall, posterior wall, roof and floor.

Anterior Wall

The anterior wall is formed by the lamina terminalis, anterior commissure and anterior column of the fornix.

Lateral Wall

The lateral wall is large and is divided by the hypothalamic sulcus into an upper thalamic part and a lower hypothalamic part.

Two thalamines are usually connected by interthalamic adhesion.

Posterior Wall

From above downwards, it is formed by supraspinal recess, habenular commissure, pineal recess in the stalk of the pineal gland and posterior commissure.

Roof

The roof is formed by the ependyma stretching between two thalamis. From the roof two choroid plexuses protrude, one on either side of the median plane.

Floor

The floor is formed by the following structures as traced anteroposteriorly: optic chiasma, infundibulum, tuber cinereum, mammillary bodies, posterior perforated substance and tegmentum of midbrain Two recesses are seen in the floor.

These are the optic recess (above) and the optic chiasma) and infundibular recess (above the infundibular stalk).

Fourth Ventricle

The fourth ventricle is found in the hindbrain. Above, it communicates through the cerebral aqueduct with the third ventricle and below it communicates with the central canal of the closed part of the medulla oblongata.

The fourth ventricle lies in front of the cerebellum and behind the lower part of the pons and upper part medulla.

The cerebellum forms the roof of the fourth ventricle and its floor is formed by the pons and upper part of the medulla. The cavity of the fourth ventricle consists of a floor, roof
and lateral wall.

Floor

In the anatomical position, the floor of the fourth ventricle is the anterior wall of the ventricle. The shape of the floor is diamond shaped and hence sometimes called rhomboid fossa.

The floor is divided into upper and lower triangular parts by bundles of transversely running fibres (striaemedullaris)

The upper triangular part is formed by the posterior surface of pons while the lower triangular part is formed by the posterior surface of the upper medulla.

Right and Left Halves

The floor of the ventricle is also divided into right and left halves by a median sulcus. On either side of the median sulcus, a longitudinal elevation is known as the median
or medial eminence.

Upper Part

• Immediately above the striae medullaris, on either side, the medial eminence shows a slight swelling known as facial colliculus. This swelling is produced due to fibres of facial nerve looping around the abducent nerve nucleus.
• At the upper level of the facial colliculus, there is a triangular depression—the superior fovea.
• Just above the superior fovea, the sulcus limitans ends in an area which is bluish and is called locus coeruleus.
• The bluish colouration is due to the presence of the melanin pigment present in the noradrenergic neurons of the nucleus coeruleus.

Lower Part

• The lower part of the floor, below the striae medullaris, shows the presence of two small triangles on either side of the median sulcus.
• These triangles are known as hypoglossal and vagal triangles.
• The vagal triangle is situated lateral to the hypoglossal triangle. The dorsal nucleus of the vagus and solitary nuclei lie deep in the vagal triangle.
• The hypoglossal nerve nucleus lies deep in the hypoglossal triangle.
• The lower part of the sulcus limitans close to the apex of the vagal triangle presents a depression—the inferior fovea.

Roof

The roof of the fourth ventricle is tent-shaped and projects posteriorly towards the cerebellum.

Upper Part

The upper part of the roof is formed on each side by superior cerebellar peduncles. The interval between these peduncles is bridged by a thin sheet of white matter—the superior medullary velum.

Lower Part

The lower part of the roof is formed, in small part, by the inferior medullary velum and in large part by tela choroidea.

• The right and left inferior medullary vela are present on each side of the nodule of the cerebellum. The inferior medullary velum is made up of a thin sheet of white matter and merges posteriorly in the white matter of the cerebellum.
• The inner surface of the inferior medullary velum is covered by ependyma while its outer surface is covered by pia mater.
• At the free margin of the inferior medullary velum, the ependyma and pia mater come close to each other and form the tela choroidea of the fourth ventricle. Thus, the lowest part of the roof of the fourth ventricle is membranous.
• Laterally, on either side, this membrane fuses with the inferior cerebellar peduncle.
• The lower part of this membrane presents a large median aperture (foramen of Magendie) through which the fourth ventricle communicates with the cerebellomedullary cistern.
• The roof also presents two lateral apertures through which the ventricle communicates with the subarachnoid space.

Lateral Walls

The upper part of the fourth ventricle is bounded laterally by the right and left superior cerebellar peduncles.

The lower part is bounded laterally by the right and left inferior cerebellar peduncles and gracile and cuneate tubercles.

Cavity of the Fourth Ventricle

The cavity of the fourth ventricle is somewhat diamond-shaped and lined by ependyma. It has superior, inferior and two lateral angles. The cavity communicates above with the third ventricle through the cerebral aqueduct.

Openings

• The fourth ventricle communicates below (at its inferior angle) with the central canal of the medulla oblongata.
• It has three openings in the roof—one median and two lateral through which it communicates with the subarachnoid space.
• Two lateral openings (foramina of Luschka), one on each side, lie in the lateral angle of the ventricle between the inferior cerebellar peduncle and the flocculus.
• Through this opening, the CSF escapes into the subarachnoid space. The choroid plexus of the fourth ventricle also protrudes through this opening.

Recesses

The cavity has many pouch-like protrusions which are known as recesses. It has a pair of lateral recesses, a single median dorsal recess and a pair of lateral dorsal recesses.

Summary

• Ventricles are CSF-filled cavities of the brain. These cavities are lined with ependyma.
• Each cerebral hemisphere contains a large C-shaped cavity known as a lateral ventricle.

Each lateral ventricle consists of a body and three horns:

• Anterior, Posterior and Inferior.
• The anterior horn extends into the frontal lobe, the posterior horn in the occipital lobe and the inferior horn in the temporal lobe.
• The third ventricle is situated in the midline and has two lateral walls, anterior wall, roof, floor and posterior wall. The cavity of the third ventricle communicates with the right and left lateral ventricles through the interventricular foramen. Below, it also communicates with the fourth ventricle through the cerebral aqueduct.
• The fourth ventricle is located between the brainstem and the cerebellum. It communicates below with the central canal.
• The floor of the fourth ventricle is diamond-shaped; hence, sometimes it is called rhomboid fossa. The fourth ventricle has three openings in the roof—one median and two lateral.

Multiple Choice Questions

Question 1. Following are parts of the lateral ventricle except

1. The central part of the body
2. Anterior horn
3. Posterior horn
4. Superior horn
5. Inferior horn

Answer: 4. Superior horn

Question 2. Which of the following facts regarding the central part of the lateral ventricle is false?

1. It extends between the interventricular foramen and the splenium
2. It has three walls—roof, floor and medial wall
3. The medial wall is formed by septum pellucidum
4. The floor is formed by the thalamus and caudate nucleus
5. None of the above

Answer: 5. None of the above

Question 3. Which of the following facts regarding the third ventricle is false?

1. It is a slit-like narrow space of diencephalon
2. It communicates with the lateral ventricle through the interventricular foramen.
3. It communicates with the fourth ventricle through the central canal
4. It has two lateral walls

Answer: 3. It communicates with the fourth ventricle through the central canal

Question 4. Which of the following structures is not seen in the floor of the fourth ventricle?

1. Stria terminalis
2. Median sulcus
3. Median eminence
4. Sulcus limitans
5. Vestibular area

Answer: 1. Stria terminalis

Question 5. Which of the following structures are present in the roof of the fourth ventricle?

1. Superior cerebellar peduncles
2. Superior medullary velum
3. Inferior medullary velum
4. Tela choroidea
5. All of the above

Answer: 5. All of the above

Limbic System

The limbic system consists of several interconnecting nuclei and cortical structures in the telencephalon and diencephalon which form a ring-like structure around the upper end of the brainstem.

Functions Of The Limbic System

The limbic system is mainly concerned with emotions, visceral responses to emotions and memory.

The specific functions of the limbic system are as follows:

• Self-preservation: Procuring food, eating and identification of danger
• Species preservation: Sex and rearing of young ones
• Feeling of pleasures that are related to our survival such as those experienced from eating and sex
• Regulation of autonomic and endocrine functions
• Learning and retention of recent memory

Components Of The Limbic System

The limbic system consists of cortical regions, which include limbic lobe and hippocampal formation, and subcortical structures such as amygdala, septal nuclei, hypothalamus
and thalamus.

Cortical Region Of The Limbic System

The cortical region of the limbic system consists of limbic lobe and hippocampal formation.

Limbic Lobe

• The term limbic lobe is used for the cortical structures that are in the form of a rim which surrounds the corpus callosum.
• The limbic lobe includes the orbitofrontal cortex, subcallosal, cingulate and parahippocampal gyri.
• These gyri encircle the upper part of the brainstem and corpus callosum on the medial surface of the cerebral hemisphere.
• The cortex of the limbic lobe has reciprocal connections with various subcortical nuclei of the limbic system.

Hippocampal Formation

The hippocampal formation consists of the hippocampus, dentate gyrus, subiculum, gyrus fasciolaris, indusium griseum and medial and lateral longitudinal striae.

Functions of Hippocampal Formation

• The most important function of hippocampus formation is the retention of information in short-term memory and its transfer into long-term memory.
• This allows the animal to compare the condition of a present threat with a similar experience in the past. This helps the animal in its survival.
• Through its connection with the septal area, hypothalamus and cingulate gyrus, the hippocampus influences emotion, endocrine and visceral functions.

Amnesia refers to loss of memory due to disease or trauma When both the hippocampi get destroyed, recent memory also gets lost.

However, long-term memory persists. This is because long-term memories are stored in various areas of the cerebral cortex.

Hippocampus

• Hippocampus is a longitudinal elevation, located in the entire length of the floor of the inferior horn of the lateral ventricle. Its anterior end is expanded and bears few ridges and grooves, resembling an animal’s foot.
• Hence, it is called pes hippocampi. When traced posteriorly, the hippocampus gradually narrows and ends below the splenium of the corpus callosum.
• The ventricular surface ofthe hippocampus is covered by a thin layer of white matter, known as an alveus.
• The alveus contains axons arising from the hippocampus and dentate gyrus. These fibres collect to form a bundle of fibres on the medial part of the hippocampus, known as fimbria. Posteriorly, fimbria continues as crus of fornix

Fornix

• Fornix consists of efferent fibres from the hippocampus. Each crus of fornix curves upwards behind the thalamus and two crura converge in midline to form the body of fornix.
• At the meeting point of two crura, decussation of fibres from two sides takes place which is known as hippocampal commissure.
• These fibres interconnect the hippocampi of the two sides. Anteriorly, the body of the fornix divides into two columns.
• Each column curves ventrally in front of the interventricular foramen and lies behind the anterior commissure. It then curves posteriorly to end in the mammillary body.

Dentate Gyrus

• The dentate gyrus is a narrow strip of grey matter. It bears a series of notches which give it a toothed appearance, hence the name dentate gyrus.
• When traced anteriorly, the dentate gyrus runs in the cleft of the uncus and then turns medially as the tail of the dentate gyrus. Posteriorly, it is continuous with gyrus fasciolaris.

Indusium Griseum and Longitudinal Striae

Indusium griseum is a thin layer of grey matter covering the upper surface of the corpus callosum. It is continuous posteriorly with the gyrus fasciolaris and anteriorly with the septal area.

Within the substance of the indusium griseum, two bundles of longitudinal running fibres are present on each of the midlines.

These fibres are called medial and lateral longitudinal striae. Both the indusium griseum and lateral longitudinal striae are hippocampal rudiments.

The large pyramidal cells in area CA1 of the hippocampus are highly sensitive to lack of oxygen and die within a few minutes.

The formation of new memory is concerned with involving area CA1 with the help of the phenomenon known as long-term potentiation.

Loss of memory and intellectual functions are severely affected if the pyramidal cells of the hippocampus are affected. This happens in a variety of conditions including Alzheimer’s disease.

Connections of Hippocampus

The connection of the hippocampus is illustrated.

Afferent Connections From the Entorhinal Area (Area 28)

• Hippocampal formation receives a variety of sensory information including olfaction because of its connections with the entorhinal area.
• In addition to the extrinsic connections between the entorhinal area and the hippocampus mentioned in the preceding text, intrinsic connections in the form of a closed circuit are also present.
• This circuit consists of the dentate gyrus to cornu ammonis (hippocampus) subiculum to the entorhinal area and back to the dentate gyrus.

Efferent Connections

The efferent fibres ofthe hippocampus form the fornix. Circuit of Papez (Hippocampal Circuit] The limbic system contains a ring (circuit) of interconnected neurons (nuclei). This circuit is depicted.

The Papez circuit interconnects the hippocampus with the hypothalamus (mammillary body through fornix), thalamus (anterior nucleus), cingulate gyrus and back to the hippocampus through cingulohippocampal fibres.

Subcortical Regions Of The Limbic System

The subcortical portions of the limbic system include the olfactory bulb, amygdala, septal nuclei, hypothalamus (mainly mammillary body), anterior nucleus of the thalamus, habenular nuclei, stria medullary thalami and midbrain reticular formation.

Amygdala

The amygdala is located near the temporal pole in the anterior part ofthe roof of the lateral ventricle.

The tail of the caudate nucleus is in contact with the amygdala in the roof of the inferior horn of the lateral ventricle.

Functions

The functions of the amygdaloid body are as follows:

• The amygdala part of the limbic system is considered to be most specifically involved with emotional experiences such as fear and anxiety.
• The emotional responses are processed in the amygdala.
• The amygdala can identify danger which is fundamental to self-preservation.

Connections

Afferent Connections

• From cortical areas: Orbitofrontal cortex, prepyriform and pyriform regions of the olfactory cortex, temporal lobe, cingulate gyrus and hippocampus.
• From the thalamus, hypothalamus and habenular nucleus
• From reticular formation.

Efferent Connections

The efferent fibres from the amygdala pass through the stria terminalis.

• To the temporal cortex and cingulate cortex,
• To reticular formation and
• To septal area and thalamus.

Stria Terminalis This begins from the posterior end of the amygdala. It is a bundle of white matter formed by efferent fibres from the amygdala.

At the interventricular foramen, most of its constituent fibres terminate in the septal area, preoptic area, anterior hypothalamus and stria medullaris thalami. The stria medullaris thalami fibres end in the habenular nuclei.

Damage to the amygdala in an experimental animal produces tameness (deprivation of affection and indifference to danger).

The PET and MRI studies have indicated that the amygdala is involved in the recognition of an emotional facial expression.

A patient with bilateral degeneration of the amygdala is not able to judge correctly the human facial expressions of sadness, happiness, surprise, fear, anger and disgust.

Septal Region

The septal region is present in front of the lamina terminalis beneath the genu and rostrum of the corpus callosum.

Septal nuclei lie deep in the cortex of the septal region. The septal region is continuous superiorly with the indusium griseum and inferiorly with the diagonal band and medial olfactory stria. The diagonal band connects the amygdaloid body with septal nuclei.

Functions

• Neuroendocrine and behavioural functions.
• Some consider the septal region a centre of orgasm.

Connections

Afferent Connections

Afferents come from amygdale, midbrain reticular formation and hippocampus

Efferent Connections

Efferents go to the hippocampus, reticular formation, habenular nuclei and cingulate gyrus.

Limbic System Summary

• The limbic system consists of many nuclei and cortical areas in the telencephalon and diencephalon which are interconnected with each other.
• The limbic system consists of cortical and subcortical structures. The cortical structures include the limbic lobe and hippocampal formation.
• The subcortical structures include the olfactory bulb, amygdala, septal nuclei, mammillary body, anterior nucleus of the thalamus, habenular nuclei, stria medullary thalami and midbrain reticular formation.
• The limbic lobe consists of the orbitofrontal cortex, subcallosal gyrus, cingulate gyrus, parahippocampus and uncus.
• Hippocampus formation consists of hippocampus, dentate gyrus, gyrus fasciolaris, indusium griseum and longitudinal striae.
• The hippocampus is a longitudinal elevation located on the floor of the inferior horn of the lateral ventricle.
• The fibres arising from the hippocampus pass through the alveus, fimbria and fornix.
• The dentate gyrus is a narrow strip of grey matter which bears a series of notches (appearing as teeth). Posteriorly, it is continuous with gyrus fasciolaris and then with indusium griseum.
• A coronal section passing through the inferior horn of the lateral ventricle also passes through the hippocampus, dentate gyrus, subiculum and parahippocampal gyrus.
• The subiculum is a transitional zone between the hippocampus (archicortex) and the parahippocampus (neocortex).
• There is a gradual change in the structure of the subiculum from four through five to a modified six-layered cortex.
• The fornix contains more than 1 million myelinated axons that originate from the pyramid cells of the hippocampus and subiculum.
• There, fibres are first collected in the alveus, then form the fimbria and ultimately the fornix.
• The amygdala is located near the temporal pole in the anteriormost part of the roof of the lateral ventricle close to the tail of the caudate nucleus. The stria terminalis is a bundle of white matter that consists of important efferent fibres of the amygdala.
• The cortex of the septal region is present in front of the lamina terminals beneath the genu and rostrum of the corpus callosum. The septal region is continuous with indusium griseum. It is connected with the medial olfactory stria and amygdala.
• Functionally, the limbic system is concerned with emotions, visceral response to emotions and recent memory.

Limbic System Multiple-Choice Questions

Question 1. The limbic system is concerned with the following functions except

1. Conscious dissemination of taste
2. Emotions
3. Visceral response to emotions
4. Memory

Answer: 1. Conscious dissemination of taste

Question 2. Which of the following cortical areas are included in the limbic lobe?

1. Orbitofrontal cortex
2. Subcallosal gyrus
3. Cingulate gyrus
4. Parahippocampal gyrus
5. All of the above

Answer: 5. All of the above

Question 3. Which of the following subcortical areas are included in the limbic system?

1. Olfactory bulb
2. Amygdala
3. Mammillary body
4. Anterior nucleus of the thalamus
5. All of the above

Answer: 5. All of the above

Question 4. The fibres of stria terminalis terminate in the following regions except

1. Septal area
2. Preoptic area
3. Anterior hypothalamus
4. Habenular nuclei
5. Anterior thalamus

Answer: 3. Anterior hypothalamus

Question 5. Following are the components of hippocampal formation except

1. Hippocampus
2. Dentate gyrus
3. Subiculum
4. Gyrus fascioliasis
5. Septal area

Answer: 3. Subiculum

Question 6. Following are the functions of hippocampal formation except

1. Formation of long-term memory
2. Retention of recent memory
3. Transfer of short-term memory into long-term memory
4. Compares the condition of the present threat with a similar experience

Answer: 1. Formation of long-term memory

Olfactory System

The olfactory system consists of the following structures:

• Olfactory epithelium with olfactory nerves
• Olfactory bulb, tract and striae
• Primary and secondary olfactory cortices

Olfactory Epithelium And Olfactory Nerve

• The olfactory epithelium is present on the roof and back of the nasal cavity.
• The olfactory neurosensory cells of the olfactory epithelium are modified bipolar neurons.
• From the apical pole of each olfactory sensory cell (neuron), a single dendrite runs towards the epithelial surface.
• From each dendrite, about 5-20 thin cilia protrude on the surface while from the basal pole of this sensory neuron, a single axon projects.
• These axons are collected to form about 15-20 olfactory nerves.
• These nerves reach the olfactory bulb through the cribriform plate ofthe ethmoid. The olfactory nerves end in the cells of the olfactory bulb.

Olfactory Bulb, Tract And Striae

• The olfactory bulb is a small, oval structure that lies above the cribriform plate of ethmoid.
• In the olfactory bulb, the incoming sensory axons synapse with the dendrites of olfactory bulb neurons (mitral cells, tuft cells, and periglomerular cells).
• The mitral cells and tuft cells are the principal cells and their axons form the olfactory tract. The periglomerular and granule cells are interneurons of the olfactory bulb.
• Millions of axons of olfactory sensory receptor cells terminate in the synaptic unit called glomeruli.
• Each glomerulus receives many afferent neurons, which synapse with the dendrites of a few principal cells (mitral and tufted cells).
• The activity of principal cells is modified by inhibitory interneurons of olfactory bulbs (granule cells and periglomerular cells).
• The axons of mitral and tufted cells run in the olfactory tract. They also send collateral branches to the neurons of the anterior olfactory nucleus.
• The fibers that originate in the anterior olfactory nucleus pass through the anterior commissure to the opposite olfactory bulb.
• The fibers that go to the opposite olfactory bulb synapse with the dendrites of interneurons. Sensory information is likely to be extensively processed and refined in the olfactory bulb before it is sent to the olfactory cortex.
• The olfactory bulb is continuous posteriorly with the olfactory tract and expands into the olfactory triangle at the anterior end of the anterior perforated substance.

The olfactory tract divides into two roots:

• Lateral and Medial olfactory striae. The lateral stria runs posterolaterally on the margin of the anterior perforated substance and carries most of the axons of the tract. These fibers enter the gyrus semilunaris, which lies anterior to the uncinate gyrus.
• The fibers of the medial olfactory stria probably terminate in the anterior perforated substance and the paraterminal gyrus. It is a rudimentary stria.
• The intermediate olfactory stria is not always present. It ends in the olfactory tubercle in the anterior perforated substance.

Primary Olfactory Cortex

The primary olfactory cortex is that region of the cerebral cortex that is responsible for conscious awareness of olfactory stimuli.

The primary olfactory cortex receives direct afferents from the lateral olfactory stria. Olfaction appears to be unique in the sensory system as the sensations reach the primary cortex without relaying in the thalamus.

However, when the primary olfactory cortex projects to the secondary cortex, information reaches through the thalamus. The primary olfactory cortex includes the following

Lateral olfactory stria, when traced backward, ends in gyrus semilunaris, which is present in front of the uncinate gyrus.

The lateral olfactory gyrus covers the lateral olfactory stria It is continuous posteriorly as the gyrus ambiance. Gyrus ambiens lies lateral to gyrus semilunaris. The dorsomedial part of the amygdala

The anterior part of the parahippocampal gyrus includes uncus (uncinate gyrus). It is included in the entorhinal area (Brodmanns area 28).

Secondary Olfactory Cortex

• The lateral part of the orbital surface of the frontal lobe is the olfactory association cortex.
• This part receives direct afferents from the primary olfactory area. The frontal cortex also receives indirect input from the olfactory cortex through the thalamus.
• The frontal and orbitofrontal cortices are known as the olfactory association area. The hypothalamus receives olfactory information through the amygdala.
• The emotional aspects of olfactory sensations are due to their limbic projections involving the hypothalamus and amygdala.
• Olfactory stimuli induce visceral response (salivation following pleasing aromas from food and nausea and vomiting following offensive smell) by modulating the activities of the autonomic nervous system.

Anosmia

Anosmia is defined as a lack of olfactory sensation. It is of two types:

1. Specific and
2. General.

Specific anosmia: Olfactory acuity varies from person to person. This may be explained as due to the absence of a specific odourant receptor on the olfactory sensory cells.

General anosmia: Complete lack of olfactory sensation Olfactory Hallucination Olfactory hallucination may be due to a lesion involving the parahippocampal gyrus, uncus, and the adjoining areas.

These olfactory hallucinations precede epileptical seizures referred to as ‘uncinate fits’.

Olfactory System Summary

1. Humans can perceive thousands of different varieties of odor.
2. The olfactory system consists of olfactory epithelium present in the nasal cavity, olfactory bulb, olfactory tract, and olfactory striae.
3. The smell is perceived in the primary and secondary olfactory cortices of the cerebrum.
4. The olfactory neurosensory cells of the olfactory epithelium can appreciate a large number of smells because there are more than 3000 different receptor proteins in their cilia.
5. The axons of olfactory sensory receptor cells terminate on mitral and tuft cells in the olfactory bulb. Their axons form the olfactory tract.

The olfactory tract divides into two roots:

1. Lateral and Medial olfactory striae.
2. The fibers of the lateral olfactory stria end in the gyrus semilunaris, which is part of the primary olfactory cortex.
3. The primary olfactory cortex is responsible for conscious awareness of olfactory stimuli.
4. The secondary olfactory cortex (olfactory association cortex) receives direct information from the primary olfactory area and is located in the frontal and orbitofrontal cortices. This area is responsible for the conscious dissemination of odor.

Olfactory System Multiple-Choice Questions

Question 1. The following types of cells are present in the olfactory epithelium except.

1. Olfactory neurosensory cells
2. Supporting cells
3. Pillar cells
4. Stem cells

Answer: 3. Pillar cells

Question 2. Which of the following neurons and interneurons are present in the olfactory bulb?

1. Granule cells
2. Periglomerular cells
3. Mitral cells
4. Tuft cells
5. All of the above

Answer: 5. All of the above

Question 3. Which of the following olfactory striae carry most ofthe axons of the olfactory tract?

1. Lateral olfactory stria
2. Medial olfactory stria
3. Intermediate olfactory stria

Answer: 1. Lateral olfactory stria

Question 4. The primary olfactory cortex includes the following except

1. Gyrus semilunaris
2. Gyrus ambient
3. The dorsomedial part of amygdalae
4. Uncus
5. The lateral part of the orbital surface of the frontal lobe

Answer: 5. Lateral part of the orbital surface of the frontal lobe

Question 5. The following cells synapse in ‘glomeruli’ of the olfactory bulb except

1. Axons of olfactory sensory receptor cells
2. Mitral cells
3. Tuft cells
4. Periglomerular cells
5. Cells of amygdalae

Answer: 5. Cells of amygdalae

Cranial Nerves Nuclei And Functional Aspects

Cranial nerves, like spinal nerves, are a part of the peripheral nervous system (PNS). 12 pairs of cranial nerves are attached to the ventral surface of the brain (except the trochlear nerve which is attached to the dorsal surface of the brain). The first two cranial nerves olfactory and optic, are attached to the forebrain.

While The Rest Of The Cranial nerves are attached to the brainstem. The Cranial nerves are usually designated by Roman numerals.

Cranial nerves 1 and 2 are pure secondary nerves while other nerves are mixed. thus a cranial nerve may contain fibers that take origin from various functional types of nuclei located in the brainstem, i.e. somatic motor, visceral motor, somatic sensory, and j visceral sensory.

Cranial Nerves Nuclei Developmental Aspects

Before discussing the functional columns in the brainstem it is important to understand the development of the brainstem. Tire brainstem and the spinal cord develop from the neural tube that appears in the embryo

Development Of Functional Columns In Spinal Cord And Brainstem

The events that take place during the development of the spinal cord and brainstem are depicted respectively. Students are suggested to read the description of these diagrams carefully before proceeding to learn the following text.

Arrangement of Nuclear Columns

The nuclear aggregates or columns of basal and alar laminae are arranged functionally in a definite sequence when traced from the medial to the lateral side.

In the spinal cord, these include the following

1. Nuclear columns of basal lamina

• General somatic efferent (GSE)
• General visceral efferent (GVE)

2. Nuclear columns of alar lamina

• General visceral afferent (GVA)
• General somatic afferent (GSA)

However, in the brainstem, two more special sensory columns are added in the alar lamina (SSA and SVA).

Similarly, one more special visceral efferent column (SVIv) is added to the basal lamina.

Cranial Nerve Nuclei In Brainstem Hindbrain

The functional components of cranial nerve nuclei are

Somatic Efferent Column

• The somatic efferent (SE) column consists of motor nuclei of the cranial nerves 3, 4, 6, and 7.
• The nuclei of this column supply the skeletal muscles derived from somites.
• The nuclei of the cranial nerves 3, 4, and 6 nerves innervate muscles responsible for the movements of the eyeball.
• The nucleus of the cranial nerve 7 innervates the muscles of the tongue.

Special Visceral Efferent Column

• This column includes also the motor nucleus nuclei ambiguus of the cranial (for, nerves and cranial nerves).
• These nuclei innervate skeletal muscles derived from the pharyngeal or branchial arches (branchiomotor).
• These branchiomotor skeletal muscles of the head and neck (muscles of the face, muscles of mastication, muscles of palate, pharynx, and larynx) are also known as special visceral muscles.

General Visceral Efferent Column

• The GVE column consists of parasympathetic motor nuclei, i.e. Edinger-Westphal nucleus, superior and inferior salivatory nuclei, and dorsal nucleus of the vagus.
• The axons from these nuclei innervate the smooth muscles of the viscera, blood vessels, and exocrine glands.
• These nuclei consist of preganglionic nerve cells whose axons terminate in the ganglia closely related to viscera.
• The postganglionic fibers from these ganglia arise and supply the smooth muscles of organs and glands.

General Visceral Afferent and Special Visceral Afferent Columns

• It is believed that both of these sensory columns have a single nucleus which is known as the ‘nucleus of the temporal tract’.
• It receives general visceral sensations from the pharynx, larynx, trachea, esophagus, and thoracic and abdominal viscera.
• It also receives the sensation of taste, a special sensation, from the tongue, epiglottis, and palate.
• This column receives sensory fibres from 7, 9, and 10 cranial nerves.

General Somatic Afferent Column

This column consists of three sensory nuclei of the 5 cranial nerves:

• The main sensory nucleus,
• Spinal nucleus and
• The mesencephalic nucleus of the trigeminal nerve.
• The main sensory and spinal nuclei are concerned with exteroceptive sensations (pain, touch, and temperature) from the region of the face.
• The mesencephalic nucleus receives proprioceptive impulses from the muscles of mastication, facial muscles, ocular muscles, and muscles of the tongue.

Special Somatic Afferent Column

• The special somatic afferent column consists of sensory nuclei:
• Vestibular and Cochlear.
• The vestibular nuclei convey impulses associated with equilibrium while the cochlear nuclei convey impulses for hearing.

Functional Components, Nuclei, Origin, Course, And Termination Of Cranial Nerves

• Students should note that a detailed description of intracranial and extracranial course and termination of an individual cranial nerve is out of the scope of this book.
• Therefore, only a brief description of the termination of cranial nerves is given here.
• Students should read the detailed description of the course and termination of these cranial nerves from a textbook on the gross anatomy of the Head and Neck.

Oculomotor Nerve

The oculomotor nerve is the third cranial nerve. It is predominantly a motor nerve that innervates the majority of extraocular muscles.

These include the superior rectus, inferior rectus, medial rectus, inferior oblique and levator palpebrae superioris.

Origin, Course, and Termination

• The oculomotor nerve emerges out on the medial aspect of the cerebral peduncle, in the interpeduncular fossa.
• Thereafter, the nerve passes through the cavernous sinus and the superior orbital fissure.
• The oculomotor nerve then enters the orbit (after passing through the superior orbital fissure) and supplies all the extraocular muscles except the lateral rectus and superior oblique.
• The main oculomotor nerve nucleus is located in the midbrain at the level of superior colliculus The oculomotor nerve has three functional components.

The lesion ofthe oculomotor nerve may present in the form of various clinical signs, as follows.

Squint: This is seen due to the unopposed action ofthe lateral rectus and superior oblique muscles. The eyeball moves downwards and outwards on the affected side.

Diplopia: Diplopia or double vision is seen when the patient tries to look medially, superiorly, or inferiorly.

Ptosis: The paralysis of the levator palpebral superioris muscle leads to drooping ofthe upper eyelid.

Loss of accommodation reflex is due to paralysis of the ciliary muscle.

Dilatation of pupil (mydriasis): Due to the damage of parasympathetic fibers, the unopposed action of sympathetic fibers leads to the dilation of the pupil.

Loss of light reflex: In cases of 3 nerve palsy, the dilated pupil fails to constrict in response to light.

Exophthalmos: The affected eyeball looks prominent as compared to a normal eye. This is due to paralysis of many extraocular muscles which keep the eyeball retracted.

Trochlear Nerve

Trochlear Nerve Location of Nucleus

The trochlear nerve nucleus is located just below the oculomotor nucleus at the level of the inferior colliculus.

Trochlear Nerve Origin, Course, and Termination

• The fibers arising from the trochlear nerve nucleus follow an unusual course, i.e. these fibres curve backward around the periaqueductal grey matter and decussate in the superior medullary velum.
• The nerve emerges outside the brain immediately caudal to the inferior colliculus on each side ofthe frenulum veil
• The nerve then runs laterally and winds forward around the cerebral peduncle lying between the posterior cerebral and superior cerebellar arteries.
• The nerve soon passes through the lateral wall of the cavernous sinus and reaches the orbit after passing through the superior orbital fissure.
• Within the orbit, the nerve supplies the superior oblique muscle.

The trochlear nerve is a mixed nerve and has two functional components:

1. SE and
2. GSA.

For functional components, nuclei, origin, course, and termination of nuclear fibers and function of the trochlear nerve,

Trochlear Nerve  Effects of Damage

• Squint: Damage to the trochlear nerve clinically produces a squint in the affected eye, and the affected eyeball in these cases moves in a superolateral direction.
• Diplopia: This occurs when the patient tries to look downwards. Thus, the patient will have difficulty in walking downstairs.

Trochlear Nerve  Clinical Observations

The patient is unable to look inferolaterally, on the affected side, when asked to do so. This is due to paralysis ofthe superior oblique muscle.

Abducent (Abducens) Nerve

Abducent Nerve Location of Nucleus

The motor nucleus of this nerve is located at the level of lower pons. It lies beneath the facial colliculus on the floor of the fourth ventricle.

Abducent Nerve Origin, Course, and Termination

• The neurons of this nucleus give origin to the fibers (axons) that pass through the tegmentum of the pons, in a ventral direction.

• The nerve comes out on the surface of the brainstem at the junction of the pons and pyramid.
• After lying in the lateral wall of the cavernous sinus, the nerve reaches the orbit through the superior orbital fissure and supplies the lateral rectus muscle.

The abducent nerve consists of two functional components:

1. GSE and
2. GSA.

For functional component, nuclei, origin, course, and termination of nuclear fibers and function of the abducent nerve.

• Medial squint: Damage to this nerve results in a clinical condition whereby the eyeball rotates medially; this condition is referred to as medial squint.
• Diplopia (double vision): It occurs when the patient tries to look on the lateral side.
• Damage to the nerve: This is suspected if the eyeball turns medially on the affected side when the patient is asked to look towards the lateral side.

Trigeminal Nerve

The trigeminal is a mixed nerve and consists of three divisions:

1. Ophthalmic,
2. Maxillary, and
3. Mandibular.

Therefore, it is called a trigeminal nerve.

Trigeminal Nerve Origin, Course, and Termination

• The trigeminal nerve is attached to the ventrolateral surface of the pons by the motor and sensory roots.
• The sensory root is big and lies lateral to the smaller motor root. Both motor and sensory roots run forward and laterally over the apex of the petrous temporal bone to reach the middle cranial Fossa.
• Here, the sensory root contains a ganglion (semilunar ganglion), which is enclosed in the recess of dura matter called a trigeminal cave. This ganglion contains pseudo-unipolar neurons, which are first-order neurons in the sensory pathways.

At its distal end, the semilunar ganglion branches into three divisions:

1. Ophthalmic,
2. Maxillary and
3. Mandibular.

The motor root passes deep to the semilunar ganglion and fuses with the mandibular nerve.

The ophthalmic and maxillary are sensory nerves while the mandibular nerve consists of both motor and Three divisions of the trigeminal ophthalmic, maxillary, and mandibular—leave the cranial cavity through the superior orbital fissure, foramen rotundum, and foramen ovale, respectively.

Ophthalmic Nerve

After its origin from the semilunar ganglion, the ophthalmic division pierces the dura and lies in the lateral wall of the cavernous sinus.

Before entering the superior orbital fissure, it divides into three branches:

1. Lacrimal,
2. Frontal and
3. Nasocilliary.

All three nerves give many branches in the orbit (For details of text and diagrams, refer to a textbook of Gross Anatomy.)

Through these branches, it supplies sensory fibers to the eyeball, conjunctiva, part of the nasal cavity, skin of the forehead, and lacrimal gland.

The ciliary ganglion is attached to the nasociliary nerve.

Maxillary Nerve

• After piercing the dura of the trigeminal cave, this nerve lies in the lateral wall of the cavernous sinus inferior to the ophthalmic division.
• The nerve leaves the skull through the foramen rotundum and enters in pterygopalatine fossa. From here, it reaches the orbit through the infraorbital fissure.
• In the orbit, it runs in the infraorbital groove and appears on the face through the infraorbital foramen. Here, it terminates by dividing by the number of branches.
• Through the pterygopalatine ganglion, it conveys the secretomotor (parasympathetic) fibers to the lacrimal gland and glands present in the nasopharyngeal mucosa.
• The nerve is sensory to the skin of the middle face, nasal cavity, gums maxillary teeth, and palate.

Mandibular Nerve

• The motor root of the trigeminal nerve fuses with the mandibular as it passes through the foramen ovale.
• After coming out through the foramen ovale, it lies in the infratemporal fossa as the trunk ofthe mandibular nerve.
• The trunk soon divides into anterior and posterior divisions. Many branches arise from the trunk, anterior, and posterior divisions. (For details of text and diagrams, refer to the textbook of Gross Anatomy.)
• As the mandibular nerve is a mixed nerve, it supplies motor fibers to the muscles of mastication and other muscles. It supplies the sensory fibers to the skin of the lower face, gum, and teeth of the lower jaw.

Location of Trigeminal Nuclei

The trigeminal nerve is represented by three sensory nuclei and a single motor nucleus. The sensory nuclei (GSA) are the main sensory nucleus, a nucleus of the spinal tract, and the mesencephalic nucleus.

The motor nucleus (SVE) is referred to as the motor nucleus of the trigeminal nerve For functional component, nuclei, origin, course, and termination of nuclear fibers and function of the trigeminal nerve.

Trigeminal Neuralgia

• In this disorder of the trigeminal nerve, episodes of intense, stabbing pain occur in the area of distribution of one of the trigeminal nerve divisions.
• The maxillary division is most frequently involved and the ophthalmic division is least affected.
• Pain usually occurs in a specific zone of the face such as around the nose and mouth. The pain lasts for a few seconds to about 1-2 minutes but occurs repeatedly. The pain may be triggered by touching an especially sensitive area of skin.
• The exact cause of this disorder is not known. It is believed that the pressure of an artery leads to demyelination of the sensory nerve fibers. The short-circuiting of electrical impulses among the demyelinated axons is considered to generate an abnormal signal of pain.
• Painkiller drugs are of limited help. Severe cases can also be treated by cutting the nerve or by transection of the spinal trigeminal tract in the lower medulla.
• As the three divisions of the trigeminal nerve are somatotopically arranged in this tract, the transaction of fibers of a single division ofthe trigeminal nerve can be achieved.

Facial Nerve

The facial nerve is a mixed nerve. It conveys the following:

1. Sensation of taste.
2. It is motor to all the skeletal muscles derived from the second pharyngeal arch.
3. The nerve is secretomolor to lacrimal, submandibular and sublingual salivary glands.

The facial nerve emerges at the lower border of the pons in the cerebellopontine angle.

It has two roots:

• Facial nerve proper (containing motor fibers, SVE) and
• Nervus intermedius (containing sensory and preganglionic parasympathetic fibers.
• The nerve soon enters the internal acoustic meatus.
• Origin, Course, and Termination

The facial nerve arises from two roots:

• Motor and Sensory. Its sensory branch is also known as nervus intermedius.
• Both the roots are attached to the lower border of the pons medial to the 8 cranial nerve.
• The facial nerve, along with the 8 cranial nerve, enters the internal acoustic meatus. In the meatus, the sensory and motor roots of the facial nerve fuse to form a single trunk.
• In the facial canal, it runs laterally above the bony labyrinth of the internal ear. It then bends sharply backward to run in the medial wall of the middle ear. This bend is thick as it contains a genicular ganglion.
• At the junction of the medial and posterior walls of the middle car, it again bends downwards to come out of the skull through the stylomastoid foramen.

Within the facial canal, the nerve has three branches:

• Greater petrosal,
• Nerve to stapedius and
• Chorda tympani.

Soon after its exit from the stylomastoid foramen, it gives posterior auricular, digastric, and stylohyoid branches.

The nerve enters the posteromedial surface of the parotid gland and within the substance of the gland, it divides into five terminal branches (temporal, zygomatic, buccal, marginal mandibular, and cervical).

For functional components, nuclei, origin, course, and termination of nuclear fibers and function of the Facial nerve,

Cortical Connections of Motor Nucleus

The motor nucleus of the facial nerve is innervated by the corticonuclear fibers (supranuclear fibers) arising from the cerebral cortex. The fibers arising from the fascial nucleus (infranuclear fibers) innervate facial muscles.

The neurons of the facial nerve supplying the muscles of the upper part of the face have double innervation, i.e. they are supplied by corticonuclear fibers from the same and opposite cerebral hemispheres.

However, the neurons of the facial nerve supplying muscles of the lower face are supplied by corticonuclear fibres of the opposite cerebral hemisphere only.

Neuroanatomical Basis of Facial Nerve Palsy

Supranuclear lesions (upper motor neuron [UMN] lesion): In the case of this lesion, only the muscles of the lower half of the face on the opposite side are paralyzed.

The muscles of the upper part of the face remain functional. This is because the muscles of the upper face receive bilateral corticonuclear connections and escape paralysis.

Nuclear and intranuclear lesions (lower motor neuron [LMN] lesion or Bell’s palsy): This kind of lesion is also known as LMN lesion and involves the axons of the facial motor nucleus.

The lesion may occur anywhere along the course of the facial nerve. The infranuclear paralysis of the facial nerve is called Bell’s palsy.

The most common site of internuclear lesion is near the stylomastoid foramen, though the nerve may get affected anywhere in the facial canal in the petrous temporal bone.

Vestibulocochlear Nerve

The vestibulocochlear nerve (8 cranial nerve) is predominantly sensory.

Anatomically and functionally, the nerve consists of two different parts:

Vestibular and Cochlear. The nerve comes out from the internal ear through the internal acoustic meatus.

After passing through the meatus, the vestibulocochlear nerve enters the brainstem at the lower border of pons posterolateral to the attachment of the facial nerve.

Lower Motor Neuron Lesion of the Vagus Nerve

The nuclear and intranuclear lesions will lead to the following clinical features depending on whether the lesion is unilateral or bilateral.

1. Unilateral lesion will present with the following features:

Paralysis of soft palate on the same side of the lesion. The soft palate elevates and the uvula deviates to the normal side due to the unopposed action of muscles on the normal side.

• Speech is affected and the ‘gag reflex’ is absent.
• Paralysis of pharyngeal muscles will lead to difficulty in swallowing.
• Unilateral paralysis of laryngeal muscles will result in
  hoarseness of voice and dyspnoea.
• Loss of sensations from the mucous membrane of the pharynx and larynx, on the side of the lesion, would result in loss of cough reflex.

Bilateral lesions will show the following features:

Complete paralysis of the larynx. Both vocal cords are paralyzed; hence, there is a complete loss of voice. Death may result due to asphyxia.

Due to complete paralysis of the pharyngeal and palatine muscles, swallowing and speech are severely affected. The gag reflex is absent.

Origin, Course, and Termination

• The vagus nerve is attached superficially in the posterolateral sulcus of the medulla by 10-12 rootlets, just below the attachment of the glossopharyngeal nerve.
• These rootlets join to form a single trunk and leave the cranial cavity through the intermediate part of the jugular foramen. The vagus nerve receives fibers from the cranial root of the accessory nerve.
• The nerve bears two sensory ganglia: Superior and Inferior. The superior ganglion is situated in the jugular foramen and the inferior ganglion is just below the foramen.
• The nerve runs downwards within the carotid sheath between the internal carotid artery and the internal jugular vein till it reaches the root of the neck.
• On the right side, it enters the thorax by crossing the right subclavian artery whereas on the left side, it enters the thorax between the left common carotid and the left subclavian arteries.
• It gives many branches in the neck for the pharynx and larynx. It supplies parasympathetic branches to thoracic viscera and terminates in the abdomen by supplying many abdominal viscera.

Accessory Nerve Nuclei

The accessory nerve is predominantly a motor nerve. The nerve consists of two distinct parts:

Cranial root and Spinal root. The cranial root (SVE) originates from the nucleus ambiguus and the spinal root (GSE) from the spinal nucleus of the accessory nerve. There is a sensory component also (GSA)

Accessory Nerve Nuclei Origin, Course, and Termination

The cranial and spinal roots of the accessory nerve take origin separately from their respective nuclei.

Cranial Root

• These fibers emerge as four to five rootlets from the posterolateral sulcus of the medulla, below the attachment of the filament of the vagus nerve. The rootlets of the cranial part join to form a trunk and then pass laterally toward the jugular foramen.
• During its course in the jugular foramen, the nerve joins the spinal root for a short distance. The two roots soon separate as they come out of the jugular foramen.
• The cranial root passes over the inferior ganglion of the vagus nerve and fuses with it to become part of the vagus nerve.
• The fibres of the cranial part of the accessory nerve are considered to supply all the intrinsic muscles of the larynx through the recurrent laryngeal branch of the vagus nerve.

Spinal Root

• The GSE fibers of the spinal root originate as five to six rootlets from the lateral aspect of the spinal cord between dorsal and ventral roots.
• These rootlets soon join to form a trunk that ascends through the vertebral canal and enters the skull through the foramen magnum.
• The nerve root comes out of the cranial cavity through the jugular foramen in association with the vagus and glossopharyngeal nerves.
• The spinal root of the accessory nerve supplies the sternocleidomastoid and trapezius on the same side.

Hypoglossal Nerve

The hypoglossal nerve is motor to all the extrinsic and intrinsic muscles of the tongue except palatoglossus (which is supplied by the vagus nerve).

Though the hypoglossal nerve is predominantly a somatic motor nerve (GSE), it also contains proprioceptive (sensory, GSA) fibers from the muscles of the tongue.

Hypoglossal Nerve Origin, Course, and Termination

• The nerve exits on the ventral surface of the medulla, between the pyramid and the olive by 15-20 rootlets.
• These rootlets of the nerve soon join and leave the skull through the hypoglossal canal.
• Extracranially, the hypoglossal nerve descends lateral to the vagus nerve and just above the hyoid bone it enters the root of the tongue.
• The hypoglossal nerve is motor to all extrinsic and intrinsic muscles of the tongue, except for the palatoglossus muscles

Hypoglossal Nerve Effect of Damage

If the hypoglossal nerve or nucleus is damaged, it leads to the following clinical effects:

• Impaired speech, chewing, and swallowing
• Deviation of tongue towards the injured side, if asked to protrude.
• Atrophy of the tongue towards the damaged side.
• Inability to protrude tongue if nerves on both sides are damaged.

Cranial Nerves Nuclei  Summary

• During the development of the neural tube, the alar lamina forms the sensory nuclei while the basal lamina forms the motor nuclei.
• The motor columns that develop in the basal lamina of the brainstem are GSE, SVE, and GVE.
• The sensory columns that develop in the alar lamina of the brainstem are GVA, SVA, GSA, and SSA.
• The somatic efferent column consists of nuclei of the cranial nerves 3, 4, 6, and 7.
• The SVE column includes the motor nuclei of cranial nerves 5,7 and nucleus ambiguus (9, 10, and 9). These nerves supply skeletal muscles derived from pharyngeal arches (branchiomotor).
• The GVE column consists of parasympathetic nuclei (secretomotor).
• GVA and SVA columns are represented by the ‘nucleus of solitary tract’. The nucleus receives sensory fibers from the viscera of neck, thorax, and abdomen; it also receives taste sensations from the tongue, epiglottis, and palate.
• The GSA column is represented by the sensory nuclei of the trigeminal nerve, namely the main sensory nucleus, spinal nucleus, and mesencephalic nucleus.
• The SSA column is represented by vestibulocochlear nuclei.
• The 3, 5, and 4 cranial nerves are predominantly motor nerves that supply extraocular muscles. These nerves are represented by SE and GVA columns. The 3 cranial nerve is additionally represented by the GVE column.
• The V cranial nerve is represented by a motor nucleus (SVE) and three sensory nuclei (GSA—mesencephalic, main sensory, and spinal nucleus).
• The trigeminal nerve is motor to muscles of mastication and it also carries sensory impulses from the skin of the scalp and face.
• The 7 nerve has the following functional columns: SVE (motor nucleus of the facial nerve), GVE (superior salivatory nucleus), and SVA (nucleus of the solitary tract). The facial nerve is motor to muscles of facial expression and secretomotor to salivary glands (except parotid). It also carries taste sensations from the anterior two-thirds of the tongue and soft palate.
• The 8 cranial nerve is predominantly a sensory nerve and the sensory component is represented by the SSA column.
• The 7 cranial nerve is a mixed nerve and consists of the following functional components: SVE (nucleus ambiguus), GVE (inferior salivatory nucleus), GVA (NTS), and SVA (NTS).
• The nerve gives motor innervation to the stylopharyngeus muscle and secretomotor innervation to the parotid gland; it also conveys general and taste sensations from the pharynx and the posterior one-third of the tongue.
• The X cranial nerve is also a mixed nerve. It is represented by the following functional components: GVE (dorsal nucleus of vagus), SVE (nucleus ambiguus), GVA (NTS), SVA (NTS), and GSA (spinal nucleus of the trigeminal nerve).
• The vagus nerve is secretomotor to glands and viscera, branchiomotor to the pharynx, larynx, and soft palate, and carries general sensation from the viscera and special sensation of taste from the epiglottis and tongue.
• The 9 cranial nerve is predominantly a motor nerve and includes the following functional components: SVE (nucleus ambiguus), GSE (spinal nucleus of the accessory nerve), and GSA (proprioceptive fibers of the spinal nerve).
• The spinal accessory nerve innervates the trapezius and sternocleidomastoid muscles.
• The 7 cranial nerve is predominantly a motor nerve that innervates the musculature of the tongue. It has the following functional components: GSE (hypoglossal nerve nucleus) and GSA (proprioceptive).

Multiple Choice Questions

Question 1. Which of the following statements about cranial nerves is/are true?

• Cranial nerves 1 and 2 are attached to the forebrain
• Cranial nerves 9 to 12 are attached to the medulla
• Cranial nerves 3 and 4 are attached to midbrain
• Cranial nerve 4 is attached to the dorsal aspect of the brain
• All of the above

Answer: 5. All of the above

Question 2. Following are tried nerve cell columns in the basal lamina (efferent or motor columns) except

1. GSE (general somatic efferent)
2. SVE (special visceral efferent)
3. GVE (general visceral efferent)
4. SSE (special somatic efferent)

Answer: 4. GVE (general visceral efferent)

Question 3. Following are tried nerve cell columns in the alar lamina (efferent or sensory columns) except

1. GVA (general visceral afferent)
2. SVA (special visceral afferent)
3. GSA (general somatic afferent)
4. SSA (special somatic afferent)
5. GSA (general sympathetic afferent)

Answer: 3. GSA (general somatic afferent)

Question 4. Which of the following cranial nerve nuclei does not belong to the general somatic efferent (GSE) column?

1. Oculomotor nucleus
2. Trochlear nucleus
3. Facial nucleus
4. Abducent nucleus
5. Hypoglossal nucleus

Answer: 3. Abducent nucleus

Question 5. The special visceral efferent (SVE) column consists of the following nerve nuclei except

1. Motor nucleus of the trigeminal nerve
2. Facial nerve nucleus
3. Nucleus ambiguus
4. The dorsal nucleus of the vagus

Answer: 3. Nucleus ambiguus

Question 6. Which of the following nuclei belongs to the GVE column?

1. Edinger-Westphal
2. Salivatory nuclei
3. Lacrimatory nucleus
4. The dorsal nucleus of the vagus
5. All of the above

Answer: 3. Lacrimatory nucleus

Question 7. The following nuclei belong to the GSA column except?

1. Sensory trigeminal nucleus
2. Mesencephalic nucleus
3. Nucleus solitary tract
4. Nucleus of the spinal tract of trigeminal

Answer: 3. Nucleus of solitary tract

Question 8. The following cranial nerve nuclei are located in the midbrain except?

1. Oculomotor nerve nucleus
2. Trochlear nerve nucleus
3. Abducent nerve nucleus
4. Edinger-Westphal nucleus
5. Mesencephalic nucleus

Answer: 3. Edinger-Westphal nucleus

Question 9. Which of the following motor (efferent) nerve nuclei are located in pons?

1. Abducent nerve nucleus
2. Facial nerve nucleus
3. Motor trigeminal nucleus
4. Salivatory nucleus
5. All of the above

Answer: 5. All of the above

Question 10. The following sensory (efferent) nuclei are located in the medulla except

1. The nucleus of the spinal tract of trigeminal
2. Vestibular nucleus
3. Cochlear nucleus
4. Nucleus of the solitary tract
5. The main sensory nucleus of trigeminal

Answer: 5. Main sensory nucleus of trigeminal

Question 11. Which of the following nuclei represents the GVA and SVS columns?

1. Nucleus solitary tract
2. The main sensory nucleus of trigeminal
3. The spinal nucleus of the trigeminal
4. Mesencephalic nucleus
5. Cochlear nucleus

Answer: 1. Nucleus of solitary tract

Question 12. Which of the following functional components is/are not present in the oculomotor nerve?

1. SE
2. GVE
3. GSA
4. GVA

Answer: 4. GVA

Question 13. Which of the following functional components are present in the trochlear nerve?

1. SE and GSA
2. SE, GVE, and GSA
3. GVE and GSA
4. None of the above

Answer: 1. SE and GSA

Question 14. Which of the following structures send their sensory proprioceptive impulses to the mesencephalic nucleus of the trigeminal nerve?

1. Upper and lower teeth
2. Periodontal ligaments and joint capsule
3. Muscles of mastication
4. Extraocular muscles
5. All of the above

Answer: 5. All of the above

Question 15. The facial nerve has the following functions except

1. It is motor to the muscle of the face
2. It conveys the sensation of taste from the anterior two-thirds of the tongue
3. It is secretomotor to lacrimal, submandibular and sublingual salivary glands
4. It carries exteroceptive sensation from the skin of the face
5. It carries exteroceptive sensation from the part of the skin of the external ear

Answer: 4. It carries exteroceptive sensation from the skin of the face

Question 16. Which of the following statements is/are true about the infranuclear facial nerve lesion in the facial canal?

1. Paralysis of facial muscles
2. Loss of taste from the anterior two-thirds of the tongue
3. Secretion from submandibular, sublingual, and lacrimal glands is affected
4. The sound seems abnormally loud
5. All of the above

Answer: 5. All of the above

Question 17. Which of the following nuclei is not connected with the glossopharyngeal nerve?

1. Nucleus ambiguus
2. Inferior salivatory nucleus
3. Nucleus of tractus solitarius
4. Lacrimatory nucleus

Answer: 4. Lacrimatory nucleus

Question 18. Which of the following nuclei contribute to the vagus nerve?

1. The dorsal nucleus of the vagus
2. Nucleus ambiguus
3. Spinal nucleus of the trigeminal nerve
4. Nucleus of the solitary tract
5. All the above

Answer: 5. All the above

Basal Nuclei

The basal nuclei, or the basal ganglia, are large masses of grey matter situated in the subcortical regions.

They are considered a part of the extrapyramidal system. This is because the motor functions of basal nuclei are independent of the activity of the pyramidal system. They play an important role in voluntary movements.

Parts Of Basal Nuclei

Basal nuclei consist of the following parts or nuclei:

1. Caudate nucleus
2. Lentiform nucleus, further divided into putamen and globus pallidus
3. Subthalamic nucleus
4. Substantia nigra

All the above nuclei are grouped under the heading ‘basal nuclei’.

This is because all of them are interconnected to form a single functional unit. A lesion in any of these nuclei results in motor disorders.

1. Caudate nucleus: The caudate nucleus is an arch-shaped mass of grey matter that surrounds the thalamus.

It is present in the lateral ventricle divided into three parts:

1. Head,
2. Body and
3. Tail

Head: The head of the caudate nucleus is a large, rounded mass present on the floor of the anterior horn of the lateral ventricle.

Laterally, the head is related to the anterior limb of the internal capsule and putamen.

Bands of grey matter, running across the anterior limb of the internal capsule, connect the head of the caudate nucleus to the putamen.

Body: The body of the caudate nucleus is narrow and long and forms the central part of the caudate nucleus.

It lies on the floor of the central part of the lateral ventricle.

Tail: The tail of the caudate nucleus is formed by the body of the caudate nucleus as it curves downwards and forwards, ‘I lie lall Is long and slender and lies In the roof of die Inferior horn of die lateral ventricle. Anteriorly, the tail hi continuous with the amygdaloid body.

2. lentiform nucleus: The lentiform nucleus Is a wedge-shaped (convex) mav of grey matter that lies deeply hurled in the cerebral hemisphere,

Unlike the caudate nucleus, the lentiform nucleus is not related to the lateral ventricle.

Medially, the die lentiform nucleus is related to the internal capsule and laterally to the external capsule.

Antcroinfcriorly, the heads of caudate and lentiform nuclei are continuous with each other.

The lentiform nucleus is further divided into two parts:

1. Medial ylobus pallidum and
2. Lateral putamen.

3. Subthalamic nucleus: The subthalamic nucleus lies just below the thalamus and above the substantia nigra. It is a small, biconvex mass of grey matter that lies intermedia! to Globus pallidum.

4. Substantia nigra: The substantia nigra is a large motor nucleus present in the midbrain. The dopamine synthesized by the substantia nigra is used as a neurotransmitter in the corpus striatum.

The deficiency of dopamine leads to a condition called Parkinson’s disease characterized by diminished movements.

The caudate nucleus and putamen are histologically identical and are collectively known as corpus striatum.

It is recent in phylogeny; hence, it is also called neostriatum. The Globus pallidum, phylogenetically, is an old structure; hence, it is called palaeostriatum.

Connections Of Basal Nuclei

• The corpus striatum (caudate nucleus and putamen receives almost all the inputs (afferents) of the basal nuclei,
• The axons of the striatum project to globus pallidus and substantia nigra.
• The axons from globus pallidus and substantia nigra constitute the major output (efferent) from the basal nuclei.

Connections of Corpus Striatum (Input Nucleus)

Afferents

The afferents of basal nuclei terminate in the corpus striatum (i.e. caudate nucleus and putamen.

From the cerebral cortex: Corticostriate fibers especially from the frontal and parietal lobes, terminate in various parts of the striatum. Thus, information regarding motor movements reaches the basal nuclei from the premotor and motor areas of the cerebral cortex.

From the thalamus: Thalamostriate fibers originate from the intralaminar nuclei of the thalamus and project to the striatum

From the substantia nigra: Nigrostriate fibers originate in the pars compacta of the substantia nigra and terminate in the putamen. These fibers use dopamine as a neurotransmitter.

Efferents

The efferents from the corpus striatum go to the output nuclei—globus pallidus and substantia nigra. These afferents are called striatopallidal and striatonigral fibers, respectively.

Both striatopallidal and striatonigral fibers are inhibitory and use GABA as their neurotransmitter.

To globus pallidus: Striopallidal fibers begin in the putamen and caudate nucleus and terminate in both the outer and the inner parts of globus pallidus.

To substantia nigra: Strionigral fibers originate in the putamen and terminate in both parts of the substantia nigra (pars reticulata and pars compacta).

Connections of Output Nuclei

The output nuclei (globus pallidus and substantia nigra) receive their afferents from the corpus striatum.

Efferent Connections

The efferent connections of globus pallidus and substanti nigra go to the thalamus, subthalamic nucleus, superior colliculus, reticular nuclei and habenular nucleus.

To the thalamus: terminate in the thalamus. The axons from the nuclei of the thalamus then terminate in the motor areas of the cerebrum. Thus, basal nuclei exert their influence on the motor areas of the cerebral cortex through the thalamus.

To the subthalamic nucleus: The fibers of globus pallidus terminate in the subthalamic nucleus. The fibers from the subthalamic nucleus, in turn, terminate on the internal segment of the globus pallidus.

To the superior colliculus: The fibres ofsubstantia nigra terminate on the superior colliculus. This connection controls eye movements

To the habenular nucleus: The fibers from the pallidum terminate in the habenular nucleus. The basal nuclei can modify the limbic system through these connections.

Functions Of Basal Nuclei

The basal nuclei have motor as well as cognitive and behavioral functions.

Motor Functions

The basal nuclei are a part of the extrapyramidal system and are concerned with regulation of the voluntary muscular activities. The important motor functions of
basal nuclei are as follows

Programming of voluntary movements: Instructions for learned muscular movements are stored in basal nuclei.

Regulation of automatic movements: The caudate nucleus and putamen control the automatic movements of skeletal muscles, such as swinging the arm while walking and laughing in response to a joke.

Regulation of muscle tone: The globus pallidus helps regulate the muscle tone required for a specific body movement.

Effect on the functions of reticular formation: This nucleus is involved in stereotyped motor functions such as locomotion.

Cognitive and Behavioural Functions

The basal nuclei also have a role in cognition (reasoning judgment and memory), mood, and non-motor behavior.

Basal Nuclei: Part of the Extrapyramidal System

• The basal nuclei receive their inputs from the cerebral cortex; these inputs carry information regarding voluntary movements.
• After programming the voluntary movements, the basal nuclei send their output back to the motor areas of the frontal cortex through the thalamus (via the thalamocortical circuit).
• Thus, the motor functions of the basal nuclei are mediated through their actions in the motor areas of the frontal cortex.
• This activity of basal nuclei is independent of the pyramidal motor system. Therefore, basal nuclei are included in the category of the extrapyramidal system.

Lesions Of Basal Nuclei

The lesions of basal nuclei lead to movement disorders characterized by involuntary movements, muscular rigidity, and immobility without paralysis.

Movement Disorders of Basal Nuclei

The movement disorders of basal nuclei are classified into two major classes:

1. Hypokinetic disorders: for example Parkinsons disease

2. Hyperkinetic disorders: Excessive and abnormal motor activity leading to involuntary movements

The involuntary movements arc of several types:

• Athetosis: Slow, sinuous, writhing movements of limbs
• Chorea: Quick, jerky, involuntary random movements of limbs and orofacial structure
• Ballism: Violent, large amplitude, proximal limb movements

Parkinson’s Disease

Parkinson’s disease is a chronic, progressive, nervous disorder. It is more common in people aged over 60 years, especially in males.

The exact cause of the disease is unknown. The disease is characterized by tremors, muscular weakness, rigidity, and a peculiar gait. This disease is also known as paralysis agitans/parkinsonism.

Symptoms

Fine tremor (pill-rolling tremor): It appears as if the patient is rolling a pill between the tips of the thumb and the index finger. This is a kind of resting tremor.

Muscle stiffness, making it difficult to start moving. This is due to increased muscle tone

Slowness of movement and absence of arm swinging during walking

Stooped posture (the body bent forward): In this posture, the elbows, knees, and back are flexed.

The gait of the patient is stiff due to the rigidity of muscles and joints.

Pathology

Parkinson’s disease results due to a deficiency of the neurotransmitter ‘dopamine’ in the striatum.

The deficiency occurs due to degeneration of dopaminergic neurons in the pars compacta of substantia nigra.

Hemiballismus

• Hemiballismus results from a small stroke lesion in the subthalamic nucleus or due to a lesion in its connections with globus pallidus.
• In a normal condition, the subthalamic nucleus is responsible for smooth voluntary movement while its lesion leads to involuntary, often violent, movement of the opposite limb.
• These movements resemble the movements of throwing; hence, it is ballism.

Chorea

Chorea is of two different types:

• Sydenham’s chorea and Huntington’s chorea. Sydenham’s chorea occurs in children. The disease is transient and recovery is full.
• The disease is associated with rheumatic fever. The proteins on the surface of the streptococcal bacteria are similar to the proteins on the membrane of neurons of the caudate and lentiform nuclei.
• Therefore, most antibodies also combine with the membranes of the neurons of basal nuclei. This results in Sydenham’s chorea.
• Huntington’s chorea is a rare genetic disorder. The onset of the disease occurs in the third to fifth decades of life.
• This disease is characterized by heritability, chorea, psychiatric disturbances, dementia, and death within 15-20 years of onset.
• The choreiform movements are involuntary movements of limbs and twitching of the face. Later, the patient becomes immobile and unable to speak or swallow.

Basal Nuclei Summary

• Basal nuclei are large masses of grey matter situated in subcortical regions. They play a role in normal voluntary movement.
• Basal nuclei consist of a caudate nucleus, lentiform nucleus, subthalamic nucleus, and substantia nigra.

The lentiform nucleus consists of two parts:

• Putamen and Globus pallidus. The putamen and caudate nucleus together form the striatum.

Substantia nigra is present in the midbrain and consists of two parts:

• Pars reticulata and
• Pars compacta.

The striatum (caudate nucleus and putamen) receives almost all the inputs (afferents) of basal nuclei; hence, it is called the input nucleus. It receives fibers from the cerebral cortex, thalamus, and substantia nigra.

The axons of the striatum project to globus pallidus and substantia nigra; hence, these are considered output nuclei of basal nuclei.

The efferents from globus pallidus and substantia nigra go to the thalamus, subthalamic nucleus, superior colliculus, and habenular nucleus.

The functions of basal nuclei include programming of normal voluntary movements, control of abnormal involuntary movement, regulation of nuclei zone, and control of reticular formation. Basal nuclei are also involved in memory and thought.

Parkinson’s disease results due to a deficiency of the neurotransmitter ‘dopamine’ in the striatum.

Dopamine is synthesized in pars compacta of substantia nigra and transmitted to striatum through nigrostriatal fibers.

Basal Nuclei Multiple Choice Questions

Question 1. Which of the following facts are true about basal nuclei?

1. They are large masses of grey matter situated in subcortical regions
2. They are a major component of the motor system
3. They belong to the extrapyramidal system
4. The lesion of basal nuclei leads to involuntary movements, muscular rigidity, and immobility without paralysis
5. All of the above

Answer: 3. The lesion of basal nuclei leads to involuntary movements, muscular rigidity, and immobility without paralysis

Question 2. Basal nuclei consist of the following except

1. Caudate nucleus
2. Lentiform nucleus
3. Subthalamic nucleus
4. Red nucleus
5. Substantia nigra

Answer: 4. Substantia nigra

Question 3. The corpus striatum consists of

1. Putamen and caudate nucleus
2. Lentiform nucleus
3. Globus pallidus and caudate nucleus
4. Caudate nucleus and substantia nigra

Answer: 1. Putamen and caudate nucleus

Question 4. Which of the following fact(s) about the lentiform nucleus is/are false?

1. It is not related to the lateral ventricle
2. Anteroposteriorly, the caudate and lentiform nuclei are continuous with each other
3. It is deeply buried in the cerebral hemisphere
4. Medially, it is related to the external capsule and laterally to the internal capsule

Answer: 4. Medially, it is related to the external capsule and laterally to the internal capsule

Question 5. Which ofthe following statement(s) about the subthalamic nucleus is/are correct?

1. It lies just below the thalamus and above the substantia nigra
2. It is closely connected with the globus pallidus
3. It contains glutaminergic neurons
4. It is involved in the smooth movements of different parts of the body
5. All of the above

Answer: 5. All of the above

Question 6. The following statements regarding substantia nigra are correct except

1. It is a large motor nucleus present in the midbrain
2. It contains glutaminergic cells
3. The neuromelanin
4. The dopamine is synthesized by the substantia nigra
5. Deficiency of dopamine leads to Parkinsons disease

Answer: 2. It contains glutaminergic cells

Question 7. Parkinson’s disease has the following symptoms:

1. Fine tremor
2. Muscle stiffness
3. Slowness of movements
4. Stooped posture
5. All of the above neurons of the substantia nigra contain

Answer: 5. All of the above neurons of substantia nigra contain

Question 8. Following are the motor functions of basal nuclei except

1. They are part of the extrapyramidal system and concerned with somatic muscular movements
2. They are concerned with programming of voluntary movements
3. They control the reticulospinal tract
4. The caudate nucleus and putamen control the automatic movements of skeletal muscles
5. They are concerned with the maintenance of equilibrium and posture

Answer: 5. They are concerned with the maintenance of equilibrium and posture.