Neuroanatomy

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.

White Matter Of Cerebrum

• Each cerebral hemisphere consists of an enormous mass of white matter lying deep in the gyri and sulci of the cerebral cortex.
• White matter consists of a large number of fibres (axons) which connect various parts of the cortex. These fibres also connect various parts of the cortex with other parts of the central nervous system (CNS).

Classification of Fibres

Based on the nature of their connections, the fibres can be divided into three different groups:

1. Association fibres: These fibres connect various parts of the cerebral cortex of the same hemisphere. They do not cross the midline and are therefore confined to a single cerebral hemisphere.

2. Commissural fibres: These fibres connect the cortical areas of the two cerebral hemispheres and in the process cross the midline. Therefore, these fibres are also referred to as cerebral commissure.

3. Projection fibres: These fibres establish connections between the cortex and other subcortical regions of the CNS (thalamus, corpus striatum, brainstem and spinal cord). These fibres may or may not cross the midline at various levels of their descent.

Association Fibres

Association fibres are the most numerous types of white fibres. They are usually grouped into short and long-association fibres.

The short association fibres pass from one part of a gyrus to another part of the same gyrus, or they may connect the adjacent gyri by looping around the intervening sulcus.

The long association fibres run for long distances, for example between two lobes of the same cerebral hemisphere. These fibres connect the functionally related regions of the cerebral cortex.

The long-association fibres are named as follows.

Cingulum: The cingulum is present deep to the cingulate gyrus on the medial surface of the cerebral hemisphere.

These fibres interconnect the cingulate gyrus, parahippocampal gyrus and septal area below the genu of the corpus callosum.

• Superior longitudinal bundle: The superior longitudinal bundle runs on the superolateral surface of the cerebral hemisphere above the level of the insula. The fibres of this bundle connect various areas of the frontal, parietal occipital and temporal lobes.
• Inferior longitudinal bundle: The inferior longitudinal bundle connects the visual association area (areas 18 and 19) of the occipital lobe with the temporal lobe.
• Uncinate fasciculus: Uncinate fasciculus connects Broca’s area and gyri on the orbital surface of the frontal lobe with the cortex of the temporal lobe. This bundle hooks around the stem of the lateral sulcus.
• Fronto-occipital fasciculus: Fronto-occipital fasciculus is located deep in the cerebral hemisphere. It connects the frontal lobe with the occipital and temporal lobes.

Commissural Fibres

Examples of the commissural fibres are corpus callosum, anterior commissure, posterior commissure, and habenular Examples of the commissural fibres are corpus callosum, anterior commissure, posterior commissure, and habenular.

Corpus Callosum

• The highest number of commissural fibres is present in the corpus callosum.
• The corpus callosum interconnects the cortical areas of one cerebral hemisphere with the corresponding cortical areas of the opposite cerebral hemisphere.
• The axons (fibres) of the corpus callosum run in both directions transversely and form a thick, wide Corpus callosum
• The highest number of commissural fibres is present in the corpus callosum.
• The corpus callosum interconnects the cortical areas of one cerebral hemisphere with the corresponding cortical areas of the opposite cerebral hemisphere.
• The axons (fibres) of the corpus callosum run in both directions transversely and form a thick, wide sheet that connects the medial surfaces of the two cerebral hemispheres.

Corpus Callosum Location: The corpus callosum lies on the floor of the longitudinal fissure. It is about 10 cm long in the anteroposterior direction.

Corpus Callosum Shape: In the sagittal section of the cerebrum, the corpus callosum is seen as an arched or a ‘C ’-‘-shaped mass of white matter.

Corpus Callosum Surfaces: The superior surface of the corpus callosum is related to the indusium griseum and is concerned with the cingulate gyrus. The inferior surface is concave and is attached to the fornix through the septum pellucidum.

Parts of the corpus callosum: In the anteroposterior direction, it consists of four parts:

1. Rostrum,
2. Genu,
3. Trunk and
4. Splenium.

Rostrum: It connects the genu to the upper end of the lamina terminalis. In the midline, its superior surface gives attachment to the septum peculium.

The fibres of the rostrum interconnect the orbital surfaces of the right and left frontal lobes.

Genu: The genu is the fold of the corpus callosum located at its anterior end. The genu continues as the trunk of the corpus callosum posteriorly. The posterior surface of the genu gives attachment to the septum pellucid, in the midline.

Its fibres pass transversely to the anterior part of the right and left frontal lobes and appear like limbs of forceps. Therefore, these fibres are also called forceps minor.

Trunk: It extends between the genu anteriorly and the splenium posteriorly. In the midline, the inferior surface of the trunk gives attachment to the septum pellucidum anteriorly and to the fornix posteriorly.

The fibres of the trunk interconnect most of the corresponding areas of the frontal and parietal lobes.

The fibres of the posteriormost part of the trunk and a few fibres of the anterior part of the splenium form the roof of the posterior horn. These fibres are known as tapetum.

The fibres of the tapetum interconnect the corresponding part of the temporal and occipital lobes.

Splenium: It is the posterior enlarged part of the corpus callosum. It ends behind as a free border overhanging the pulvinars, pineal gland and tectum of the midbrain.

The fibres of splenium interconnect the corresponding part of the right and left parietal, occipital and temporal lobes.

These fibres of splenium which connect two occipital lobes curve backwards as forceps major. The fibres of forceps major bulge in the posterior horn and produce an elevation on its medial wall that is known as bulbs of the posterior horn.

Blood supply of corpus callosum: Most of the corpus callosum is supplied by the anterior cerebral artery. The posteriormost part of the corpus callosum is also supplied by the posterior cerebral artery.

Functions of the corpus callosum: The fibres of the corpus callosum and other commissures provide connections between two cerebral hemispheres.

Thus, any sensory information or knowledge is collected and shared by both the cerebral hemispheres. For example, if a person learns a task with one hand, then information is transferable to another hand due to commissural connections.

The congenitally absent or surgically transected corpus callosum results in an isolated cerebral hemisphere (two separate brains). This condition is referred to as split-brain.

2. Anterior commissure: The anterior commissure is a bundle of axons that crosses the midline in the lamina terminalis, anterior to the column of the fornix.

It connects the anterior perforated substance, olfactory tract, and middle and inferior temporal gyri of two sides. Thus, most of the right and left temporal lobes
are connected through the anterior commissure rather than the corpus callosum.

Posterior commissure: The posterior commissure crosses the midline in the lower stalk of the pineal gland. This commissure connects the interstitial nucleus of Cajal, superior colliculi and pretectal nuclei of two sides.

4. Habenular commissure: This bundle is present in the upper stalk of the pineal gland and connects the habenular nuclei of the two sides.

5. Hippocampal commissure: The hippocampal commissure is also known as the commissure of fornix. It interconnects two crura of fornix, i.e. two hippocampi with each other.

6. Optic chiasma: The fibres from the right and left nasal halves of the retina cross in the optic chiasma.

Projection Fibres

• The projection fibres establish connections between the cerebral cortex and the subcortical regions of the CNS. The projection fibres run in both directions.
• In the cerebrum, the projection fibres are present in the forms of corona radiata and internal capsule.

Corona radiata: The fibres of corona radiata are continuous below the internal capsule which is present around the periphery of the lentiform nucleus.

Above the level of the internal capsule, the fibres spread out in a fan-like fashion into the cerebral cortex. Corona radiata consists of both afferent and efferent fibres.

The fibres of corona radiata are intersected by the fibres of corpus callosum (commissural fibres) and are intermingled with the association fibres.

Internal capsule: The projection fibres of the corona radiata are concentrated in the form of an internal capsule.

The internal capsule is present between large masses of grey matter (nuclei) of the cerebrum. The internal capsule is discussed in detail as a separate topic in the subsequent text.

External capsule: The external capsule is a thin layer of white matter between putamen and claustrum. It consists of projection fibres connecting the cerebral cortex with the putamen and reticular nuclei.

Fornix: The fornix.

Internal Capsule

The internal capsule is an aggregation of projection fibres which are arranged compactly in the cerebrum.

Internal Capsule Extent: The fibres of the internal capsule are continuous above as corona radiata and below as the crus cerebri of the midbrain

Internal Capsule Relations: ‘Hie lentiform nucleus is present on the lateral aspect of the internal capsule while the thalamus and caudate nucleus are present on its medial aspect.

Internal Capsule Shape: In a horizontal section of the cerebrum, the internal capsule appears like a ‘V’ with its concavity directed laterally.

The internal capsule consists of both ascending and descending (afferents and efferents) fibres which interconnect the cerebral hemisphere with the brainstem and spinal cord.

Internal Capsule Parts

• The internal capsule consists of an anterior limb, a genu, a posterior limb, a retrolentiform part and a sublentiform part.
• The anterior limb is bounded laterally by the lentiform nucleus and medially by the head of the caudate nucleus.
• The genu is located medial to the apex of the lentiform nucleus. It is a bent between the anterior and posterior limbs of the internal capsule.
• The posterior limb is situated between the lentiform nucleus laterally and the thalamus medially.
• The retrolentiform part is situated behind the lentiform nucleus.
• The sub-lentiform part consists of fibres that pass beneath the posterior part of the lentiform nucleus.

Fibres in the Internal Capsule

The various parts of the internal capsule consist of both efferent (corticofugal/motor) and afferent (corticospinal/ sensory) fibres. These are presented.

Sensory Fibres

The sensory fibres or the thalamic radiation establish a reciprocal connection between the thalamus and the cerebral cortex.

Anterior thalamic radiation: These fibres are situated in the anterior limb of the internal capsule.

They establish connections between the mediodorsal thalamic nucleus and the prefrontal cortex.

Superior thalamic radiation: This is situated in the genu and posterior limb of the internal capsule.

Fibres provide a connection between the ventral nuclei of the thalamus and motor, premotor and supplementary motor areas of the frontal lobe and sensory areas of the parietal lobe.

Posterior thalamic radiation: These fibres connect the thalamus with the visual cortex in the occipital lobe.

These fibres are present in the retrolentiform part of the internal capsule and connect the lateral geniculate body with the visual cortex (optic radiation).

Inferior thalamic radiation: These fibres run in the sublentiform part of the internal capsule that connects the medial geniculate body with the superior temporal gyrus (auditory area).

Motor Fibres

• The motor fibres are the projection fibres, also called centrifugal fibres.
• The efferent fibres present in the internal capsule are corticopontine, pyramidal and extrapyramidal

Corticopontine fibres: They originate from all four lobes of the cerebral cortex:

• Frontal, parietal, occipital and temporal. These fibres terminate in the nuclei points located in the basal part of the pons. Accordingly, these are called frontopontine, parietopontine, occipitopontine and temporopontine fibres.
• Fibres of the pyramidal motor system: The corticonuclear and corticospinal fibres originate in the motor, premotor, supplementary motor and cingulate areas of the frontal lobe.
• The genu of the internal capsule is occupied by the corticonuclear fibres while the posterior limb has the corticospinal fibres.
• Corticonuclear fibres terminate on the motor nuclei of cranial nerves of the opposite side while corticospinal fibres terminate on the anterior horn cells of the opposite side of the spinal cord.

Extrapyramidal fibres: These are corticorubral, corticostriate, corticoreticular and corticonigral fibres. Most of these arise from the motor areas of the frontal lobe and terminate in red nucleus, caudate nucleus, putamen, reticular formation and olivary nuclei.

The extrapyramidal fibres are situated in the posterior limb of the internal capsule and accompany the fibres of the pyramidal system.

Arterial Supply

• The internal capsule consists of a compact bundle of motor and sensory fibres.
• Any occlusion of blood vessels supplying the internal capsule can potentially lead to a serious clinical outcome (paralysis and sensory loss on the opposite side of the body).

Arteries supplying various parts of the internal capsule are described as follows:

Anterior limb: The upper part of the anterior limb is supplied by the striate branches of the middle cerebral artery.

The lower part of the anterior limb is supplied by the medial striate artery, a branch of the Anterior cerebral artery (also called the recurrent artery of Huebner).

Genu: The genu of the internal capsule is supplied by a striate branch of the middle cerebral artery, medial striate artery, a striate branch of the posterior communicating artery and direct branches from the internal carotid artery.

Posterior limb: The posterior limb of the internal capsule is supplied by the striate branches of the middle cerebral artery including a large Charcot artery and striate branches of the anterior choroidal artery.

Retrolentiform part: This is supplied by the striate branches of the posterior cerebral artery.

Sublentiform part: This part of the internal capsule is supplied by the striate branches ofthe posterior cerebral artery, striate branches of the anterior choroid artery and branches from the internal carotid artery.

• The ascending sensory and descending motor axons course through the Internal capsule. Occlusion of the vascular supply of the internal capsule can result in paralysis and loss of general sensation on the opposite side of the body (hemiplegia).
• The most common lesion affecting the Internal capsule Is haemorrhage or thrombosis.
• This occurs most commonly due to rupture of the lateral striate branch of the middle cerebral artery which supplies the posterior limb of the internal capsule.
• This artery is also known as the artery of cerebral haemorrhage or Charcot’s artery of cerebral haemorrhage. This is the most common artery to rupture because it remains under constant high pressure and has no collateral vessels.
• The rupture of this artery leads to the upper motor neuron type of paralysis, on the opposite side of the body (hemiplegia).

White Matter Of Cerebrum Summary

The white matter of the cerebrum is divided into three groups:

1. Association, Commissural and Projection.
2. The association fibres contain axons that conduct nerve impulses between gyri in the same hemisphere.
3. The commissural fibres conduct nerve impulses between the corresponding areas of the right and left cerebral hemispheres.
4. The projection fibres connect the cerebral hemisphere with the lower parts of the CNS.
5. The corpus callosum has the highest number of commissural fibres. It has four parts:
6. Rostrum, Genu, Trunk and Splenium.
7. A congenitally absent or transected corpus callosum results in an isolated cerebral hemisphere (split brain).
8. The projection of corona radiata is continuous below the internal capsule. Parts of the internal capsule are the anterior limb, genu, posterior limb, retrolentiform and sublentiform.
9. The internal capsule consists of both ascending (sensory) and descending (motor) fibres.
10. As the internal capsule consists of a compact bundle of motor and sensory fibres, occlusion of blood supply can result in paralysis and loss of sensation on the opposite half of the body (hemiplegia).

White Matter Of Cerebrum Multiple Choice Questions

Question 1. Which of the following statement(s) is/are true?

1. White matter consists of a large number of fibres (axons)
2. White matter connects various parts of the cortex
3. White matter connects the cerebral cortex with other parts of the CNS
4. Many of these fibres pass through the internal capsule
5. All of the above

Answer: 2. White matter connects various parts of the cortex

Question 2. The white matter of the cerebrum is divided into the following groups except

1. Association fibres
2. Commissural fibres
3. Projection fibres
4. Decussating fibres

Answer: 4. Decussating fibres

Question 3. The following statements about association fibre(s) are/are true except

1. Association fibres are the most numerous white fibres
2. Short association fibres run between two cerebral lobes
3. The cingulum is an example of long-association fibres
4. Long association fibres are arranged in distinct bundles

Answer: 2. Short association fibres run between two cerebral lobes

Question 4. The following are examples of commissural fibres except

1. Cingulum
2. Corpus callosum
3. Anterior commissure
4. Optic chiasma
5. Habenular commissure

Answer: 1. Cingulum

Question 5. Which of the following statements about corpus callosum is false?

1. Axons of the corpus callosum run in both directions
2. Corpus callosum is located about 6 cm away from the occipital pole
3. Much of the cortices of the temporal lobe of two sides connect through the corpus callosum
4. Corpus callosum consists of four parts—rostrum, genu, trunk and splenium

Answer: 3. Much of the cortices of the temporal lobe of two sides connect through the corpus callosum

Question 6. The following facts about corpus callosum are true except

1. The fibres of the rostrum connect the orbital surface of the right and left frontal lobes
2. The fibres of genu are arranged like the limbs of forceps and are called forceps minor
3. The fibres of the posteriormost part of the trunk form ‘tapetum’
4. The fibres of splenium curve backwards to form forceps major
5. The fibres of forceps major connect two parietal lobes

Answer: 5. The fibres of forceps major connect two parietal lobes

Question 7. The blood supply of the corpus callosum is due to the following arteries:

1. Anterior cerebral artery
2. Middle cerebral artery
3. Posterior cerebral artery
4. Central branches from cerebral arteries

Answer: 1. Anterior cerebral artery

Question 8. Which of the following facts about projection fibres are true?

1. Projection fibres establish connections between the cerebral cortex and the subcortical regions of the CNS
2. Projection fibres run in both directions
3. In cerebrum, projection fibres are present in corona radiata and internal capsule
4. Projection fibres below the corona are concentrated in the internal capsule
5. All of the above

Answer: 5. All of the above

Question 9. The internal capsule consists of the following parts except

1. Anterior limb
2. Posterior limb
3. Inferior limb
4. Genu
5. Retrolentiform part

Answer: 3. Inferior limb

Question 10. Which of the following are extrapyramidal fibres of the internal capsule?

1. Corticorubral
2. Corticostriatal
3. Corticoreticular
4. Cortico-olivary
5. All of the above

Answer: 5. All of the above

Functional Cerebral Areas Of Cerebral Cortex

The cerebral cortex has many areas that have been assigned to perform specific functions. These areas are usually described as sulci and gyri.

Brodmann’s Mapping

The best-known scheme to map out the functional area of the cerebral cortex is that of Brodmann. He mapped the ” cerebral cortex in 47 such areas and indicated each area by a number.

Functional Areas Of Cerebral Cortex

Animal experiments and clinical pathological studies have indicated that the human cerebral cortex possesses three different types of functional areas

1. Sensory,
2. Motor and
3. Association areas.

Sensory areas: These areas are concerned with the perception of general somatic sensations (pain, touch and temperature), special sensations (hearing, vision, taste and smell) and equilibrium. The sensory areas of the cerebral cortex receive fibres from the thalamus.

The primary somatic sensory area, which receives pain, touch and temperature, is located in the postcentral gyrus (areas 3, 2 and 1).

Similarly, the sensory area for vision is located in the occipital lobe (area 17) while the area for hearing is located in the temporal lobe (areas 41 and 42).

Motor areas: These areas are responsible for the contraction of skeletal muscles during electrical stimulation. The precentral gyrus (area 4) is an example of a primary motor area which sends projection fibres to the brainstem and spinal cord.

Association areas: Besides the motor and sensory areas, a large part of the cerebral cortex (up to 70% of the neocortex) is referred to as the association cortex.

Association areas are usually present adjacent to the primary sensory areas. The function of the association cortex is to integrate various types of sensory information and to direct behaviour, communication and intellect.

Primary Sensory And Association Areas Of the Cerebral Cortex

Primary Sensory And Association Areas Of the Cerebral Cortex are as follows:

• Sensory and association areas of the parietal lobe
• Sensory and association areas of the occipital lobe
• Olfactory areas

Sensory and Association Areas of the Parietal Lobe

The parietal lobe has the following sensory and association areas:

• Area for general somatic sensation
• Area for a taste sensation
• Vestibular area
• Sensory speech area

Area for General Somatic Sensation

To perceive general somatic sensations, there is a primary area and an association area defined on the cerebral cortex.

Somatic Sensory Area

• This area is located in the postcentral gyrus on the superolateral surface and in the posterior part of the paracentral lobule on the medial surface of the cerebral hemisphere.
• This area is defined as areas 3, 1 and 2 on Brodmann’s cytoarchitecture map. The conscious perception of pain, touch, temperature and proprioception takes place here.
• The opposite half of the body is represented upside down on the primary somatic area.
• The primary sensory area receives its afferents from the ventral posterior nucleus of the thalamus through the internal capsule (as the thalamocortical tract).

Somatic Sensory Association Area

• This area of cerebral cortex corresponds to Brodmann’s areas 5 and 7 and is located in the ‘superior parietal lobule’, on the lateral surface, and in the ‘precuneus’, on the medial surface.
• This association area is concerned with the analysis and integration of general sensation based on experience.

Area for Taste Sensation

The area for taste sensation (area 43) is located at the inferior end of the postcentral gyrus, in the superior wall of the posterior ramus of the lateral sulcus. This area is located close to the general sensory area for the tongue.

Vestibular Area

The area concerned with the vestibular function, i.e. vestibular area, is located at the anterior end of the intraparietal sulcus and in the postcentral gyrus.

Functionally, the vestibular area is concerned with the motor regulation needed for the maintenance of equilibrium.

Sensory Speech Area of Wernicke

Wernicke’s sensory speech area is limited only to the posterior part of area 22 (superior temporal gyrus; The sensory speech area or Wernicke’s area is located in the dominant cerebral hemisphere.

Language functions: Wernicke’s area plays an important role in language functions.

The sensory speech centre of Wernicke’s is concerned with the interpretation of spoken and written words including signals and symbols.

Wernicke’s area (area 22) with the help of areas 39 and 40 by recognising spoken words and the meaning of writing by seeing written words interprets the meaning of speech

Language areas are located in the normally dominant left cerebral hemisphere which also contains Broca’s speech area (motor speech area).

Wernicke’s area is necessary for language comprehension while Broca’s (area 44) is concerned with language production Broca’s area activates the mouth, tongue palate and vocal cord regions of the motor cortex for articulation of speech.

Aphasia

Aphasia is a disorder affecting the ability to speak or read. A lesion of the sensory speech area of Wernicke’s leads to a person’s inability to understand both written and spoken words even though his vision and hearing are normal. This condition is called sensory aphasia.

Dyslexia

Dyslexia is a disorder of adults affecting the comprehension and use of words.

Developmental Dyslexia

• Developmental dyslexia is seen in children. These children have difficulty in reading and writing. Their writing looks uneven and disorganised.
• Sometimes, letters are written as reversed. This is due to a problem in processing visual information. These children possess normal or above-normal intelligence.

Sensory and Association Areas of the Occipital Lobe

• The occipital lobe of the cerebral hemisphere has a primary visual area (area 17) which is concerned with the ability to perceive visual impulses.
• Surrounding the visual area (areas 18 and 19) is an association area called the visual association area or visuopsychic area.
• The lesion of the visual association area leads to a condition called visual agnosia in which a person is unable to identify an object or a person seen in the past.

Sensory and Association Areas of the Temporal Lobe

• The temporal lobe of the cerebral hemisphere has an area that is concerned with hearing. This area is referred to as the primary auditory area.
• It is located on the superior surface of the superior temporal gyrus Surrounding the primary auditory area is an association area (area 22) which is concerned with the interpretation of sounds based on past experiences.

Olfactory Areas

The areas of the brain which are concerned with the function of olfaction are limen insulae, uncus, amygdaloid body and entorhinal cortex. All these structures are a part of the limbic system.

Functional Areas Of Frontal Cortex

The frontal cortex is not only concerned with motor activities but also plays an important role in judgement and foresight. This lobe also plays an important role in the development of the personality of an individual.

1. Primary motor area: The primary motor area is located in the precentral gyrus, in the anterior wall of the central sulcus and the anterior part of the paracentral lobule on the medial surface of the hemisphere.

This area is designated as Area 4 of Broca. The primary motor area initiates and controls the contraction of skeletal muscles that are mainly located on the opposite side of the body. The fibres arising from the primary motor area run in corticospinal, corticonuclear and corticoreticular tracts.

In the primary motor area, the muscles of various parts of the body are represented upside down. The sequence of representation in the precentral gyrus.

In addition to area 4, the cerebral cortex also has a supplementary motor area (area 6) and a cingulate motor area located on the medial surface of the cerebral hemisphere.

2. Premotor area: The premotor area corresponds to area 6 of Brodmann. It is situated in front of the primary motor area occupying the posterior-most part of the superior, middle and inferior frontal gyri.

Based on experience, the premotor and supplementary motor areas can programme skilled motor activity. The premotor area directs the primary motor area to execute the skilled movements.

3. Frontal eye field: This area is present in the posterior part of the middle frontal gyrus which corresponds to the lower part of Bradman’s area 8. It receives afferents from the visual association areas and sends efferents to the contralateral nuclei of the extraocular muscles. The frontal eye field controls the voluntary conjugate scanning movement of the eyes.

4. Prefrontal cortex: The part of the frontal lobe located anterior to the motor and premotor areas is included in the prefrontal cortex.

This roughly corresponds to Brodmann areas 9, 10, 11 and 12. The electrical stimulation of the prefrontal cortex does not elicit motor response.

Functionally, it acts as an association cortex. Its functions are as follows:

• It is the storehouse of all the past experiences.
• It controls the behaviour based on judgement and foresight.
• Through its connections with the limbic system, the area helps to develop the personality of a person.

5. Motor speech area of Broca: The motor speech area is located in the inferior frontal gyrus; this area corresponds with the pars triangularis (area 45) and pars opercularis (area 44) of the dominant cerebral hemisphere.

In right-handed individuals, the motor speech area is located in the left cerebral hemisphere, while in the case of left-handed persons this area is present in the right cerebral hemisphere.

Broca’s area, through its connections with the primary motor area (area 4), is responsible for the production of speech. A lesion of the motor speech area leads to an inability to speak (motor aphasia).

Lesion of the Prefrontal Cortex

• A bilateral lesion of the prefrontal cortex leads to a change in personality.
• In a normal subject, frustration, tension and anxiety are generated in the prefrontal cortex.
• Therefore, after a lesion of the prefrontal cortex, the patient no longer suffers from depression, anxiety and pain.
• The patient does not care for the norms of social life and becomes rude and inconsiderate to others. He becomes careless, lazy and incapable of judging the consequences of reckless actions.
• It should be noted that despite profound personality changes, the memory and intelligence of a person are not affected.

Functional Cerebral Areas Of Cerebral Cortex Summary

• The cerebral cortex consists of three different types of functional areas:
• Sensory, Motor and Association.
• Brodmann mapped the cerebral cortex in 47 areas based on its differing histological structure and function.
• The sensory areas of the cerebral cortex receive and interpret sensory impulses, the motor areas initiate movement and the association areas deal with functions such as intelligence, memory emotion, reasoning, judgment and personality.
• The primary somatic sensory area (areas 3, 1, 2) is located in the postcentral gyrus.
• The sensory speech area consists of area 22. This area is also known as the sensory speech area of Wernicke. Other areas which help in sensory speech are areas 39 and 40.
• The primary visual area is located in area 1 7 while areas 1 8 and 1 9 are known as visual association areas.
• The primary auditory area is located in areas 41 and 42 on the superior surface of the superior temporal gyrus.
• The primary motor area is located in the precentral gyrus (area 4). This area initiates and controls the contraction of skeletal muscles on the opposite side of the body.
• The frontal eye field is located in the posterior part of the middle frontal gyrus (area 8). This area controls conjugate movements of the eyes.

Functional Cerebral Areas Of Cerebral Cortex Multiple Choice Questions

Question 1. The following different types of functional areas are located in the cerebral cortex except

1. Sensory areas
2. Motor areas
3. Association areas
4. Interpretation area

Answer: 4. Interpretation area

Question 2. The following sensory areas are present in the parietal lobe except

1. General somatic sensory area
2. Vestibular area
3. Broca’s motor area of speech
4. Area of taste sensation

Answer: 3. Brocas motor area of speech

Question 3. The following functional areas are present in the frontal lobe except

1. Frontal eye field
2. Premotor area
3. Primary motor area
4. Broca’s motor area of speech
5. Sensory area of speech

Answer: 5. Sensory area of speech

Question 4. Which of the following area(s) of the cerebral cortex are concerned with the recognition of painful stimuli from teeth?

1. Precentral gyrus
2. Postcentral gyrus
3. Superior temporal gyrus
4. Cingulate gyrus

Answer: 2. Postcentral gyrus

Question 5. The primary auditory cortex is located in which lobe of the cerebrum?

1. Frontal
2. Parietal
3. Occipital
4. Temporal
5. Insular

Answer: 4. Insular

Question 6. Damage to the frontal lobe, just anterior to the central sulcus, would affect which of the following functions?

1. Somatic motor function
2. Somatic sensory function
3. Vision
4. Hearing
5. Taste sensation

Answer: 1. Somatic motor function

Question 7. The motor speech area is located in which of the following gyrus?

1. Superior frontal gyrus
2. Middle frontal gyrus
3. Inferior frontal gyrus
4. None of the above

Answer: 3. Inferior frontal gyrus

Question 8. Lesions of uncus are associated with

1. Visual hallucination
2. Auditory hallucination
3. Olfactory hallucination
4. None of the above

Answer: 3. Olfactory hallucination

Question 9. All the following statements about the Brocas area are true except

1. Lesion of Broca’s area in right-handed persons will not show a significant deficit
2. It is concerned with the production of speech
3. The area is located between the horizontal and ascending rami of the lateral sulcus
4. Lesion results in paralysis of muscles involved in the production of speech

Answer: 4. Lesion results in paralysis of muscles involved in the production of speech

Question 10. Which of the following statement(s) about the motor area of the cerebral cortex is/are false?

1. It lies in the precentral gyrus and paracentral lobule
2. The motor area is supplied by the middle cerebral artery
3. In this area, the body is represented upside down
4. The part of the body is represented by an area proportional to the size of the part

Answer: 4. The part of the body is represented by an area proportional to the size of the part

Cerebral Hemispheres

The cerebrum is the largest part of the brain that occupies the anterior and middle cranial fossae of the skull. It consists of two cerebral hemispheres which are partially separated from each other by a longitudinal fissure.

At the bottom of the longitudinal fissure, the right and left cerebral hemispheres are connected by a great commissure of white fibers known as the corpus callosum.

Each cerebral hemisphere is covered by a layer of grey matter, known as the cerebral cortex. It shows complicated folds known as gyri and grooves between gyri are called sulci.

The formation of gyri and sulci is an attempt to increase the surface area of the cerebral cortex which has to be accommodated into the cranial cavity.

Deep into the cortex, the central core of each cerebral hemisphere is formed by white matter.

Within the white matter of the cerebral hemisphere, large masses of grey matter are present which are called basal nuclei (mainly caudate and lentiform nuclei).

Each cerebral hemisphere has an extensive cavity known as the lateral ventricle.

External Features Of The Cerebral Hemisphere

Each cerebral hemisphere presents with three surfaces, three borders, and three poles. These borders, surfaces, and poles can be easily identified with the help of figures showing the lateral aspect, medial aspect, and inferior aspect of the cerebral hemisphere.

A coronal section passing through the hemisphere will also help in identifying these features.

Surfaces

Each cerebral hemisphere consists of superolateral, medial, and inferior surfaces.

Superolateral surface: The superolateral surface is a large and convex surface that lies between superomedial and inferolateral borders. The convexity faces superolaterally.

Medial surface: The medial surface is seen in the midsagittal section of the brain. It lies between the superomedial and the inferomedial borders.

Inferior surface: The inferior surface is divided into the orbital surface and tentorial surface by the stem of the lateral sulcus.

The orbital surface is formed by the frontal lobe while the tentorial surface is formed by the inferior surfaces of the temporal and occipital lobes.

Borders

Each cerebral hemisphere consists of superomedial, inferolateral, superciliary, and inferomedial borders.

Superomedial border: The superomedial border is the upper border of the cerebral hemisphere and extends between the frontal and occipital poles. It separates a large superolateral surface from the medial surface.

Inferolateral border: The inferolateral border separates the superolateral surface from the tentorial surface and extends between the temporal and occipital poles.

Superciliary border: The superciliary border separates the superolateral surface from the orbital surface of the frontal lobe. It extends between the frontal pole and the stem of the lateral sulcus.

Inferomedial border: The inferomedial border is subdivided into medial orbital and medial occipital borders. The medial orbital border separates the orbital surface from the medial surface while the medial occipital border separates the tentorial surface from the medial surface.

Poles

Students are requested to identify the frontal, occipital, and temporal poles in the cerebral hemisphere with the help of

Important Sulci on the Superolateral Surface of the Cerebral Hemisphere. The cerebral hemisphere is divided into four lobes with the help of major sulci on the superolateral surface.

These important sulci are central sulcus, posterior ramus of lateral sulcus, parieto-occipital sulcus and pre-occipital notch. Identify these important sulci on the superolateral surface.

Lobes Of The Cerebral Hemisphere

With the help of four major landmark sulci and two imaginary lines, the cerebral hemisphere is divided into four lobes.

These are the frontal, parietal, temporal, and occipital lobes on the superolateral surface of the hemisphere. Various lobes on the superolateral surface are determined as follows:

The frontal lobe is bounded by the central sulcus (behind) and by the posterior ramus of the lateral sulcus (below).

The parietal lobe lies behind the central sulcus. Posteriorly, it is bounded by the first imaginary line and below by the posterior ramus of the lateral sulcus and the second imaginary line.

The temporal lobe is situated below the posterior ramus of the lateral sulcus and the second imaginary line. Posteriorly, this lobe is limited till the first imaginary line.

The occipital lobe lies behind the first imaginary line. The boundaries of the frontal, parietal, temporal, and occipital lobes on the medial and inferior surfaces of the cerebral hemisphere are shown respectively.

Sulci and Gyri of the Cerebral Hemisphere

Except for the major sulci and gyri mentioned in the preceding text, individual variation may be seen in the presence or absence of many other gyri and sulci.

A brief account of the major sulci and gyri on various surfaces of the cerebral hemisphere is as follows.

Superolateral Surface

The superolateral surface of the cerebral hemisphere presents important sulci and gyri in each of the lobes. Students should learn these sulci and gyri with the help of

Medial Surface

The medial surface shows the following major sulci callosal, cingulate, parieto-occipital, supraspinal, and calcarine. These sulci are located around the corpus callosum.

Inferior Surface

The inferior surface of the cerebral hemisphere is divided into two surfaces by the stem of the lateral sulcus: Orbital and Tentorial. The orbital surface lies anterior to the stem of the lateral sulcus while the tentorial surface lies posterior to it.

Cerebral Hemispheres Summary

• The cerebrum is situated on the diencephalon and brainstem. It is the largest part of the brain. It consists of two cerebral hemispheres.
• Each cerebral hemisphere is covered by a layer of grey matter, known as the cerebral cortex.
• The cerebral hemisphere shows the presence ofsulci and gyri. The formation ofsulci and gyri is an attempt to increase the surface area of the cerebral cortex.
• Each cerebral hemisphere consists of three surfaces (superolateral, medial, and inferior), three borders (superomedial, inferomedial, and inferolateral), and three poles (frontal, parietal, and occipital).

Each cerebral hemisphere consists of four lobes:

• Frontal,
• Parietal, Temporal and Occipital.
• Each lobe of the cerebral hemisphere shows many sulci and gyri on various surfaces of the cerebral hemisphere.

Cerebral Hemispheres Multiple Choice Questions

Question 1. Which of the following statements about the cerebral hemisphere is false?

• It is present in the anterior and middle cranial fossae
• The cerebral cortex shows complicated foldings called gyri
• Deep in the cerebral cortex, the white matter contains large masses of grey matter called caudate and lentiform nuclei
• Each cerebral hemisphere contains a cavity known as the third ventricle
• The right and left cerebral hemispheres are connected by corpus callosum

Answer: 4. The right and left cerebral hemispheres are connected by the corpus callosum

Question 2. Which of the following statements about the cerebral hemisphere is/are true?

1. It shows three surfaces—superolateral, medial, and inferior
2. It shows three borders—superomedial, inferomedial, and inferolateral
3. It shows three poles—frontal, occipital, and temporal
4. All of the above

Answer: 4. All of the above

Question 3. The cerebral hemisphere is divided into the following lobes

1. Frontal
2. Orbital
3. Parietal
4. Temporal
5. Occipital

Answer: 2. Orbital

Question 4. The lentiform nucleus is situated deep in the insular cortex

1. It is a hidden part of the cerebral cortex
2. It consists of long and short gyri
3. The lentiform nucleus is situated deep in the insular cortex
4. The insula is involved in motor and sensory activities of the autonomic nervous system
5. All of the above

Answer: 5. All of the above

Question 5. The following major sulci are present on the medial surface of the cerebral hemisphere except for?

1. Callosal
2. Cingulate
3. Parieto-occipital
4. Interparietal
5. Calcarine

Answer: 4. Interparietal

Question 6. Which of the following major sulci are present on the inferior surface of the cerebral hemisphere?

1. Orbital
2. Olfactory
3. Collateral
4. Occipito-temporal
5. All of the above

Answer: 5. All of the above

Question 7. Which of the following statement(s) about the parahippocampal gyrus is/are true?

1. It lies between the medial occipital border and the collateral sulcus
2. It is continuous posteriorly with the lingual gyrus
3. The anterior end of the parahippocampal gyrus shows a hook-like projection called an uncus
4. All of the above

Answer: 2. It is continuous posteriorly with the lingual gyrus

Thalamus, Metathalamus And Epithalamus

The diencephalon forms the central core of grey matter surrounded by cerebral hemispheres. Each half of the diencephalon is located between the midbrain and the cerebrum.

It extends from the region of the interventricular foramen to the region of the posterior commissure. Laterally, it is bounded by the posterior limb of the internal capsule.

The third ventricle may be regarded as the cavity of the diencephalon that divides it into two symmetrical parts.

Components Of Diencephalon

The hypothalamic sulcus divides the diencephalon into dorsal and ventral parts. The dorsal diencephalon lies above the hypothalamic sulcus while the ventral diencephalon below the sulcus.

Dorsal Diencephalon

The dorsal diencephalon consists of:

• Thalamus
• Metathalamus (medial and lateral geniculate bodies)
• Epithalamus (habenular nucleus and commissure, pineal gland and posterior commissure)

Ventral Diencephalon

The ventral diencephalon consists of:

• Hypothalamus: It lies below the hypothalamic sulcus
• Subthalamus: It lies below the posterior part of the thalamus and consists of the subthalamic nucleus

Thalamus

The thalamus is the largest part of the diencephalon. It consists of paired egg-shaped oval masses of grey matter, situated one on each side ofthe third ventricle. It is about 4 cm long and placed obliquely.

External Features

Each thalamus has two ends (anterior and posterior) and four surfaces (superior, inferior, medial and lateral;

Ends

Anterior end: The anterior end (pole) is narrow, placed close to the midline and lies just behind the interventricular foramen.

Posterior end: The posterior end (pole) is wide and directed dorsolaterally.

It is situated above and lateral to the superior colliculus. This expanded end of the thalamus is called the pulvinar. The medial and lateral geniculate bodies are present on the inferior aspect of the pulvinar

Surfaces

Medial surface: The medial surface forms the upper part of the third ventricle, i.e. above the hypothalamic sulcus.

The hypothalamic sulcus extends from the interventricular foramen to the upper end of the cerebral aqueduct.

This surface is lined by ependyma. The medial surfaces of the right and left thalami are usually interconnected by a band of grey matter called interthalamic adhesion.

Superior (dorsal) surface: The superior (dorsal) surface is a slightly convex and curved surface. It extends between the stria medullaris thalami (medially) and the caudate nucleus (laterally).

The superior surface is separated from the caudate nucleus by a thalamostriate vein and a bundle of fibres called stria terminalis The superior surface of the thalamus forms the floor of the central part of the lateral ventricle.

3. Lateral surface: The lateral surface of the thalamus concerns the external medullary lamina, reticular nucleus and the posterior limb of the internal capsule

Inferior surface: The inferior surface of the thalamus concerns the superior part of the hypothalamus (anteriorly) and subthalamus (posteriorly).

Internal Structure

The internal structure of the thalamus consists of

• White matter and
• Grey matter

White Matter

The white matter covering the lateral surface of the thalamus is called the external medullary lamina

Grey Matter

Each thalamus is mainly composed of grey matter which is subdivided into three main parts by a Y-shaped vertical sheet of white matter known as the internal medullary lamina

Anterior part

1. Medial part
2. Lateral part

Thalamic Nuclei

The three parts of the thalamus (anterior, medial and lateral parts) are further subdivided into various nuclei,

Anterior Part

The nuclei in the anterior part of the thalamus lie between the two limbs of ‘Y’.

Medial Part

The medial part of the thalamus lies medial to the internal medullary lamina. This part consists of a large medial dorsal nucleus and a smaller medial ventral nucleus or midline nucleus.

Lateral Part

The lateral part is subdivided into many nuclei. These nuclei in simple terms may be classified into ventral and lateral groups of nuclei

White Matter

The white matter covering the lateral surface of the thalamus is called the external medullary lamina.

Grey Matter

Each thalamus is mainly composed of grey matter which is subdivided into three main parts by a Y-shaped vertical sheet of white matter known as the internal medullary lamina.

1. Anterior part
2. Medial part
3. Lateral part

Thalamic Nuclei

The three parts of the thalamus (anterior, medial and lateral parts) are further subdivided into various nuclei

Anterior Part

The nuclei in the anterior part of the thalamus lie between the two limbs of ‘Y.

Medial Part

The medial part of the thalamus lies medial to the internal medullary lamina. This part consists of a large medial dorsal nucleus and a smaller medial ventral nucleus or midline nucleus.

Lateral Part

The lateral part is subdivided into many nuclei. These nuclei in simple terms may be classified into ventral and lateral groups of nuclei.

Ventral group of nuclei: In the anteroposterior direction, this group consists of three nuclei:

Ventral anterior nucleus, Ventral lateral nucleus and Ventral posterior nucleus. The ventral posterior nucleus is further divided into the ventral posterolateral nucleus and the ventral posteromedial nucleus.

Lateral group of nuclei: From the anterior to the posterior side, this group consists of three nuclei lateral dorsal nucleus, lateral
posterior nucleus and pulvinar.

Other Nuclei

In addition to the anterior, medial and lateral parts, the thalamus also contains the following nuclei.

Intralaminar Nuclei

Intralaminar nuclei are a collection of grey matter-forming nuclei within the internal medullary lamina of thalamus The important nucleus of this group is known as the centromedian nucleus.

Medial and Lateral Geniculate Bodies

These were previously considered a part of the metathalamus but are now considered an integral part of the thalamus.

Connections of Thalamus: An Overview

The thalamus has extensive afferent and efferent connections.

Afferents

The following afferents bring various kinds of impulses to the thalamus.

• Visual impulses: These are brought to the lateral geniculate body through the optic tract.
• Auditory impulses: These are brought to the medial geniculate body through the lateral lemniscus.
• Olfactory impulses: These are brought to the thalamus indirectly through the amygdaloid complex.
• Taste impulses: These are brought through the solitariothalamic tract.
• Exteroceptive impulses: The impulses carrying pain, touch and temperature sensations are brought through the spinothalamic and trigeminothalamic tracts.
• Proprioceptive impulses: The sensory impulses from muscles, joints and tendons are brought to the thalamus through the medial lemniscus and trigeminothalamic tracts.
• Visceral information: This information is brought through the hypothalamus and reticular formation.
• In addition to the above afferents, the thalamus also receives afferents from all parts of the cerebrum, cerebellum and corpus striatum.

Efferents

The efferents from the thalamus project to the following structures:

Cerebral cortex: The thalamocortical fibres project to all parts of the cortex.

• Hypothalamus, cerebellum, corpus striatum and reticular formation With its extensive afferent and efferent connections, the thalamus is regarded as a great integrating centre and is believed to perceive the sensations of crude pain and temperature.

The connections and functions of important nuclei of the thalamus are summarized

Functions of Thalamus

Thalamic nuclei act as primary relay nuclei. All sensory impulses (except smell) terminate in various nuclei of the thalamus.

From here, the sensory impulses project to different specific cortical areas through thalamocortical radiations.

Ultimately, the cerebral cortex is responsible for the interpretation of various kinds of sensory stimuli.

However, in case of the destruction of the cerebral cortex, the thalamus can appreciate the sense of crude pain and temperature.

The thalamus serves as an integrative centre for motor functions.

Through the ascending reticular activating system (ARAS), the intralaminar nuclei of the thalamus regulate the state of consciousness, alertness and attention.

Thalamic Syndrome

The thalamic syndrome occurs due to a vascular lesion of the artery supplying to the thalamus, i.e. thalamogeniculate branch of the posterior cerebral artery.

In this condition, emotional instability (spontaneous laughing or crying) and disturbances of sensation, i.e. hypersensitivity, are seen. A spontaneous pain on the opposite side of the body is also noticed. This syndrome may be associated with hemiparesis if the lesion also involves the internal capsule.

Metathalamus

The metathalamus consists of medial and lateral geniculate bodies.

Medial Geniculate Body

The medial geniculate body is a small ovoid mass of grey matter on the inferior aspect of the pulvinar.

It consists of medial, ventral and dorsal nuclei. It receives auditory information from the inferior colliculus through the inferior brachium. Its connections are summarised.

Lateral Geniculate Body

The lateral geniculate body is a small ovoid mass of grey matter on the inferior aspect of the pulvinar.

It is connected with the superior colliculus through the superior brachium. The lateral geniculate body consists of an inverted U-shaped lateral geniculate nucleus.

It is composed of six layers of nerve cells in the coronal section. These laminae are numbered 1-6 from the ventral to the dorsal side. Layers 1 and 2 consist of large cells (magnocellular layers) and layers 4-6 have smaller neurons (parvocellular laminae).

Afferents

The lateral geniculate body receives fibres from the retina. The nasal fibres from the opposite retina (crossed fibres) Terminate in layers 1, 4 and 6.

The ipsilateral fibres from the temporal half of the retina (uncrossed fibres) terminate in layers 2, 3 and 5. Thus, each lateral geniculate body receives visual information from the opposite field of vision.

Efferents

The efferents from the lateral geniculate body terminate in the visual areas of the occipital lobe (areas 17, 18 and 19) through optic radiation (geniculocalcarine tract)

Epithalamus

The epithalamus consists of the pineal gland, habenular nucleus, stria medullaris and posterior commissure.

Pineal Gland

The pineal gland is a small conical structure present in the posterior wall of the third ventricle. It lies below the splenium of the corpus callosum in a depression between two superior colliculi.

It is attached to the posterior wall of the third ventricle by a stalk (pineal stalk). The pineal stalk divides anteriorly into superior and inferior laminae to enclose the pineal recess of the third ventricle.

The superior lamina of the pineal stalk contains a habenular commissure and the inferior lamina contains the posterior commissure.

The pineal gland was previously considered a rudimentary gland but now it is a well-established endocrine gland which secretes various hormones.

Functions

The pinealocyte secretes serotonin and melatonin, which influence the activities of other endocrine glands.

The secretion of the hormone melatonin is associated with circadian rhythm and is influenced by light. Thus, the pineal gland acts as a biological clock.

Habenular Nucleus, Stria Medullaris Thalami and Posterior Commissure

The habenular nuclei are situated in the habenular triangle. These nuclei are regarded as cell stations in olfactory and visual pathways. These nuclei are also involved in the limbic system.

The stria medullaris thalami are a bundle of white fibres. These fibres are afferent to the habenular nuclei.

The posterior commissure is the band of white fibres which crosses the midline by passing through the inferior lamina of the stalk of the pineal gland. Many nuclei are present concerning the fibres of the posterior commissure.

Thalamus, Metathalamus And Epithalamus Summary

• The diencephalon is located between the cerebrum (above) and the midbrain (below).
• The third ventricle is the cavity of the diencephalon that divides it into two symmetrical parts.
• The hypothalamic sulcus divides the diencephalon into ventral and dorsal parts.
• The dorsal part of the diencephalon lies above the sulcus and consists of the thalamus, metathalamus and epithalamus.
• The ventral part of the diencephalon lies below the hypothalamic sulcus and consists of the subthalamus and hypothalamus.

Thalamus

• The thalamus of each side is a large, egg-shaped mass of grey matter, situated on either side of the third ventricle.
• Each thalamus is subdivided into three main parts by a Y-shaped vertical sheet of white matter known as the internal medullary lamina.
• The three main subdivisions of the thalamus are the Anterior, Medial and lateral parts.

Anterior part

• The medial part consists of the medial dorsal and midline nucleus.
• The lateral part is further divided into ventral and lateral groups of nuclei. The lateral group consists of lateral dorsal, lateral posterior and pulvinar.
• The ventral group consists of the ventral anterior, ventral lateral and ventral posterior.
• Intralaminar nuclei are present within the internal medullary lamina.
• The thalamus has extensive afferent and efferent connections. It receives visual impulses, auditory impulses, olfactory impulses, taste impulses, exteroceptive impulses, proprioceptive impulses and visceral information.
• The efferents of the thalamus go to cerebral cortex, hypothalamus, cerebellum, corpus striatum and reticular formation.
• Thalamic nuclei are the principal relay station for sensory impulses that reach the cerebral cortex from various parts of the CNS.

Medial and lateral geniculate bodies

• These bodies are present on the inferior aspect of the pulvinar.
• The medial geniculate body is connected with the inferior colliculus through the inferior brachium.
• The efferents of the medial geniculate body project to the auditory area of the cortex. The medial geniculate body is the relay centre in the auditory pathway.
• The lateral geniculate body is connected with the superior colliculus through the superior brachium. It receives fibres from the retina and sends efferents to the visual cortex in the occipital lobe.
• The lateral geniculate body acts as a final relay station in the visual path.

Epithalamus

• The epithalamus consists of pineal gland, habenular nucleus, stria medullaris and posterior commissure.
• The pineal gland is a small conical structure present in the posterior wall of the third ventricle. The gland secretes serotonin and melatonin which influence the activities of other endocrine glands

Thalamus, Metathalamus And Epithalamus Multiple Choice Questions

Question 1. Which of the following statements about diencephalon are true?

1. Each half of the diencephalon is located between the cerebrum (above) and the midbrain (below)
2. It extends from the region of the interventricular foramen to the region of the posterior commissure.
3. Laterally, it is bounded by the posterior limb of the internal capsule
4. The third ventricle is regarded as the cavity of the diencephalon
5. All of the above

Answer: 5. All of the above

Question 2. The diencephalon consists of the following subdivisions except

1. Dorsal thalamus
2. Metathalamus
3. Epithalamus
4. Lateral thalamus
5. Subthalamus
6. Hypothalamus

Answer: 4. Lateral thalamus

Question 3. The following features are seen on the medial surface of the thalamus except

1. Hypothalamic sulcus
2. Stria medullaris thalami
3. Taenia thalami
4. Stria terminalis

Answer: 4. Stria terminalis

Question 4. The grey matter of the thalamus is subdivided into the following parts except

1. Anterior part
2. Posterior part
3. Medial part
4. Lateral part

Answer: 2. Posterior part

Question 5. Which of the following are the nuclei of the ventral group of the lateral part of the thalamus?

1. Ventral anterior
2. Ventral lateral
3. Ventral posterolateral
4. Ventral posteromedial
5. All of the above

Answer: 3. Ventral posterolateral

Question 6. Which of the following statements is true?

1. The nuclei of the anterior group of the thalamus are concerned with memory and emotion as they are part of the limbic system
2. The nuclei of the medial group of the thalamus are connected with the prefrontal cortex; thus, the person becomes aware of emotions
3. The lateral group of nuclei projects to the postcentral gyrus. Thus, this group of nuclei is a relay station for exteroceptive sensations
4. All of the above

Answer: 4. All of the above

Question 6. The following facts about the medial geniculate body are true except

1. It is a relay centre in the auditory pathway
2. It receives fibres from the inferior colliculus of the same side through the inferior brachium
3. It does not receive fibres from the inferior colliculus of the opposite side
4. It consists of three nuclei—medial, ventral and dorsal nuclei

Answer: 4. It consists of three nuclei—medial, ventral and dorsal nuclei

Question 7. The following facts about the lateral geniculate body are true except

1. It is connected with the superior colliculus through the superior brachium
2. It is a relay station in the visual pathway
3. It receives fibres from the retina on the same side
4. The nucleus of the lateral geniculate body consists of six layers of nerve cells.

Answer: 3. It receives fibres from the retina on the same side

Question 8. The following facts about the lateral geniculate body are true except

1. It is connected with the superior colliculus through the superior brachium
2. It is a relay station in the visual pathway
3. It receives fibres from the retina on the same side
4. The nucleus of the lateral geniculate body consists of six layers of nerve cells.

Answer: 3. It receives fibres from the retina of the same side.

Diencephalon-2 Subthalamus And Hypothalamus

The ventral part of the diencephalon lies below the hypothalamic sulcus and consists of the subthalamus and hypothalamus.

Ventral Thalamus (Subthalamus)

The ventral thalamus was previously described as the subthalamus.

However, many nuclei of the subthalamus, for functional reasons, are now considered a part of basal nuclei. The ventral thalamus appears to be the upward continuation of the tegmentum of the midbrain.

It is bounded medially and ventrally by the hypothalamus and laterally by the lowest part of the internal capsule. The ventral thalamus contains two main nuclei:

1. Zona incerta
2. Reticular nucleus

Zona Incerta

Zona incerta is a thin layer of grey matter that is interposed between the thalamus and the subthalamic nucleus. It is continuous with the reticular nucleus Above.

Reticular Nucleus

• The reticular nucleus is a thin layer of grey matter situated lateral to the dorsal thalamus.
• It is separated from the thalamus by a thin layer of white matter known as the external medullary lamina. Inferiorly, it is continuous with zona incerta.
• Laterally, the reticular nucleus lies concerning the posterior limb of the internal capsule.
• This nucleus has nothing to do with the reticular formation of the brainstem. It is believed that the reticular nucleus may play a role in the regulation of thalamic activities.

Hypothalamus

• The hypothalamus, on the right and left sides, lies below the thalamus and is separated from the latter by the hypothalamic sulcus. It is present in the lateral wall and the floor ofthe third ventricle.
• The hypothalamus is concerned with many important functions which include autonomic, visceral, and endocrine functions. Most of the hypothalamus is hidden (not visible) to the naked eye when a specimen of the brain is examined.
• However, some parts of the hypothalamus can be seen on the external (ventral) surface of the brain.
• These visible parts of the hypothalamus are located in the interpeduncular fossa.
• These include optic chiasma, the median eminence of the tuber cinereum, infundibulum, and mammillary bodies. All these structures also form the floor of the third ventricle.

Boundaries of Hypothalamus

In the midsagittal section of the brain; also, the extent of the hypothalamus can be visualized as follows:

Anterior boundary: Anteriorly, the hypothalamus is bounded by lamina terminalis. The lamina terminalis extends from the anterior commissure to the optic chiasma.

Posterior boundary: Posteriorly, the hypothalamus extends up to a vertical plane posterior to mammillary bodies.

Superior boundary: Superiorly, the hypothalamus is separated from the thalamus by the subthalamic sulcus.

Inferior boundary: Inferiorly, the hypothalamus is bounded by structures that form the floor of the third ventricle and is visible externally in the interpeduncular fossa.

Lateral Boundary: Lterally, The hypothalamus concerning the internal capsule

Medial boundary: Medically, the hypothwamulues form the ventrolateral wall of the cavity of the third ventricle below the hypothalamic sulcus.

Subdivisions of Hypothalamus

The hypothalamus (on both the right and the left sides) consists of grey and white matter.

There are many distinct nerve cell areas known as nuclei. To locate the position of these nuclei, the hypothalamus is divided into two different planes:

Mediolateral subdivision: Based on the presence of prominent myelinated fibers of the column of the fornix and mamillothalamic tract, the hypothalamus is divided into medial and lateral zones. The medial zone contains many small nuclei while the lateral zone contains only a few.

Anteroposterior subdivision: The hypothalamus can also be divided anteroposteriorly into four regions:

1. Preoptic, Supraoptic, Infundibulotuberal and Mammillary regions
2. Nuclei in Various Regions of Hypothalamus

The hypothalamus is divided into four regions:

Prcoptic region: The preoptic region lies adjacent to the lamina terminalis. It has a preoptic nucleus, which extends in both medial and lateral zones.

Supraoptic region: The supraoptic region of the hypothalamus lies above the optic chiasma.

In the medial zone, it has three nuclei:

1. Paraventricular,
2. Anterior and
3. Suprachiasmatic

In the lateral zone, the supraoptic nucleus is present The lateral zone also contains a large lateral nucleus extending through the supraoptic, tuberal, and mammillary regions.

Infundibulotuberal region: The infundibulotuberal region consists of infundibulum, tuber cinereum and the region above it.

The medial zone of this region contains a dorsomedial nucleus, ventromedial nucleus, infundibular nucleus, and a small premammillary nucleus. The lateral zone of this region consists of a small tube nucleus.

Mammillary region: The mammillary region consists of the posterior nucleus and mammillary nucleus in the medial zone and the tuberomammillary nucleus in the lateral Zone. The Lateral Zone Extending Through The Supraptic, Infundibular, and Mammilary regions contains a large lateral nucleus.

The physiological significance of the nuclei in the hypothalamus is summarised.

Nervous Connection of Hypothalamus

The afferent and efferent connections of the hypothalamus are described in the following text.

Afferent Connections

The hypothalamus receives the afferent connections from the following structure

Afferent from the spinal cord: Reticular formation and collaterals of sensory lemnisci terminate in the lateral hypothalamus.

Afferent from the brainstem: From visceral (autonomic) nuclei and nuclei of the solitary tract to various hypothalamic nuclei.

Afferent from the limbic system: From the hippocampus through the fornix to the mammillary body.

Afferent from the retina: To suprachiasmatic nuclei.

Afferent from the thalamus: To various nuclei of the hypothalamus.

Efferent Connections

The efferent fibers from the hypothalamus go to the same structures from where it has received afferent fibers:

Efferents to hippocampal formation: Through the medial forebrain bundle

Efferents to the amygdaloid nuclear complex: Through stria terminalis.

Efferents to the thalamus and tegmentum: Through the mamillothalamic tract and mamillotegmental tract.

Efferents to the autonomic motor neurons of the brainstem and spinal cord: Through the medial forebrain bundle to the autonomic nuclei of the brainstem and spinal cord (sympathetic T1 to L2 and parasympathetic S2 to S4).

Efferent connections to the pituitary gland: This is described in detail separately.

Hypothalamic Control Of The Pituitary Gland

• The endocrine secretions from the pituitary gland are under the direct control of the hypothalamus.
• This control is because the median eminence and part of the infundibulum (both are parts of the hypothalamus) secrete certain substances that control the release or inhibition of hormones from pars distalis.
• These substances are called releasing and release-inhibiting factors or hormones.
• The releasing and release-inhibiting factors (hormones) are produced by a group of neurons (nuclei) situated in the median eminence and upper part of the infundibulum.
• The axons of these neurons end concerning capillaries present in these areas of the hypothalamus and pour their hormones into blood circulation see in the following text).
• The production of releasing and release-inhibiting hormones is under the control of signals from the nervous system and/or chemical changes in the blood (feedback mechanism).

Hypothalama-Hypophyseal Portal System

• The releasing or release-inhibiting factors from the hypothalamus reach the pars distalis (anterior pituitary) through the portal system of blood capillaries.
• This system is called the hypothalami-hypophyseal portal System the system is called the hypothalamic hypophyseal portal system.
• The release of release-inhibiting factors is taken into the primary capillary plexus of the portal system
• From the primary plexus, these factors reach the secondary plexus situated in the anterior pituitary where they act on the cells of the anterior pituitary gland,
• The hormones secreted by the anterior pituitary gland pass into the secondary plexus of the portal system.
• These hormones then pass into the anterior hypophyseal veins for distribution to target tissues throughout the body.

Connection of Hypothalamus with Neurohypophysis (Posterior Pituitary)

• Neurohypophysis consists of structures such as median eminence, infundibular stalk, and a posterior (neural) lobe of the pituitary gland.
• The supraoptic and paraventricular nuclei of the hypothalamus synthesize vasopressin and oxytocin hormones, respectively.
• The axons from these nuclei project to the posterior lobe of the pituitary through the infundibular stalk.
• The arrival of an action potential results in the release of hormones from Herring bodies which diffuse through capillaries to enter the general circulation.
• The hormone ‘vasopressin’ (ADH) helps in the restriction of water loss from the kidney while oxytocin causes the contraction of smooth muscles during childbirth.

Functions Of Hypothalamus

The hypothalamus, in general, serves to marry functions. These are as follows:

• Controls the autonomic nervous system (both sympathetic and parasympathetic) and acts as an integrating center.
• The limbic system sets emotional states and controls sexual desire and behavior.
• Creates new memory
• Regulates body temperature (the anterior hypothalamus facilitates heat loss by vasodilatation and sweating while the posterior hypothalamus conserves the body heat by vasoconstriction and shivering)
• Serves as a link between nervous and endocrine systems to regulate growth, metabolism, reproductive and stress responses
• Secretes hormones such as ADH and oxytocin
• Controls appetitive drives such as thirst and hunger (control food and water intake)
• Controls circadian rhythms
• Regulates sleep and wakefulness states.

Diabetes Insipidus

• Diabetes insipidus is a condition in which there is increased secretion of dilute urine (polyuria).
• This condition may result due to destruction of the supraoptic nucleus which is specifically concerned with the maintenance of body water balance.
• This nucleus produces ADH which helps in the reabsorption of water from the distal convoluted and collecting tubules of the kidney. Although ADH is produced in the hypothalamus, it is stored and secreted by the posterior pituitary.
• Therefore, it is usually considered a pituitary hormone. In the absence or reduced secretion of this hormone, water is not absorbed, and polyuria results.
• This also results in increased water intake (polydipsia). This condition is known as hypothalamic syndrome.
• The reduced secretion of ADH may be due to the destruction of the supraoptic nucleus, hypothalami-hypophyseal tract, or posterior pituitary (due to a tumor near the hypothalamus, pituitary tumor, surgery, and radiation therapy, or head injury).
• Diabetes insipidus may also occur due to the disease of the kidney in which it fails to respond to ADH.

Subthalamus And Hypothalamus Summary

• The hypothalamus is situated below the hypothalamic sulcus in the lateral wall and across the floor of the third ventricle.
• Some parts of the hypothalamus are visible on the external (ventral) surface of the brain, which are located in the interpeduncular fossa.
• The hypothalamus consists of many nuclei. To locate the position of these nuclei, the hypothalamus is divided into two different planes:
• Mediolaterally and Anteroposteriorly.
• From the medial to the lateral side, it is divided into medial and lateral zones because of the presence of columns of the fornix and mamillothalamic tract between the two zones.

Anteroposteriorly, it is divided into four regions:

• Pre-optic,
• Supraoptic,
• Infundibulotuberal and
• Mammillary regions.

The following nuclei are present in each region of the hypothalamus:

• Preoptic region—
• preoptic nucleus
• Supraoptic region—paraventricular, anterior, and suprachiasmatic nuclei in the medial zone and lateral nucleus in the lateral zone.
• Infundibulotuberal region—dorsomedial nucleus, ventromedial nucleus, infundibular nucleus, pre mammillary nucleus in the medial zone, and lateral tuberal nucleus in the lateral zone
• Mammillary region—posterior nucleus and mammillary nucleus in the medial zone; tuberomammillary nucleus and large lateral nucleus in the lateral zone
• The afferent connections of the hypothalamus are from various sources, i.e. limbic system, cerebral cortex, visual and olfactory systems, thalamus, ascending visceral and somatic sensory systems from the brainstem and spinal cord.
• The efferents of the hypothalamus go to most of the above-mentioned sources.
• The hypothalamus is concerned with many important autonomic, visceral, and endocrine functions.

Multiple Choice Questions

Question 1. Which of the following facts about habenular nuclei is are false?

1. They are situated in the habenular triangle medial to the pulvinar above the superior colliculus
2. They are regarded as cell stations in olfactory and visceral pathways
3. Afferents of habenular nuclei run in stria medullary thalami
4. Nuclei of two sides are connected with habenular commissure
5. None of the above

Answer: 5. None of the above

Question 2. The following parts of the hypothalamus are visible on the the external surface of the brain (in the interpeduncular fossa) except

1. Lamina terminalis
2. Optic chiasma
3. Median eminence of tuber cinereum
4. Infundibulum
5. Mamillary body

Answer: 1. Lamina terminalis

Question 3. In the midsagittal section of the brain, the following structures indicate the extent of hypothalamus except

1. Anteriorly—lamina terminalis
2. Posteriorly—vertical plane posterior to mammillary bodies
3. Superiorly—subthalamic sulcus
4. Inferiorly—structures visible in the interpeduncular fossa
5. Medially—floor of the third ventricle

Answer: 3. Inferiorly—structures visible in the interpeduncular fossa

Question 4. The hypothalamus is divided anteroposteriorly into the following regions except

1. Preoptic
2. Supraoptic Infraoptic
3. Infundibulotuberal
4. MammiUary regions

Answer: 3. Infundibulotuberal

Question 5. Which of the following nuclei is/are present in the hypothalamus?

1. Preoptic
2. Paraventricular
3. Supraoptic
4. Dorsomedial
5. Ventromedial
6. All ofthe above

Answer: 6. All of the above

Question 6. The following are the connections of the mammillary body except

1. Afferents from the hippocampus through the fornix to the mammillary body
2. Mammillary peduncle from tegmental nuclei to the mammillary body
3. Mammillothalamic tract
4. Mammillotegmental tract
5. Mammillospinal tract

Answer: 3. Mammillothalamic tract

Question 7. The hormone vasopressin (antidiuretic) is synthesized by

1. Paraventricular nucleus
2. Supraoptic nucleus
3. Suprachiasmatic nucleus
4. Preoptic nucleus

Answer: 2. Supraoptic nucleus

Question 8. The releasing and release-inhibiting hormones produced by

1. Nuclei of median eminence and upper part of infundibulum
2. Nuclei located in neurohypophysis
3. Adenohypophysis
4. Herring bodies

Answer: 1. Nuclei of the median eminence and upper part of the infundibulum

Question 9. The following statement(s) about the hypothalamohypophyseal portal system is/are false:

1. The release or release-inhibiting factors from the hypothalamus reach the anterior pituitary by the portal system of blood capillaries
2. The portal system consists of two sets of capillary networks
3. The first capillary network is present in median eminence and infundibulum
4. The second capillary plexus is present in the anterior pituitary
5. None of the above

Answer: 5. None of the above

Question 10. The following functions may be attributed to the hypothalamus except

1. Control of circadian rhythms
2. Control of food and water intake
3. Control of coordinated muscular movement during walking
4. Secretion of hormones
5. Control of autonomic functions

Answer: Control of coordinated muscular movement during walking

Cerebellum

• The cerebellum is a part of the hindbrain. It lies behind the pons and the upper medulla. The fourth ventricle separates the cerebellum from the pons and the medulla.
• The cerebellum is concerned with motor activities without any conscious awareness. The functions of the cerebellum include maintenance of posture and balance, control of muscle tone and coordination of activities of various muscle groups.

External Features Of Cerebellum

The cerebellum consists of two cerebellar hemispheres and a central part called vermis. The vermis unites the two cerebellar hemispheres.

Surfaces of Cerebellum

• The cerebellum presents superior and inferior surfaces. The superior surface is flattened and presents a superior vermis in the midline.
• The inferior surface of the cerebellum is convex and shows a deep depression between the two hemispheres known as vallecula. The inferior vermis lies on the floor of the vallecula.

Fissures and Lobes of Cerebellum

The cerebellum is divided into lobes with the help of two deep fissures:

1. Primary fissure and
2. Posterolateral fissure.

Posterolateral fissure.

The posterolateral fissure separates the posterior lobe of the cerebellum from the flocculonodular lobe. The flocculonodular lobe is present on the inferior surface of the cerebellum. The above two fissures divide the cerebellum into three lobes.

Horizontal Fissure

The horizontal fissure is important as it demarcates the superior cerebellar surface from the inferior one.

Other Fissures and Lobules

The anterior and posterior lobes are further subdivided into many lobules with the help of other fissures. The names of these lobules are provided

Cerebellar Tonsils

• The cerebellar tonsils are small lobules located on the inferior surface of the cerebellum.
• The cerebellar tonsils are important structures clinically because, in cases of raised intracranial pressure, the cerebellar tonsils may herniate into the foramen magnum.
• The protruded tonsil may block the circulation of the cerebrospinal fluid between the subarachnoid space in the cranial cavity and the subarachnoid space around the spinal cord.
• The herniated tonsils may also compress the vital centres of the brainstem; if severe, they may result in the death of an individual.

Relation of the Cerebellum To The Fourth Ventricle

The relation of the cerebellum to the brainstem is shown in the midsagittal section. The fourth ventricle lies between the pons and the upper medulla anteriorly and the cerebellum posteriorly. The roof of the fourth ventricle trick- is formed above by the superior medullary velum.

Cerebellar Cortex

• The surface of the cerebellum is covered by grey matter, called the cerebellar cortex. The cortex is marked by a series of slender, parallel ridges called folia.
• These ridges are narrow leaf-like bands which are arranged transversely on the surface of the cerebellar cortex.
• The midsagittal section of the hindbrain also demonstrates the central core of white matter which shows complex tree-like branchings. This arrangement is known as ‘arbour vitae’. This branching pattern is covered by grey matter (cerebellar cortex.

Cerebellar Peduncles

The cerebellum is attached to the brainstem by three pairs of cerebellar peduncles. The superior cerebella peduncles connect it to the midbrain, the middle connects it to the pons And their inferior peduncle connects it to the spinal cord.

The afferent and efferent fibres enter and leave the cerebellum through this peduncle.

Phylogenetic Classification Of Cerebellum

Based on evolution and function, the cerebellum can be divided into three parts; Archicerebellum paleocerebellum and neocerebellum

Archicerebelium

• The archicerebellum is phylogenetically the oldest part of the cerebellum. Anatomically, it is represented by the flocculonodular lobe and the lingula of the anterior lobe.
• Functionally, the archicerebellum is related to the vestibular system and its main function is to maintain posture and balance.

Paleocerebellum

• Paleocerebellum consists of the anterior lobe (except lingual), uvula and pyramid of the posterior lobe.
• As this part of the cerebellum is functionally related to the spinal cord, it is also known as the spinocerebellum. Paleocerebellum regulates the muscle tone.

Neocerebellum

• The neocerebellum consists of the posterior lobe (except the uvula and pyramid). It is functionally related to the cerebral cortex. It receives inputs from the contralateral cerebra cortex via corticopontocerebellar pathways.
• This part of the cerebellum is also known as pontocerebellum and is concerned with the coordination of the voluntary motor function.

Functional Division Of Cerebellum

• The functional divisions of the cerebellum are vestibulocerebellum, spinocerebellum and pontocerebellum.
• The vestibulocerebellum consists of the flocculonodular lobe and receives inputs from the vestibular nerve and nuclei.
• The spinocerebellum consists of a major part of the vertical and paranormal zones.
• The pontocerebellum consists of the lateral zone of the cerebellar hemisphere.
• To a certain extent, these functional divisions of the cerebellum correspond to phylogenetic divisions (i.e. archicerebellum, paleocerebellum and neocerebellum)
• The functional division, functional anatomy and phylogenetic classification of cerebellum are correlated.

Internal Structure Of Cerebellum

The interior of the cerebellum is composed ofthe following: A thin layer of grey matter, the cerebellar cortex. The cerebellar cortex is folded into folia (gyri).

Deep into the cerebellar cortex, the core of the cerebellum is formed by white matter.

Deep within the cerebellar white matter, four pairs of nuclei are present.

Cerebellar Cortex

The histological structure of the grey matter of the cerebellar cortex is uniform throughout the cerebellum.

The cortex is composed of three layers:

• Molecular,
• Purkinje and
• Granular layers.

Deep to the granular layer, the cerebellar cortex lies in contact with the white matter.

The afferent fibres to the cerebellar cortex are of two different types:

• Climbing and Mossy fibres while efferents are axons of Purkinje cells. The efferent fibres terminate in the cerebellar nuclei.
• Students should learn the microscopic structure of the cerebral cortex from a textbook of histology

Cerebellar Nuclei

Four pairs of cerebellar nuclei are present deep within the cerebellar white matter.

These nuclei consist of most of the neurons, which give origin to the efferent fibres of the cerebellum. From the medial to the lateral side, they are named as follows:

• Fastigial
• Globose
• Emboliform
• Dentate

The connections and functions of these nuclei.

White Matter of the Cerebellum

A large body of white matter is present in the centre of each cerebellar hemisphere. Intracerebellar nuclei are present within the white matter of each cerebellar hemisphere.

The dentate nucleus occupies a large area on the lateral part of white matter.

The white matter of the cerebellum consists of two types of fibres: Intrinsic and Extrinsic.

1. Intrinsic fibres: The axons of the Purkinje cells, which originate in the cerebellar cortex and terminate in the intracerebellar nuclei are the intrinsic fibres of the cerebellum.

Extrinsic fibres: The extrinsic fibres connect the cerebellum with other parts of the central nervous system—brain and spinal cord.

These fibres, afferents and efferents, enter or exit the cerebellum through the cerebellar peduncles.

The cerebellar peduncles connect the cerebellum with the medulla oblongata, pons and midbrain. The fibres of three peduncles.

Functions Of Cerebellum

1. The cerebellum controls the muscular contraction efficiently, in an automatic manner and at an unconscious level.

2. The main function of the cerebellum is the coordination of motor activities so that the voluntary movements are smooth, balanced and accurate.

This is achieved by:

Maintenance of posture and balance with the help of the vestibulocerebellum, control of muscle tone with the help of the spinocerebellum and coordination and integration of various muscle groups, required for a given motor act, with the help of pontocerebellum.

3. The cerebellum helps in learning motor skills, with gradual training.

The lesions of the cerebellum are classified into the following categories:

• Lesion of vermis (spinocerebellum)
• Lesion of the flocculonodular lobe (archicerebellar syndrome)
• Lesion of the lateral hemisphere (neocerebellar syndrome)
• Lesion of Vermis

Patients show the following disorders:

Ataxia: It is defined as an uncoordinated sequence of movement. The patient stands with the legs spread apart walks on a wide base (wadding gait) and sways from side to side or backwards. Therefore, he takes the help of a wall while walking.

Cerebellar nystagmus: It is defined as conjugate involuntary oscillatory movements of the eyes. Nystagmus occurs due to the interruption of connections of the vermis with the ocular motor nuclei.

Speech disorder: This occurs due to a lack of coordination among muscles of speech (i.e. asynergy of speech muscles). Archicerebellum Syndrome Archicerebellum syndrome is due to a lesion of the flocculonodular lobe and uvula.

This produces truncal stance and gait ataxia. It also produces cerebellar nystagmus, vertigo and vomiting.

Neocerebeliar Syndrome

Neocerebeliar syndrome is due to a lesion of afferent pathways, cortex, intracerebellar nuclei of the cerebellar hemisphere or efferent pathways (superior cerebellar peduncle).

This syndrome shows the following signs:

Ataxia: The movements are not smooth but intermittent and jerky.

Dysmetria: It is defined as the inability to measure the distance correctly for reaching the intended target.

When the patient tries to touch an object, the finger overshoots the mark or deviates from it (described as past pointing), for example finger-nose test.

Asynergy: In the performance of fine movements, there is a lack of coordination among different muscle groups. This leads to a succession of mechanical or puppet-like movements.

Hypotonia: This results in muscle weakness and rapid fatigability. This is due to loss of deep cerebellar nuclei leading to hypotonia of peripheral muscles.

Intentional tremor: When a purposeful movement (finger-nose test) is attempted, then an involuntary, rhythmical wavering movement of the hand occurs.

Speech disorder: This is due to asynergy involving muscles used in speech. Speech becomes thick and monotonous.

Nystagmus: It is present if the vermis is also involved

Cerebellum Summary

• The cerebellum is part of the hindbrain. It lies behind the pons and the medulla in the posterior cranial fossa.
• It consists of two large lateral masses (cerebellar hemispheres) and a central part called vermis.
• The cerebellum is divided into many lobes (i.e. anterior, posterior and flocculonodular) with the help of primary fissure and posterolateral fissure. The posterior (middle) lobe is the largest and is present on both the surfaces of the cerebellum (i.e. superior and inferior surfaces).
• The surface of the cerebellum is covered by grey matter called the cerebral cortex. The grey matter is arranged in parallel ridges called folia.
• Based on evolution, the cerebellum can be divided into three parts: archicerebellum, paleocerebellum and neocerebellum.
• This classification of cerebellum roughly correlates with the functional classification of cerebellum, i.e. vestibulocerebellum, spinocerebellum and pontocerebellum.
• The archicerebellum is represented by the flocculonodular lobe and lingual. It is the oldest part of the cerebellum.
• Paleocerebellum consists of the anterior lobe, uvula and pyramid of the posterior lobe. The neocerebellum consists of the posterior lobe (except the uvula and pyramid). It is the most recent in development.
• The cerebellar cortex (grey matter) is folded into many folia (gyri). The histological structure of the cortex is uniform throughout the cerebellum. The cortex is composed of three layers: the molecular layer, the Purkinje layer and the granular layer.
• Deep in the cerebellar cortex is the presence of white matter. Four pairs of cerebellar nuclei (fastigial, globose, emboliform and dentate) are present deep within the cerebellar white matter.
• The white matter of the cerebellum consists of extrinsic and intrinsic fibres. The intrinsic fibres are mostly axons of Purkinje cells which originate in the cerebellar cortex and terminate in the intracerebellar nuclei.
• The extrinsic fibres connect the cerebellum with other parts of the central nervous system (brain and spinal cord).
• The extrinsic fibres are afferents and efferents. These fibres pass out or come into the cerebellum through three cerebellar peduncles on each side.
• The inferior, middle and superior cerebellar peduncles connect the cerebellum with the medulla oblongata, pons and midbrain, respectively.
• For the constituent fibres of various peduncles.
• The main functions of the cerebellum can be summarised as follows:
  • Maintenance of posture and balance with the help of vestibulocerebellum
  • Control of muscle tone with the help of spinocerebellum
  • Coordination and integration of various muscle groups (required for a given motor act) are achieved with the help of pontocerebellum.

Multiple Choice Questions

Question 1. Which of the following statements about the cerebellum is false?

1. It occupies the posterior cranial fossa
2. It lies below the tentorial cerebella
3. It lies behind the pons and medulla
4. It is part of the brainstem
5. It is mainly concerned with the maintenance of posture and balance

Answer: 4. It is mainly concerned with the maintenance of posture and balance

Question 2. All the statements regarding the cerebellum are correct except?

1. It consists of two lobes (hemispheres) and a vermis
2. It consists of superior and inferior surfaces
3. There are two deep fissures—primary and posterolateral
4. The posterolateral separates the anterior lobe of the cerebellum from the posterior lobe
5. The flocculonodular lobe constitutes a very small part of the cerebellum

Answer: 4. The flocculonodular lobe constitutes a very small part of the cerebellum

Question 3. Which of the following statements about the cerebellar peduncle is true?

1. The superior cerebellar peduncle connects it to the midbrain
2. The middle peduncle connects it to the pons
3. The inferior peduncle connects it to the medulla
4. All of the above

Answer: 4. All of the above

Question 4. Which of the following facts regarding the input (afferent) fibres of the cerebellum is false?

1. Afferent fibres are of two types—climbing and mossy
2. Climbing fibres originate from the inferior olivary complex
3. Mossy fibres originate in vestibular nuclei, pontine nuclei and spinal cord
4. Mossy fibres ascend to the molecular layer of the cerebellar cortex and form synaptic contacts with the dendritic tree of Purkinje cells

Answer: 4. Mossy fibres ascend to the molecular layer of cerebellar cortex and form synaptic contacts with the dendritic tree of Purkinje cells

Question 5. The following nuclei are present in each cerebellar hemisphere except

1. Fastigial
2. Vestibular
3. Globose
4. Emboliformis
5. Dentate

Answer: 2. Vestibular

Question 6. The following afferent fibres (tracts) are present in the inferior cerebellar peduncle except

1. Posterior spinocerebellar
2. Olivocerebellar
3. Vestibulocerebellar
4. Anterior spinocerebellar
5. Trigeminocerebellar

Answer: 4. Trigeminocerebellar

Question 7. The following are the functional divisions of the cerebellum except

1. Vestibulocerebellum
2. Spinocerebellum
3. Pontocerebellum
4. Olivocerebellum

Answer: 4. Olivocerebellum

Question 5. The following are the functions of the cerebellum except

1. Maintenance of posture and balance
2. Coordination of exteroceptive sensory impulses
3. Control of muscle tone
4. Coordination of various muscle groups required for a given motor act

Answer: 2. Coordination of exteroceptive sensory impulses

Question 6. Most of the efferents of the cerebellum are axons of

1. Cerebellar nuclei
2. Golgi cells
3. Basket cells
4. Purkinje cells

Answer: 4. Purkinje cells

Question 10. The branches of the following arteries are responsible for the supply of blood:

1. Vertebral artery
2. Vertebral and basilar arteries
3. Basilar artery
4. Internal carotid artery
5. Vertebral and internal carotid arteries

Answer: 2. Vertebral and basilar arteries

Question 11. Most of the afferent fibres to the cerebellum make synaptic contact with

1. Cerebellar nuclei
2. Golgi cells
3. Granule cells
4. Purkinje cells
5. Basket cells

Answer: 2. Granule cells

Reticular Formation

‘Reticular formation is the collection of a group of neurons and intersecting bundles of fibers present inside the brainstem.

This particular arrangement gives rise to a reticular or net-like structure that is seen in the transverse section of the brainstem.

Extent And Location Of Reticular Formation

The reticular formation extends throughout the brainstem and is continuous above the subthalamus and reticular nuclei of the thalamus.

Inferiorly, it is continuous with the cervical part of the spinal cord. Students should note that the reticular nucleus of the thalamus is not a part of the reticular formation.

Composition Of Reticular Formation

The reticular formation is an ill-defined collection of neurons and fibers with multiple connections.

It also has neurons with multiple synaptic contacts with various ascending and descending tracts passing through the brainstem.

Nuclei Of Reticular Formation

Many ill-defined collections of neurons are recognized in the reticular formation and these collections are called nuclei.

These nuclei are arranged in vertical columns and are briefly classified into the following three zones (columns):

1. Median zone
2. Medical Zone
3. Lateral zone

Some important nuclei of all three zones are given in

Connections Of Reticular Formation

• Reticular formation has extensive afferent and efferent connections with almost all parts of the CNS.
• The pathways originating or terminating in reticular formation are both ascending and descending. Some of these pathways are crossed while others are uncrossed.
• The nuclei in the lateral parts of reticular formation are mainly sensory while those in the medial part are mainly motor.
• Most of the connections of reticular formation, as presented here, are based on the group of nuclei secreting a specific neurotransmitter.

The connections of serotonergic raphe nuclei, cholinergic nuclei, and noradrenergic nuclei are presented separately (see in the following text).

Chemoarchitectonic Organisation Of Reticular Formation

• Although the preceding description of reticular formation has indicated collections of many nuclei, these nuclei are ill-defined.
• Modern techniques (immunocytochemistry) have defined the reticular formation as consisting of a well-defined group of nerve cells (nuclei) with specific Neurotransmitters.

Serotonergic nuclei: Serotonergic nuclei synthesize serotonin (5-hydroxytryptamine). The ‘raphe’ nuclei synthesize serotonin and use it as a synaptic neurotransmitter.

The raphe nuclei of the midbrain and upper pons project into the forebrain and help in the regulation of the wake-sleep cycle, food intake, thermoregulation, and sexual behavior.

Cholinergic nuclei: Cholinergic nuclei are involved in the synthesis of acetylcholine. Acetylcholine is used as a neurotransmitter by two nuclei.

Pedunculopontine nucleus and Lateral dorsal tegmental nucleus. These nuclei are also involved in consciousness and arousal.

Catecholamine nuclei: Chemoarchitectonically, catecholamine nuclei are of three distinct types: Noradrenergic, Adrenergic, and Dopaminergic. They synthesize noradrenaline, adrenaline, and dopamine, respectively.

Noradrenergic nucleus: The noradrenergic nucleus coeruleus’ is the largest group of noradrenergic neurons. ‘Coeruleus’ influences arousal.

Adrenergic neurons: The best-known adrenergic neurons in the medulla are within the nucleus ambiguus, the nucleus of the tractus solitarius, and the dorsal motor nucleus of the vagus.

Dopaminergic neurons: The dopaminergic neurons in the brainstem are found to be present in the midbrain (in the substantia nigra and the tegmentum of the midbrain). These neurons are involved in behavioral response.

Functions Of Reticular Formation

The reticular formation can be divided functionally into medial and lateral regions.

The medial reticular region is regarded as an ‘effector’ (motor) area while the lateral zone is referred to as the ‘sensory’ part.

Specifically, the functions of reticular formation are as follows:

1. Somatic motor function: The facilitatory and inhibitory activities of lateral and medial reticulospinal tracts (RSTs) act by influencing muscle tone to ensure smooth body movements.

2. Motor functions of cranial nerves: The lateral zone of reticular formation has many interneurons. These interneurons form a local reflex circuit with the motor nuclei, of various cranial nerves, thereby influencing the motor activity of the cranial nerves.

Effect on the functions of the X nerve: The visceral motor functions of vagus nerves are coordinated by the ventrolateral medullary reticular formation by the reflex mechanism.

The cardiovascular responses, respiratory responses, and gastrointestinal responses of reticular formation are mediated through the vagus nerve.

Effect on the functions of 5, 7, and 12 nerves:

The neurons of the reticular formation adjacent to trigeminal, facial, and hypoglossal motor nuclei form reflex circuits with the motor nuclei of these cranial nerves.

These neurons of the reticular formation of eating involve lip movements (facial nucleus), chewing (trigeminal nucleus), and movements of the tongue (hypoglossal nucleus).

Effect on the functions of the 3 nerve: The nuclei of the reticular formation that are located close to the nuclei of the III nerve coordinate various complicated eye movements.

3. Visceral motor functions: The reticular formation of the brainstem has areas (centers) that regulate the motor functions of various systems (visceral motor functions). These systems include cardiovascular, respiratory, and gastrointestinal.

Respiratory centers: The dorsal medullary area is the inspiratory center and consists of gigantocellular reticular cells.

The expiratory center is situated medially in the parvocellular region of the medulla and includes the nucleus ambiguus. The process of inspiration and expiration is controlled by pneumatic and apneustic centers.

Cardiac and vasomotor areas: Stimulation of this gigantocellular nucleus causes slowing of heart rate and lowering of blood pressure while stimulation of the nucleus in the lateral zone (parvocellular) leads to an increase in heart rate and blood pressure.

Transmission of pain: Pain transmission through the spinoreticular tract follows the spine reticulothalamo cortical pathway.

Regulation of perception of pain: The descending pathways (reticulospinal) from the nucleus raphe magnus of reticular formation modify the perception of pain. The reticulospinal tract mostly terminates in substantia gelatinosa at all levels of the spinal cord.

Sleep and wakefulness

• Sleep is a state of temporary loss of consciousness or altered consciousness from which a person can be aroused. During sleep, the cerebral cortex is the least active.
• Sleep occurs due to diminished activity of ARAS (vide infra). Raphe nuclei contain serotonergic neurons which have an inhibitory effect on the cerebral cortex. This leads to the promotion of sleep.

Wakefulness:

• The reticular formation plays an important role in waking up from sleep and also in maintaining the state of consciousness (wakefulness).
• Stimulation of some part of the reticular formation (locus coeruleus, pedunculopontine, and oral pontine nucleus) increases the activity of the cerebral cortex which helps in the maintenance of consciousness (wakefulness and alertness).
• This part of the reticular formation is known as the reticular activating system (RAS).

ARAS:

• Many reticular formation nuclei receive the sensation from various somatic sensory ascending
• The efferents from these nuclei project to the widespread areas of the cerebral cortex. This leads to a generalized increase in cortical activity. This system of reticular formation is called ascending RAS (ARAS).
• A sleeping person can be aroused from sleep by increased activity of ARAS. ARAS can be stimulated by various somatic sensory stimuli (except a sense of smell) such as touch or pressure on the skin, painful stimuli, bright light, loud noise, or the sound of the alarm clock.

Coma

• Coma is a state of unconsciousness with little or no response to stimuli. It may be caused by extensive damage to the cerebral cortex or by lesions of the brainstem (bilateral damage to ARAS at rostral pons and midbrain).
• In the case of the lesion of the lower brainstem, unconsciousness is accompanied by respiratory and cardiovascular disturbances as these centers are situated in the medulla.
• In the case of patients with light coma, brainstem and spinal reflexes persist but in cases of deep coma, all reflexes are lost. Deep coma is usually fatal as respiratory and cardiovascular failure leads to death.

Summary

• Reticular formation of the brainstem is the collection of a group of neurons and intersecting bundles of fibers. It occupies the dorsal part of the brainstem.
• The reticular formation extends throughout the brainstem, from the thalamus to the spinal cord.

In the reticular formation of the brainstem, many ill-defined nuclei are observed. These nuclei are present in three zones (columns):

1. Median,
2. Medial and Lateral.

• The median zone nuclei are located in the midline, which synthesize and secrete serotonin (serotonergic). These nuclei are concerned with sleep and suppression of pain.
• The medial zone nuclei are situated on either side of the median zone. They give origin to the reticulospinal tract and central tegmental tract.
• The lateral zone nuclei are situated lateral to the medial zone. They are pedunculopontine, nucleus coeruleus, parabrachial, and parvocellular nuclei.
• The major afferent and efferent connections of reticular formation are presented.
• The medial region of reticular formation is regarded as a motor area while the lateral zone is sensory in function.
• The medial zone has long ascending and descending tracts. These tracts modulate the action of neurons involved in movements and posture, autonomic function, pain, and arousal.
• The lateral zone of reticular formation contains small interneurons that form local reflex circuits with the motor nuclei of various cranial nerves.
• Vascular, cardiac, respiratory, and gastrointestinal responses of reticular formation are mediated through the vagus nerve.
• Reticular formation plays an important role in awakening from sleep and then in maintaining the state ofconsciousness (wakefulness).

Reticular Formation Multiple Choice Questions

Question 1. Which of the following facts about reticular formation is false?

• It appears net-like in the transverse section of
• It appears net-like in the transverse section of the brainstem.
• It is a collection of groups of neurons and intersecting bundles of fibers
• It extends throughout the brainstem
• The nucleus ambiguus is located within the territory of the reticular formation
• None of the above

Answer: 5. None of the above

Question 2. Which of the following statements about the extent of reticular formation is false?

1. It extends throughout the brainstem
2. It is continuous above with the subthalamus and reticular nuclei of the thalamus
3. Below, it is continuous in the cervical part of the spinal cord
4. In the cervical part of the spinal cord, it is situated in the lateral funiculus at the junction of grey and white matter
5. None of the above

Answer: 5. None of the above

Question 3. Which of the following statements about reticular formation is false?

1. It is an ill-defined collection of neurons and fibers with multiple connections
2. Most of these neurons are Golgi type 2 neurons
3. These neurons have long dendrites and short axons cL They have multiple synaptic contacts with various ascending and descending tracts
4. None of the above

Answer: 3. These neurons have long dendrites and short axons cL They have multiple synaptic contact with various ascending and descending tracts

Question 4. Following are the columns (zones) of nuclei of reticular formation except

1. Median zone
2. Medial zone
3. Lateral zone
4. Posterolateral zone

Answer: 4. Posterolateral zone

Question 5. Following are the nuclei of the median zone except

1. Nucleus raphe dorsalis
2. Nucleus interpeduncular
3. Nucleus raphe magnus
4. Nucleus raphe pontine

Answer: 3. Nucleus raphe magnus

Question 6. Chemoarchitectonically, which of the following nuclear groups exist?

1. Serotonergic nuclei
2. Cholinergic nuclei
3. Catecholamine nuclei
4. All of the above

Answer: 4. All of the above

Question 7. Which of the following statements about the reticular formation is true?

1. Nuclei of the medial part of reticular formation are mainly motor
2. The nuclei of the lateral part are mainly sensory
3. The lateral part receives sensory information from collaterals of the spinothalamic tract, trigeminothalamic tract, and lateral lemniscus
4. Giant cell area of reticular formation is the main motor area of reticular formation
5. All of the above

Answer: 5. All of the above

Question 8. Following are the descending (efferent) tracts of the reticular formation except

1. Lateral reticulospinal tract
2. Medial reticulospinal tract
3. Dorsomedial reticulospinal tract
4. Raphespinal tract

Answer: 3. Dorsomedial reticulospinal tract

Question 9. Which of the following statements about reticular formation is false?

1. It consists of a multisynaptic pathway
2. It is confined to the midbrain and pons
3. It influences the level of consciousness and alertness
4. It consists of a diffuse network of fibers and matter (nuclei)

Answer: 2. It is confined to the midbrain and pons