Neuroanatomy

Brainstem Midbrain

• The midbrain is the uppermost part of the brainstem, connecting the pons and the cerebellum with the forebrain.
• It measures about 2 cm in length. The midbrain is traversed by a cerebral aqueduct, which connects the third ventricle to the fourth ventricle.
• The midbrain contains nuclei of origin for cranial nerves III (oculomotor) and IV (trochlear).

Brainstem Midbrain External Features

The midbrain presents ventral, dorsal, and right and left lateral surfaces.

Ventral Surface

The ventral surface shows the presence of two crura cerebri.

The crura cerebri (cerebral peduncles) are rope-like thick bundles of white fibres.

The two crura cerebri diverge from each other when traced from pons towards the cerebral hemispheres.

They form the lateral boundaries of the fossa called the interpeduncular fossa. The interpeduncular fossa contains the posterior perforated substance and mammillary bodies.

The 3 cranial nerve (oculomotor) is seen to arise from the medial aspect of the crus cerebri. The 5 cranial nerve (trochlear) comes to the ventral side from the lateral aspect of the midbrain.

Each crus cerebri is crossed transversely from above downwards by the optic tract (as it passes backwards towards the lateral geniculate body), posterior cerebral artery and superior cerebellar artery.

Dorsal Surface

• The dorsal surface ofthe midbrain presents four rounded eminences called colliculi or corpora quadrigemina
• The superior pair of eminences is called the superior colliculi and the inferior pair is referred to as the inferior colliculi.

• The 4 cranial nerves (trochlear) take origin just below the inferior colliculi.
• After taking origin, the pair of trochlear nerves run forwards across the lateral aspect of the midbrain to appear on its ventral aspect where they lie between the superior cerebellar and posterior cerebral arteries

Lateral Surface

• On each lateral surface ofthe midbrain, two thick bands of white fibres are seen. These are known as brachium colliculi.
• There is a brachium for each ofthe superior and inferior colliculi.
• The brachium of the superior colliculus passes upwards, forwards and laterally to the lateral geniculate body and optic tract.
• The brachium of the inferior colliculus, on each side, connects the inferior colliculus to the medial geniculate body.

Brainstem Midbrain Internal Structure

To understand the internal structure of the midbrain we will have to look at its transverse section The most prominent structure seen in this section is the cerebral aqueduct.

This duct is surrounded by central grey matter.

A transverse line drawn through the cerebral aqueduct divides the midbrain into two parts:

1. The part lying behind the transverse line is called the tectum
2. The part lying ventral to this line is made up of right and left cerebral peduncles.

Tectum

1. The tectum is the dorsal part of the midbrain. Its structure is different at the upper and lower levels of midbrain.
2. At the upper level, the tectum consists of a pair of superior colliculi and at the lower level, it has a pair of inferior colliculi.

Cerebral Peduncle

Each of the cerebral peduncles, from the ventral to the dorsal side, is made up of three parts. These are as follows:

1. Crus cerebri (also called basis peduncle)
2. Substantia nigra
3. Tegmentum

The structure of crus cerebri and substantia nigra remains the same throughout the length of the midbrain. However, the structure ofthe tegmentum is different at the Upper and lower levels of midbrain.

Crus Cerebri

The crus cerebri is semilunar in section and consists of a dense mass of descending white fibres.

The crus cerebri is continuous above the internal capsule and below the basilar part of the pons.

The fibres in the crus cerebri consist of the following: corticopontine fibres, corticospinal fibres and corticonuclear fibres

The corticospinal and corticonuclear fibres occupy the intermediate three-fifths of the crus cerebri while the corticopontine fibres occupy the latter one-fifth and medial one-fifth of the crus cerebri.

Corticospinal fibres are motor fibres originating in the precentral gyrus of the cerebral cortex. These fibres pass through the basilar part of the pons to enter the pyramid of the medulla.

Corticonuclear fibres also arise from the precentral gyrus and terminate in the motor nuclei of cranial nerves of the opposite and same side.

Corticopontine fibres terminate by forming synapses with the pontine nuclei located in the pons.

The axons of pontine nuclei then pass to the cerebellar hemisphere of the opposite side through the middle cerebella.

Substantia Nigra

The substantia nigra is a band of dark grey matter which is present dorsal to crus cerebri and ventral to tegmentum.

It appears dark because of the presence of pigmented nerve cells. These nerve cells contain the melanin pigment, and dopamine is synthesised in the substantia nigra.

Tegmentum

• The tegmentum is the region of the midbrain that lies between the substantia nigra and tectum.
• It consists of grey and white matter. The tegmentum of the midbrain is an upward continuation of the tegmentum of the pons.
• As the structure of crus cerebri and substantia nigra remains the same throughout the midbrain, we shall learn about the structure of tegmentum and tectum at lower and upper levels (i.e. at the level of inferior and superior colliculi)
• Transverse Section of Midbrain at the Lower Level [Level of Inferior Colliculus)
• This level corresponds to the level of the inferior colliculus

Tegmentum

The arrangement of grey and white matter in the tegmentum at the level of inferior colliculus is described in the following text.

Arrangement of Grey Matter

The grey matter present in this section is identified as follows:

1. Periaqueductal grey matter: The grey matter surrounding the cerebral aqueduct is known as periaqueductal grey (central grey) matter. At this level, it contains trochlear (motor nuclei) and mesencephalic (sensory) nuclei.

Trochlear nerve nucleus: The trochlear nudeus is situated in the ventral periaqueductal grey matter near the midline.

This nucleus is situated immediately posterior to the medial longitudinal bundle (MLB). The fibres of the trochlear nerve run dorsally and decussate before coming out on the dorsal surface of the midbrain.

The mesencephalic nucleus of the trigeminal nerve: The mesencephalic nucleus lies in the lateral part of the central grey matter.

The neurons of this nucleus receive proprioceptive impulses from the muscles of mastication, facial muscles, ocular muscles and teeth.

2. Nudeus of reticular formation: This consists of many small nuclei located in the area of the midbrain reticular formation.

Arrangement of White Matter

The tegmentum of the midbrain contains all the white bundles as seen in the tegmentum of the upper pons.

The white fibre bundles that are seen in this region are given in the following text:

Decussation of superior cerebellar peduncles: This is seen in the midline ventral to the central grey matter. After decussation, the fibres form ascending and descending tracts.

Arrangement of various lemnisci: The medial, trigeminal, spinal and lateral lemnisci are arranged dorsomedial to substantia nigra, in order from medial to lateral side. The lateral lemniscus ends in the inferior colliculus.

The other white fibres present near the midline of the tegmentum are MLB, tectospinal tract, decussation of the superior cerebellar peduncle (described in the preceding text) and rubrospinal tract, in order from the dorsal to the ventral side preceding text) and rubrospinal tract, in order from the dorsal to the ventral side.

Tectum

The tectum is the dorsal part of the midbrain, and the transverse section of the midbrain at the lower level passes through the inferior colliculus.

Inferior colliculus: The inferior colliculus is a large nucleus of the auditory pathway (acts as a relay station) and is associated with auditory and audiovisual reflexes

Transverse Section of Midbrain at the Upper Level (Level of Superior Colliculus)

Tegmentum

The characteristics of tegmentum as seen in the transverse section of the midbrain at the level of superior colliculus are as follows

Arrangement of Grey Matter

The grey matter depicts the following important nuclei:

Oculomotor nerve nucleus: It is situated ventral to periaqueductal grey matter close to midline. The axons of the oculomotor nucleus pass ventrally, traversing the red nucleus, and come out as an oculomotor nerve in the interpeduncular fossa.

Edinger-Westphal nucleus: This is a visceral motor nucleus which is situated dorsal to the oculomotor nuclear complex.

This nucleus gives rise to preganglionic parasympathetic fibres which travel with the oculomotor nerve to relay in the ciliary ganglion. The postganglionic fibres supply the ciliary muscle and the constrictor papillae muscle of the iris.

Mesencephalic nucleus: This nucleus maintains the same position as in the lower level (in the lateral part of central grey matter).

Red nucleus: This is the most prominent collection of neurons in the tegmentum. It is an oval column of cells extending in the upper half of the midbrain. It lies dorsal to substantia nigra and close to midline.

The afferent fibres reach the red nucleus from the cerebellum (through the superior cerebellar peduncle), cerebrum, basal ganglia, superior colliculus, hypothalamus, substantia nigra and spinal cord.

The efferent fibres from the red nucleus cross in the ventral tegmental decussation and then go to the spinal cord (rubrospinal tract), cranial nerve motor nuclei 3, 4, 5, 6, 7 (supranuclear), olivary nucleus, reticular formation (rubroreticular), substantia nigra, cerebral cortex and thalamus.

The red nucleus is considered an important motor nucleus of the extrapyramidal system (maintaining posture and muscle tone).

Arrangement of White Matter

The following groups of white fibres, in the form of tracts and lemnisci, are seen:

Lemnisci: Medial, trigeminal and spinal lemnisci occupy the ventral part of the tegmentum, in order from the medial to the lateral side. The lateral lemniscus is not found at this level as it has already terminated in the inferior colliculus.

Medial longitudinal bundle: The MLB is placed just in front of the oculomotor nucleus, in the paramedian position.

Dorsal tegmental decussation—tectospinal and tectobulbar tracts: This decussation is present in front of the MLB at this level. The fibres of the tectospinal tract arise from the superior colliculi and decussate in the median plane.

After decussation, the fibres descend as the tectospinal tract which is present in front ofthe MLB, in the paramedian position.

Some fibres of this tract end in the motor cranial nerve nuclei (3, 4 and 6) inside the brainstem. This tract is known as the retrobulbar tract and is considered a pathway for reflex movements of the eye in response to visual stimuli.

Ventral tegmental decussation and rubrospinal tract: The fibres of the rubrospinal tract arise from the lower part of the red nucleus and soon cross to the opposite side.

The crossing of fibres lies in the ventral part of the tegmentum and is known as ventral tegmental decussation.

After decussation, these fibres descend through the brainstem as a rubrospinal tract. The fibres of the rubrospinal tract terminate on the anterior horn cells of spinal grey matter.

Tectum

The tectum is the dorsal part of the midbrain which, at this level, consists of the superior colliculus and pretectal nucleus.

• Superior colliculus: The superior colliculus has a complex structure. It is made up of seven alternating layers of white and grey matter. It receives fibres from the retina and other sources and sends fibres to the brainstem and spinal cord through the tectobulbar and tectospinal tracts. It is concerned with retlex movements of the head and neck in response to visual stimulus.
• Pretectal nucleus: The pretectal nucleus lies lateral to the superior colliculus from where it extends upwards till the level of posterior commissure. The pretectal nuclei receive fibres from both retinae ami visual cortex through the optic tract and superior brachium. The pretectal nuclei send efferents (axons) to the Edinger-Westphal nucleus on the same and opposite side.

Brainstem Midbrain Summary

The midbrain is the uppermost part of the brainstem, connecting the pons and the cerebellum with the forebrain. It contains nuclei of origin for cranial nerves 3 and 4.

External features

• The midbrain presents ventral, dorsal and lateral surfaces. The ventral surface shows the presence of two crura cerebri which enclose posterior perforated substance and mammillary bodies. The 3 cranial nerves arise from the medial aspect of the cerebri.
• The dorsal surface of the midbrain presents a superior and an inferior pair of colliculi.
• The 4 cranial nerves (trochlear) originate from either side of the midline, just below the inferior colliculi.

The lateral surface of the midbrain shows two thick bands of white fibres:

1. Superior and
2. Inferior brachium.

Internal structure

On the section of a midbrain, a transverse line drawn through the cerebral aqueduct divides the midbrain into two parts:

1. Cerebral peduncle and Tectum.
2. Cerebral peduncle

The cerebral peduncle is further divided, in the ventrodorsal direction, into crus cerebri, substantial nigra and tegmentum. The structure of crus cerebri and substantia nigra remains the same throughout the length of the midbrain.

However, the structure of the tegmentum and tectum varies at the upper and lower levels of the midbrain.

• Crus cerebri: It consists of a dense mass of descending white fibres, that is, corticospinal, corticonuclear and corticopontine.
• Substantia nigra: It is a band of dark grey matter containing melanin and iron pigments in nerve cells. It is present just posterior to the crus cerebri. It synthesises the neurotransmitter dopamine.
• Tegmentum: It consists of grey and white matter and is situated between substantia nigra and tectum.

At the level of the inferior colliculus, the following features are seen in the tegmentum:

The central grey matter contains trochlear and mesencephalic nuclei. Trochlear nerves come out of the dorsal aspect of the midbrain. The reticular formation is present ventrolateral to central grey matter.

Decussation of superior cerebellar peduncles is seen in the midline ventral to central grey matter.

The arrangement of various lemnisci (ML, TL, SL and LL) is dorsomedial to substantia nigra.

The medial longitudinal bundle, tectospinal tract, decussation of the superior cerebellar peduncle and rubrospinal tract are seen in the paramedian position.

At the level of the superior colliculus, the following features are seen in the tegmentum:

1. The oculomotor nerve nucleus is situated ventral to periaqueductal grey in the midline. The Edinger-Westphal nucleus is dorsal to the oculomotor nucleus.
2. The red nucleus is situated dorsal to substantia nigra close to midline. It is a motor nucleus of the extrapyramidal system. The mesencephalic nucleus maintains the same position as in the lower level.
3. The medial, trigeminal and spinal lemnisci are located in the ventral part of the tegmentum, in order from the medial to the lateral side.
4. The medial longitudinal bundle is situated ventral to the oculomotor nucleus in the paramedian position.
5. Dorsal and ventral tegmental decussation is located in the midline of the tegmentum.

Tectum

Three different collections of grey matter are found in tectum:

1. Superior colliculus,
2. Inferior colliculus and
3. Pretectal nucleus.

Superior colliculus: The superior colliculus is a flattened mass formed by seven alternating layers of white and grey matter.

Inferior colliculus: It is a large nucleus of the auditory pathway and is associated with auditory and audiovisual reflexes.

Pretectal nucleus: The pretectal nucleus is situated superolateral to the superior colliculus.

It is a small mass of diffuse neurons. Stimulation of one pretectal nucleus leads to constriction of the pupil in both eyes because it innervates the Edinger-Westphal nuclei of both sides.

Brainstem Midbrain Multiple-Choice Questions

Question 1. The following structures cross crus cerebri from above downwards except

1. Optic tract
2. Posterior cerebral artery
3. Superior cerebral artery
4. Trochlear nerve

Answer: 4. Trochlear nerve

Question 2. The cranial nerve which takes origin from the dorsal aspect of the brainstem is

1. Oculomotor nerve
2. Trochlear nerve
3. Accessory nerve
4. Vagus

Answer: 2. Trochlear nerve

Question 3. Which of the following statements is false?

1. Inferior colliculi are centres for auditory pathways and auditory reflexes
2. The brachium of the inferior colliculus connects the inferior colliculus to the medial geniculate body
3. The brachium conveys auditory fibres to the medial geniculate body
4. From the medial geniculate body, fibres travel in optic radiation

Answer: 4. From the medial geniculate body, fibres travel in the optic radiation

Question 4. In a cross-section, each cerebral peduncle, from the ventral to the dorsal side, is made up of the following except

1. Crus cerebri
2. Substantia nigra
3. Tegmentum
4. Tectum

Answer: 4. Tectum

Question 5. Which of the following statements about crus cerebri is false?

1. Fibres in crus cerebri are corticospinal, corticonuclear and corticopontine
2. Corticospinal and corticonuclear fibres occupy about medial three-fifths of crus cerebri
3. The lateral one-fifth is occupied by corticopontine fibres descending from the temporal, occipital and parietal lobes
4. Corticopontine fibres belong to the extrapyramidal system

Answer: 2. Corticospinal and corticonuclear fibres occupy about medial three-fifths of crus cerebri

Question 6. Which of the following statements about substantia nigra is false?

1. It is a band of dark grey matter
2. It extends throughout the length of the midbrain
3. It is connected mainly to the corpus striatum
4. Dopamine synthesised by it reaches the corpus striatum
5. None of the above

Answer: 5. None of the above

Question 7. Which of the following statements about substantia nigra is true?

1. It synthesises dopamine and substance P
2. It also synthesises GABA, 5-HT and ENK
3. In Parkinsonism, the synthesis and transport of dopamine are defective
4. In Huntington’s disease, the production of GABA is reduced
5. All of the above

Answer: 5. All of the above

Question 8. The tegmentum of the midbrain at the level of inferior colliculus contains the following nuclei except

1. Periaqueductal grey
2. Trochlear nucleus
3. Edinger-Westphal nucleus
4. Mesencephalic nucleus
5. The dorsal nucleus of Raphe

Answer: 3. Mesencephalic nucleus

Question 9. Which of the following structures of white matter is present near the midline of the tegmentum at the level of inferior colliculus?

1. Medial longitudinal bundle
2. Tectospinal tract
3. Decussation of the superior cerebellar peduncle
4. Rubrospinal tract
5. All of the above

Answer: 5. All of the above

Question 10. Which of the following nuclei is absent in the tegmentum of the midbrain at the level of superior colliculus?

1. Oculomotor nerve nucleus
2. Edinger-Westphal nucleus
3. Superior salivatory nucleus
4. Red nucleus
5. Mesencephalic nucleus

Answer: 3. Superior salivatory nucleus

Question 11. Which ofthe following lemniscus is absent in the tegmentum of the midbrain at the level of the superior colliculus?

1. Medial lemniscus
2. Trigeminal lemniscus
3. Spinal lemniscus
4. Lateral lemniscus

Answer: 4. Lateral lemniscus

Question 12. Which of the following statements about the inferior colliculus is false?

1. It receives afferent from lateral lemniscus
2. The lateral lemniscus brings proprioceptive impulses to the inferior colliculus
3. It sends efferents to the medial geniculate body
4. The medial geniculate body in turn sends fibres to the auditory cortex in the temporal lobe.

Answer: 2. The lateral lemniscus brings proprioceptive impulses to the inferior colliculus.

Anatomy Of The Brainstem Pons

The ventral surface is convex and bounded by upper and lower borders. This surface presents transversely running ridges (fibres). Laterally, these ridges come closer to form a bundle, the middle cerebellar peduncle.

The point of junction between the anterior surface of the pons and the middle cerebellar peduncle is marked by the emergence of the trigeminal nerve. The ventral surface also presents a shallow groove in the midline, known as the basilar groove. This groove lodges the ‘basilar artery’.

On the ventral aspect, three cranial nerves (6, 7 and 8) are seen to emerge from the lower border of the pons.

The posterior surface of the pons is formed by the upper part of the floor of the fourth ventricle. The posterior surface of the pons is related to the cerebellum

Brainstem Pons External Features

Pons is a part of the brainstem, situated between the medulla, below, and the midbrain, above. It lies in front of the cerebellum. It is about 2.5 cm long and presents a ventral and a dorsal surface.

Brainstem Pons Internal Structure

A transverse section passing through the pons is divided into ventral (basilar) and posterior (tegmentum) parts.

Structure of the Basilar Part

The basilar part of the pons consists of descending longitudinal fibres, transverse pontine fibres and pontine nuclei.

Descending longitudinal fibres: The descending longitudinal fibres consist of corticospinal, corticonuclear and corticopontine fibres.

Pontine nuclei: The pontine nuclei are small masses of grey matter scattered between longitudinal (vertical) and transversely arranged fibres.

Anatomy Of The Brainstem

Corticopontine fibres from various lobes of the cerebral cortex end in pontine nuclei.

Transverse pontine fibres: Transverse pontine fibres are pontocerebellar fibres that run transversely across the midline of the pons.

The pontocerebellar fibres (which form the middle cerebellar peduncle) are a part of the corticopontocerebellar pathway.

Structure of the Tegmental Part

The tegmental (dorsal) part of the pons is the upward continuation of the medullary reticular formation.

The white matter in the tegmentum consists of various ascending and descending tracts.

The structure of the tegmentum is different in the upper and lower parts of the pons. Hence, it is customary to study the internal structure of the tegmental part of pons at two different levels.

1. Caudal (lower) Part transverse section passing through the facial colliculus
2. Cranial (upper) part, transverse section passing through trigeminal nuclei.

Transverse Section Through The Tegmentum Of Lower Pons

Arrangement of Grey Matter

Anatomy Of The Brainstem

The floor of the fourth ventricle, at this level, is lined by grey matter and shows the presence of two cranial nerve nuclei.

Abducent and Vestibular.

Also seen are the cranial nerve nuclei of the trigeminal (spinal nucleus of the trigeminal nerve) and facial nerve, at deeper levels.

• The abducent nerve nucleus lies beneath the facial colliculus and lateral to the medial longitudinal bundle.
• The facial colliculus is formed due to a complicated loop formed by the fibres of the facial nerve winding around the abducent nucleus.
• The vestibular nuclei complex receives afferent fibres from the vestibular division of the vestibulocochlear nerve.
• There are two cochlear nuclei which are designated as ‘ventral’ and ‘dorsal’. The ventral and dorsal nuclei lie on the ventral and dorsal aspects of the inferior cerebellar peduncle, respectively.
• The motor nucleus of the facial nerve is present in the reticular formation, medial to the nucleus of the spinal tract of the trigeminal nerve.
• The salivatory nuclei are usually divided into superior salivatory, inferior salivatory and lacrimatory nuclei. The nuclei.
• These nuclei send secretomotor fibres via facial and glossopharyngeal nerves to various salivary glands.
• The nucleus of the spinal tract of the trigeminal nerve and the tract is present ventromedial to the inferior cerebellar peduncle and lateral to the nucleus of the facial nerve.

Nucleus of tracts solitarius: At the level ofthe lower pons, this nucleus lies lateral to the superior salivatory nucleus.

• The reticular nuclei are a small collection of grey matter scattered in the network of white fibres (reticular formation).
• The nucleus of the trapezoid body is situated in the the ventral part of the tegmentum.
• This nucleus is placed in the auditory pathway and its fibres constitute the lateral lemniscus.

Arrangement Of White Matter

Trapezoid Body

Brain Stem Parts

The trapezoid body is formed by the fibres of both ventral The trapezoid body is formed by fibres of both ventral.

The fibres of the trapezoid body relay in the trapezoid nucleus and the superior olivary complex. The efferents of these nuclei form the lateral lemniscus (ascending auditory pathways).

Brain Stem Parts

Medial, Trigeminal, Spinal and Lateral Lemnisci

• These lemnisci are situated posterior to the trapezoid body.
• The medial lemniscus is the most medial and now oriented transversely. It was formed by the decussation of internal arcuate fibres in the medulla.
• The trigeminal lemniscus begins to form at this level and is situated lateral to the medial lemniscus. It is formed by the axons arising from the contralateral spinal nucleus of the trigeminal nerve. This tract conveys the exteroceptive impulses (pain, touch, temperature) from the area of supply of the trigeminal nerve (face, nose, mouth, tongue, conjunctiva, etc.
• The spinal lemniscus is formed by the fibres of lateral spinothalamic and spinotectal tracts. It lies lateral to the trigeminal lemniscus.
• The lateral lemniscus begins in the lower half of the pons. Is part of the auditory pathway. It lies most laterally in the lemniscal band, i.e. lateral to the spinal lemniscus.
• The ventral spinocerebellar tract is situated dorsolateral to the lateral lemniscus.
• The spinal nucleus and the tract of the trigeminal nerve are situated lateral to the facial nerve nucleus.
• The lateralmost area of the tegmentum is occupied by the inferior cerebellar peduncle.

Transverse Section Through The Tegmentum of Upper Pons

The transverse section through the upper part of the pons corresponds to the level of trigeminal nuclei. This level Passes through the 1116 motor and Principal Sensory nuclei of the trigeminal nerve.

Brainstem Nuclei

The dorsal part of the tegmentum now contains the cavity of the fourth ventricle, which is bounded dorsolaterally on either side by a superior cerebellar peduncle and roofed by the superior medullary velum.

Arrangement of Grey Matter in Tegmentum

• The following nuclei are seen in the tegmentum.
• The principal (superior) sensory nucleus of the trigeminal nerve: It is situated lateral to the motor nucleus. Below this level, the nucleus is continuous with the spinal nucleus of the trigeminal nerve.
• Motor nucleus of the trigeminal nerve: This nucleus lies medial to the sensory nucleus on the floor

Brainstem Pons Summary

• External features of pons: The pons lie between the medulla and the midbrain and are situated in front of the cerebellum.
• It presents ventral and dorsal surfaces. The ventral surface of the pons is convex and presents a shallow groove in the midline—the basilar groove.
• On each side, the ventral surface is continuous with the middle cerebellar peduncle and shows the attachment of the V cranial nerve.
• The posterior surface of the pons is formed by the upper part of the floor of the fourth ventricle. Three cranial nerves (4, 7 and 8) emerge from the pontomedullary junction.

Brainstem Nuclei

Internal structure of pons: The transverse section passing through pons is divided into basilar and tegmental parts.

• The structure of the basilar part of the pons consists of descending fibres (corticospinal, corticonuclear and corticopontine) and transverse pontine fibres (pontocerebellar).
• The pontine nuclei are small masses of grey matter scattered between longitudinally and transversely arranged fibres.
• The structure of the tegmental part of pons consists of ascending and descending tracts, cranial nerve nuclei of 5 to 8 nerves, and reticular formation.
• The structure of the basilar part of the pons remains constant throughout the pons. However, the structure of the tegmentum differs in the upper and lower parts of the pons.

Transverse section through the lower pons: The tegmentum, at the level of facial colliculus (lower pons), shows the following features:

Arrangement of grey matter in tegmentum: The floor of the fourth ventricle, at this level, shows the presence of abducent and vestibular nerve nuclei.

Brainstem Nuclei

• The ventral dorsal and cochlear nuclei are located on the dorsal and ventral aspects of the inferior cerebellar peduncle, respectively.
• The motor nucleus of the facial nerve is present in the area of reticular formation. The facial nerve follows an unusual course before it comes out at the pontomedullary junction.
• The nucleus of the spinal tract of the trigeminal nerve is located ventromedial to the inferior cerebellar peduncle.
• The nucleus of tractus solitarius and the superior salivatory nucleus is situated ventromedial to the facial nucleus.

Arrangement of white matter: The trapezoid body is seen at the junction of basilar and tegmental parts. Posterior to the trapezoid body, four lemnisci (medial, trigeminal, spinal and lateral) are arranged from the medial to the lateral side.

The medial longitudinal bundle, tectospinal tract and rubrospinal tract are situated at the paramedian position. The inferior cerebellar peduncle is present in the lateralmost area of the tegmentum.

Transverse section through the upper pons

Arrangement of grey matter in tegmentum: The principal (superior) sensory nucleus of the trigeminal nerve is situated lateral to the motor nucleus.

The motor nucleus of the trigeminal nerve lies medial to the sensory nucleus on the floor of the fourth ventricle. The nucleus of the lateral lemniscus is present medial to the lateral lemniscus.

Arrangement of white matter: The medial, trigeminal, spinal and lateral lemnisci are in the same position.

The ventral spinocerebellar tract now forms the superior cerebellar peduncle. The medial longitudinal bundle, tectospinal tract and rubrospinal tracts are seen in the paramedian position.

 

Brainstem Medulla Oblongata

The brainstem consists of the medulla oblongata, pons and midbrain. Superiorly, the brainstem is continuous with the structures forming the forebrain— thalamus, hypothalamus and cerebral hemispheres. Inferiorly, it is continuous with the spinal cord.

External Features Of Brainstem

The three parts of the brainstem—midbrain, pons and medulla—are connected to the cerebellum posteriorly with the help of the superior, middle and inferior cerebellar peduncles, respectively.

Posteriorly, the upper parts of the medulla and pons are separated from the cerebellum by the fourth ventricle. The cavity of the fourth ventricle is continuous superiorly with the cerebral aqueduct of the midbrain.

Inferiorly, the cavity of the fourth ventricle is continuous with the central canal of the spinal cord. The brainstem gives attachment to cranial nerves 3 to 7.

Medulla Oblongata Shape

Cranial nerves 3 and 4 are attached to the midbrain, 5 to the pons, 6 to 8 are attached at the junction of pons and medulla and 9 to 7 are attached to the medulla.

The brainstem gives passage to various ascending and descending tracts connecting the forebrain with the spinal cord.

The brainstem also contains centres for the control of consciousness, respiration and the cardiovascular system (vital centres).

Vital centres control essential autonomic functions such as cardiac activities (cardiac centre), blood pressure (vasomotor centre), respiratory rate (respiratory centre) and vomiting (vomiting centre).

Medulla

The medulla oblongata or simply medulla is about 3 cm long and 2 cm wide. It is conical in shape, broad above and narrow’ below’ It is continuous above with the pons and below with the spinal cord. The medulla consists of two parts:

Lower closed part and Upper open part. The lower closed part contains the central canal, which is in continuation below the central canal of the spinal cord. In the upper open part ofthe medulla, the central canal expands to form the lower part of the cavity of the fourth ventricle.

Medulla External Features

The medulla presents anterior (ventral) and posterior (dorsal) surfaces. On the anterior aspect, there is the presence of the anterior median fissure. This fissure is continuous below the anterior median fissure of the spinal cord.

Medulla Oblongata Anatomy

The posterior surface of the closed part of the medulla shows the presence of a midline sulcus known as the posterior median sulcus. This sulcus is present only in the closed part and is an upward continuation of the posterior median sulcus of the spinal cord.

Medulla Anterior Aspect

• On either side of the anterior median fissure, a swelling or bulge is present which is called a pyramid.
• The pyramid consists of corticospinal fibres. In the lower part of the medulla, most of these pyramidal fibres cross to the opposite side and this crossing constitutes pyramidal decussation.
• Lateral to the pyramid, an oval swelling is present which is called the olive. This bulge is due to the presence of the underlying inferior olivary nucleus.
• Between the pyramid and olive, a sulcus is present, known as anterolateral sulcus. This sulcus gives attachment to the rootlets of the hypoglossal nerve.
• Similar to the anterolateral sulcus, another sulcus is present posterolateral to the olive, known as posterolateral sulcus.
• This sulcus is situated between the olive and the inferior cerebellar peduncle. The posterolateral sulcus gives attachment to 9, 10 and 11 cranial nerves.

Medulla Oblongata Anatomy

Medulla Posterior Aspect

• The posterior surface of the lower part (closed part) of the medulla lies between the posteromedian and posterolateral sulci.
• This surface is in upward continuation of the fasciculi gracilis and cuneatus of the spinal cord and presents elevations, the gracile and cuneate tubercles on either side of the posteromedian sulcus.
• Beneath the gracile and cuneate tubercles, the corresponding gracile and cuneate nuclei are present.
• Between the fasciculus cuneatus and rootlets of the accessory nerve, a swelling may be seen which is referred to as the tuberculum cinereum.
• The posterior surface of the upper part of the medulla contributes to the lower part of the floor of the fourth ventricle.

Medulla Internal Structure

To study the internal structure of the medulla, transverse sections (TS) are studied at the following levels:

• TS of the medulla at the level of the pyramidal decussation
• TS of the medulla at the level of sensory decussation.
• TS of the medulla at the level of olives.

Transverse Section of Medulla at the Level of Pyramidal Decussation

This level of pyramidal decussation is the transitional zone between the spinal cord and the medulla. An extensive rearrangement of grey and white matter is observed at this level

Medulla Oblongata Diagram

White Matter at the Level of Pyramidal Decussation

• At this level, a majority (about 75-95%) of fibres constituting the pyramid cross the median plane in front of the central canal.
• After crossing the median plane, the pyramidal fibres pass backwards and laterally, to reach the lateral white column (funiculus) of the spinal cord and form the lateral corticospinal tract.
• The remaining 25% of the pyramidal fibres remain uncrossed and descend below in the anterior white funiculus as the anterior corticospinal tract
• The gracile and cuneate fasciculi start terminating in the caudal end of the gracile and cuneate nuclei, respectively.
• The posterior and anterior spinocerebellar tracts are similar to those of the spinal cord.

Nuclei (Grey Matter] at the Level of Pyramidal Decussation

Medulla Oblongata Diagram

• Because of the pyramidal decussation, the central canal and central grey matter are displaced more posteriorly.

 

• Due to the passage of decussating pyramidal fibres, the anterior horn gets detached from the central grey matter. This detached part forms the spinal nucleus of the accessory nerve.
• The caudal end of the nucleus of the spinal tract of the trigeminal nerve and its tract are seen at this level. Below this, the nucleus becomes continuous with the substantia gelatinosa of the spinal cord (dorsal grey horn).
• Nuclei gracilis and cuneatus start appearing at this level and lie within the substance of fasciculi gracilis and cuneatus, respectively. These nuclei appear as the posterior extension of the central grey matter.

Transverse Section of Medulla at the Level of Sensory Decussation

This level of medulla is situated just above the level of pyramidal decussation in the rostral portion of the closed part. Extensive changes are seen in the arrangement of grey and white matter at this level.

Medulla Oblongata Diagram

Arrangement of Grey Matter

• The readers are requested to read the following discussion.
• Nuclei gracilis and cuneatus: These nuclei, which had started appearing at an earlier level, are now large.
• They lie deep to fasciculi gracilis and cuneatus, respectively. At this level, nuclei gracilis and cuneatus are seen to have detached from the central grey matter.
• Accessory cuneate: This nucleus is situated just dorsolateral to the nucleus cuneatus. The nucleus of the spinal tract of the trigeminal nerve: It is situated ventrolateral to the nucleus cuneatus.
• Nucleus ambiguus: It is situated in the area of reticular formation medial to the nucleus of the spinal tract of the trigeminal nerve. The lowest part of the inferior olivary nucleus appears at this level posterior to the pyramid.
• The small mass of grey matter lies on the anterior aspect of the pyramid. This is the arcuate nucleus.
• Inside the central grey matter (surrounding the central canal), three nuclei are present:
• Hypoglossal nucleus,
• The dorsal nucleus of the vagus and
• The nucleus of the solitary tract
• Arrangement of White Matter

Functions Of Medulla Oblongata

The white matter is organised in the form of the following fascicles or bundles:

Fasciculi gracilis and cuneatus: These fasciculi are situated posterior to nuclei gracilis and cuneatus.

The second-order neuron fibres arise from gracile and cuneate nuclei and cross to the opposite side in front of the central grey matter. These fibres are known as internal arcuate fibres.

Sensory decussation: The internal arcuate fibres of two sides decussate in the median plane. This is called sensory decussation.

After decussation, these fibres run upwards close to the median plane as the medial lemniscus. The fibres constituting the medial lemniscus terminate in the thalamus.

Medial longitudinal bundle: This bundle lies posterior to the medial lemniscus and anterior to the hypoglossal nucleus. It consists of ascending and descending fibres connecting various cranial nerve nuclei (3, 4, 6 and 8).

Spinal tract of the trigeminal nerve: This tract lies superficial to the nucleus of the spinal tract of the trigeminal nerve on the surface of the medulla.

Anterior spinocerebellar and posterior spinocerebellar tracts: These tracts are situated in the ventrolateral area of the medulla, superficially.

Reticular formation: This consists of a network of scattered nerve cells and nerve fibres. The reticular formation is present throughout the brainstem.

Functions Of Medulla Oblongata

Transverse Section of Medulla at the Level of Olive

• This level of the transverse section of the medulla passes ventrally through olives and dorsally through the lower part of the fourth ventricle.
• The arrangement of grey and white matter shows many changes. The most striking change is that the central canal disappeared while the fourth ventricle is seen.
• The central grey matter now spreads over the ventricular floor.

Arrangement of Grey Matter

The total amount of grey matter has increased at this level because of the appearance of many new nuclei.

Hypoglossal nucleus, dorsal vagal nucleus and nucleus solitaries have changed their position due to the appearance of the fourth ventricle.

Vestibular nuclei: Lateral to these nuclei, the cauda ends of vestibular nuclei are seen.

Cochlear nuclei: The ventral and dorsal cochlear nuclei are situated on the ventral and dorsal aspects of the inferior cerebellar peduncle, respectively ends vestibular nuclei are seen.

Cochlear nuclei: The ventral and dorsal cochlear nuclei are situated on the ventral and dorsal aspects of the inferior cerebellar peduncle, and respectively nucleus of the trigeminal nerve and its tract are situated ventromedial to the inferior cerebellar peduncle.

Nucleus ambiguus: The nucleus ambiguus is situated in the area of reticular formation.

Olivary nuclei: The ventrolateral area of the section is occupied by the olivary nuclei.

Arcuate nucleus: This is seen on the anteromedial aspect of the pyramid.

Functions Of Medulla Oblongata

Arrangement of White Matter

Inferior cerebellar peduncle: This peduncle is situated in the posterolateral corner of the section. It is mainly formed by the fibres of the dorsal spinocerebellar tract and olivocerebellar tract.

Ventral spinocerebellar tract: The ventral spinocerebellar tract is situated near the surface, ventral to the inferior cerebellar peduncle.

Ascending tracts: Ascending tracts such as anterior spinothalamic, lateral spinothalamic and spinotectal tracts are deeply placed.

In the same area, descending tracts lie, for example tectospinal, rubrospinal, reticulospinal and vestibulospinal.

Pyramids: They occupy the same anterior part of the medulla on either side of the anterior median fissure.

Medial lemniscus, medial longitudinal fasciculus and tectospinal tract: Posterior to the pyramid and in the para-median position, the medial lemniscus, tectospinal tract and medial longitudinal fasciculus are placed in the anteroposterior direction.

Reticular formation: It is present in the ventrolateral part of the transverse section (same place as seen at the level of sensory decussation).

Lesions of the medulla may result due to a variety of causes, i.e. injury, congenital anomalies, raised pressure in the posterior cranial fossa usually due to tumour, demyelinating diseases and vascular lesions. The vascular lesions are the most common.

Injury to Medulla

• The medulla may be injured due to a hard blow on the back of the head or upper neck.
• This kind of injury is usually fatal because the medulla contains vital centres such as respiratory and cardiovascular. Damage to the respiratory centres can rapidly lead to death.
• If the injury is nonfatal, it may affect the cranial nerves that take origin from the medulla. This may affect the function of the cranial nerves on the same side of the injury.
• As the tracts are closely packed in the medulla, a nonfatal injury may produce the lesion of these tracts.
• This may result in paralysis of the muscles on the opposite side (due to damage to the corticospinal tract) and loss of sensation on the opposite side (due to damage to ascending sensory tracts).

Vascular Lesions of Medulla

• Vascular lesions are the most frequently seen along with the lesions of the medulla. The vascular lesion may occur due to thrombosis or haemorrhage.
• Haemorrhage in the medulla is serious because escaping blood destroys the vital centres in the reticular formation, i.e. centres controlling functions of respiration, circulation and consciousness.
• However, thrombosis causes a smaller (localised) lesion in the medulla, and the signs and symptoms of these lesions help to locate the site of destruction.
• Two well-known syndromes, resulting from the occlusion of medullary vessels, are medial medullary syndrome and lateral medullary syndrome.

 

Brainstem Medulla Oblongata Summary

• The brainstem consists of the medulla, pons and midbrain. It is continuous above the forebrain and below the spinal cord.
• The brainstem is connected posteriorly to the cerebellum with the help of superior, middle and inferior cerebellar peduncles.
• The medulla, pons and cerebellum are collectively known as hindbrain.
• The brainstem gives attachment to cranial nerves 3 to 8.
• The medulla is conical in shape, and related anteriorly to the basilar part of the occipital bone is the posterior cranial fossa.
• As indicated the medulla is divided into an upper open part and a lower closed part.
• The ventral surface shows the presence of anterior median fissure, pyramid, anterolateral sulcus, olive and posterolateral sulcus.
• The anterolateral and posterolateral sulci give attachments to cranial nerves 9 to 12.
• The dorsal surface of the lower (closed) part of the medulla shows the presence of posteromedian sulcus, gracile tubercle, cuneate tubercle and posterolateral sulcus.
• The transverse section at the level of pyramidal decussation resembles the spinal cord. The most striking feature is
  pyramidal decussation and appearance of the lateral corticospinal tract.
• The dorsal grey horn of the spinal cord is replaced by the nucleus of the spinal tract of the trigeminal nerve.
• The caudal ends of gracile and cuneatus nuclei start appearing in the posterior grey column.
• The transverse section at the level of sensory decussation shows three nuclei in the central grey matter: hypoglossal, dorsal vagal and nucleus of the solitary tract.
• Nuclei gracilis and cuneatus are now a separate mass of grey matter on the posterior aspect of the section.
• The nucleus of the spinal tract of the trigeminal is situated in the lateral part of the sections. The nucleus ambiguus is situated in the area of the reticular formation.
• The fibres arising from gracile and cuneate nuclei are known as internal arcuate fibres.
• The internal arcuate fibres of two sides decussate in the median plane, which is known as sensory decussation. After decussation, these fibres form the medial lemniscus.
• The medial longitudinal bundle is located anterior to the hypoglossal nucleus. In the anterior area of the section are pyramids on either side of the anterior median fissure containing corticospinal fibres.
• The anterior and posterior spinocerebellar tracts are situated in the ventrolateral area of the medulla near the surface.
• The section passing through the open part of the medulla (lower part of the fourth ventricle) shows the most striking change; that is, instead of the central canal, the fourth ventricle is seen.
• Due to the opening of the central canal, the position of the hypoglossal nucleus, vagal nucleus and nucleus solitarius has changed.
• Note the location of vestibular and cochlear nuclei in the lateral part of the section. The nucleus ambiguus and spinal nucleus of the trigeminal have maintained the same position as in the lower section.
• The olivary nuclear complex (inferior, medial and dorsal olivary nuclei) is located in the ventrolateral area of the
  section.
• The arrangement of white matter is almost the same as at a lower level. The appearance of the inferior cerebellar peduncle is a new feature at this level.
• Lesions of the medulla may result due to a variety of causes: injury, congenital anomaly and vascular lesions.

 

Nervous System Definition

• The nervous system mainly consists of the brain and the spinal cord, which are situated in the midline of the body.
• The brain is present within the skull and the spinal cord in the vertebral canal of the vertebral column.
• Various systems of the body are connected with the brain and spinal cord through nerves.
• The nerves that originate from the brain are called cranial nerves. Tirere are 12 pairs of cranial nerves.
• The nerves that originate from the spinal cord are called spinal nerves. There are 31 pairs of such nerves arising from the right and left sides of the cord.

There are 31 pairs of such nerves arising from the right and left sides of the cord The nervous system is composed of trillions of specialized cells and their processes.

These cells are called neurons or nerve cells. The processes of nerve cells are called axons and dendrites.

Supporting cells are also present in the nervous system, known as neuroglia.

Components Of Nervous System

Functions Of The Nervous System

The nervous system has three basic functions: Sensory Motor and integrative.

• Nervous System Sensory Function

Neurons that carry sensory impulses towards the brain and spinal cord are called sensory neurons or afferent neurons.

These neurons allow the nervous system to detect changes in the internal and external environments.

• Nervous System Integrative Function

The sensory impulses received by the brain are analyzed within the brain. The brain makes appropriate decisions and acts accordingly.

The neurons that serve this function are called interneurons (association neurons).

These neurons not only analyze and integrate the sensory impulse but also store the information.

• Nervous System Motor Function

The motor functions of the nervous system are carried out by motor neurons. These neurons carry information from the brain or spinal cord to the effector organs (muscles and glands).

This motor activity occurs in response to integration of sensory impulses in the CNS.

Classification Of The Nervous System

The nervous system can be classified on a structural or functional basis.

1. Nervous System Structural classification: Structurally, the nervous system is classified as follows.

• Central nervous system: The CNS consists of the brain and spinal cord. Various parts ofthe brain and spinal cord.
• Peripheral nervous system: The peripheral nervous system (PNS) consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. It also includes their associated ganglia and splanchnic nerves. Ganglia are a collection of nerve cells outside the CNS. Details of the PNS.

Nervous System Functional classification: Functionally, the nervous system is classified as follows.

• Somatic nervous system: The somatic nervous system innervates the somatic structures of the body (head, neck, limbs, and trunk) and mediates somatic sensory and somatic motor functions. ‘1his system responds to the changes in the external environment of the body.
• Autonomic nervous system: The autonomic nervous system (ANS) innervates viscera, glands, and blood vessels and is involved in the control of visceral functions.
  • Thus, this system maintains the internal environment of the body. ANS is present within the CNS and PNS.
  • This system is further subdivided into sympathetic and parasympathetic nervous systems
• Enteric nervous system: The enteric nervous system (ENS) is defined as the system of neurons that is found within the wall of GIT.
  • It consists of sensory and motor enteric neurons and their processes. There are about 100 million enteric neurons in the plexuses of GIT.
  • Functions of ENS include both sensory and motor. The sensory neurons of ENS monitor the stretching of the walls of the intestine and chemical changes within the gastrointestinal tract.
  • The motor neurons of ENS control the contraction of the smooth muscle and secretion of the gastrointestinal gland and endocrine cells associated with GIT.
  • A comparison of the components and functions of the somatic nervous system, ANS, and HNS is presented in

Nervous System Summary

• The nervous system controls and coordinates the functioning of various other systems of the body. It controls the body’s activities through nerve impulses conducted along the axons of neurons.
• The CNS consists of the brain and spinal cord. Various systems of the body are connected with the brain and spinal cord through nerves that take origin from the brain and spinal cord.
• The PNS consists of 12 pairs of cranial and 31 pairs of spinal nerves.
• Functionally, the nervous system is classified into somatic, autonomic, and enteric nervous systems.
• The somatic nervous system innervates somatic structures of the body (head, neck, limbs, and trunk).
• The ANS innervates viscera, glands, and blood vessels. It is subdivided into sympathetic and parasympathetic nervous systems.
• The ENS consists of neurons and enteric glial cells in the plexus of the gastrointestinal tract.

Nervous System Multiple Choice Questions

Question 1. Homeostasis of the body is maintained by

1. Endocrine and reproductive systems
2. Immune system
3. Endocrine and nervous systems
4. Circulatory and respiratory systems

Answer: 3. Endocrine and nervous systems

Question 2. The nervous system is concerned with the following function(s):

1. It receives information
2. It analyses information
3. It takes decisions
4. It responds quickly
5. All of the above

Answer: 5. All of the above

Question 3. Which of the following statements is correct?

1. All the cranial nerves are attached to the ventral aspect of the brain
2. One cranial nerve is also attached to the ventral aspect of the cervical spinal cord
3. All cranial nerves are attached to the dorsal aspect of the brain
4. Only one cranial nerve is attached to the dorsal aspect of the brain

Answer: 4. Only one cranial nerve is attached to the dorsal aspect of the brain

Question 4. The nervous tissues consist of the following except

1. Neurons and their processes
2. Neuroglia
3. Connective tissue
4. Bloodvessels

Answer: 3. Connective tissue

Question 5. The peripheral nervous system consists of all of the following except

1. Cranial nerves
2. Spinal nerves
3. Spinal cord
4. Ganglia

Answer: Spinal cord

Question 16. Which of the following statements is false?

1. There are 7 pairs of cervical spinal nerves
2. There are 12 pairs of cranial nerves
3. There are 31 pairs of dorsal root ganglia
4. All the cranial nerves are not associated with ganglia

Answer: 1. There are 7 pairs of cervical spinal nerves

Ascending Descending Tracts Of The Spinal Cord

• The ascending and descending fibers in the spinal cord are organized in the form of ‘tracts’ or ‘fasciculus’.
• These tracts are present in the anterior, lateral, and posterior funiculi (columns of white matter) of the spinal cord.

Ascending Tracts

Ascending Tracts

• The ascending tracts or the sensory tracts in the spinal cord carry somatic and visceral sensations.
• The somatic sensations include pain, touch, temperature, vibration, and pressure.
• The visceral sensory impulses are mainly in the form of pain and stretch sensations. Most visceral sensory and some somatic sensory impulses do not reach the level of consciousness.

Sensory Pathway

The sensations are first received by receptors, which are sensory end organs. Each receptor receives a stimulus and transforms it into a nerve impulse.

The nerve impulses are then carried through a pathway, which usually consists of three sensory neurons arranged in sequence:

• First-order,
• Second-order and
• Third-order neurons.

First-Order Neuron

The first-order sensory neurons are located in the dorsal root ganglia (DRG) of the spinal nerves and the corresponding root ganglion of the cranial nerves.

Second-Order Neuron

The cell bodies of second-order sensory neurons lie in the dorsal grey column (horn) of the spinal cord.

The axonal processes of the second-order sensor)7 neurons, as a general rule, cross to the opposite side before reaching the thalamus.

Hence, sensations from the left side of the body go to the right thalamus and vice versa.

As a result, the left side of the brain receives sensory information from the right side of the body while the right side of the brain senses the left side of the body.

Ascending Tracts

Third-Order Neuron

The cell bodies of third-order sensory neurons are located in the thalamus.

The axons of third-order sensory neurons from the thalamus project to the sensor)7 cortex of the cerebrum (postcentral gyrus; areas 3, 1, 2). Thus, general sensations are perceived in the postcentral gyrus in Various ascending tracts.

Ascending Tracts Of The Dorsal White Column

The ascending tracts located in the dorsal white column or funiculus of the spinal cord are fasciculus gracilis and fasciculus cuneatus.

Fasciculus Gracilis and Fasciculus Cuneatus

Fasciculus gracilis and fasciculus cuneatus carry sensations of conscious proprioception, fine touch, vibration and wo-point discrimination.

The lower spinal cord shows only fasciculus gracilis while the upper spinal cord shows both gracilis and cuneatus fasciculi.

Ascending Tracts

Sensory Pathway

The sensory pathway involves the participation of three sets of neurons: First-order, Second-order, and third-order sensory neurons.

First-Order Sensory Neurons

They lie in DRG. The central processes of the dorsal nerve root ascend in the dorsal funiculus without relaying in the posterior horn. The fibers of gracile and cuneate fasciculi terminate on the cell bodies of gracile and cuneate nuclei, respectively.

Spinal Cord

Second-Order Sensory Neurons

Nuclei gracilis and cuneatus contain second-order sensory neurons. These nuclei are located in the medulla. Their axons curve ventromedially as internal arcuate fibers.

These fibers decussate in the midline with the fibers arising from the nuclei located on the opposite side.

After decussation, these fibers then turn upwards to form a medial lemniscus.

This decussation is known as great sensory decussation. The fibers of the medial lemniscus ascend through the medulla, pons, and midbrain to terminate in the ventral-posterolateral nucleus of the thalamus.

Third-Order Sensory Neurons

They are located in the ventral-posterolateral nucleus of the thalamus. The axons of third-order neurons end in the sensory area of the cerebral cortex (postcentral gyrus; areas 2, 1, 3).

Lesions of the Posterior Column (Tabes Dorsalis)

• The lesion of the posterior column will diminish or abolish the discriminating tactile and kinaesthetic sense.
• The loss of sense of position in the lower extremities greatly affects the equilibrium, stance, and gait.
• Usually, posterior column lesions are associated with syphilis infection.
• The degenerative disease of the posterior column due to syphilis is called tabes dorsalis.

Ascending Tracts

Ascending Tracts Of The Lateral White Column

The major ascending tracts of the lateral white column or funiculus are described as follows:

• Posterior spinocerebellar tract
• Anterior spinocerebellar tract
• Lateral spinothalamic tract

Posterior Spinocerebellar Tract

The posterior or dorsal spinocerebellar tract is uncrossed and carries unconscious proprioceptive impulses from joints, muscles, and tendons.

This tract forms a part of the neural pathway that carries these impulses from the lower limb and caudal part of the body to the cerebellum.

Origin, Course, and Termination

The pathway involves two sets of neurons. This is in contrast to the sensory pathways going to the cerebral cortex, which usually consists of three neurons.

First-Order Neurons

These are located in the DRG of the spinal nerves

Second-Order Neurons

• The second-order neurons of this tract are large neurons of Clarke’s column. This nucleus extends from C8 to L3 spinal segments.
• Though these fibers are second-order neurons, they do not cross to the opposite side.
• These fibers ascend as a posterior spinocerebellar tract in the lateral white column of the spinal cord and end in the cerebellum through the inferior cerebellar peduncle.

Second-Order Neurons Functions

• The function of the dorsal spinocerebellar tract is to convey unconscious proprioceptive impulses from a single (individual) muscle.
• This information is useful in posture maintenance and body movements.
• These tracts also carry unconscious exteroceptive impulses (touch and pressure) to the cerebellum, which are utilized for the coordination of posture.

Anterior Spinocerebellar Tract

The anterior or ventral spinocerebellar tract is a crossed tract carrying proprioceptive and exteroceptive impulses from the lower limbs.

• Anterior Spinocerebellar Tract Origin, Course, and Termination

There are two sets of neurons which are involved in this pathway.

Ascending Tracts

• Anterior Spinocerebellar Tract First-Order Neuron

This lies in the DRG of the coccygeal, sacral, and lumbar spinal nerves.

• Anterior Spinocerebellar Tract Second-Order Neuron

The cell body of the second-order neuron is located in the dorsal horn.

The axons of the second-order neuron immediately cross to the opposite side and give origin to the ventral spinocerebellar tract.

Anterior Spinocerebellar Tract Function

Similar to a dorsal spinocerebellar tract, this tract also carries unconscious proprioceptive and exteroceptive impulses from the lower limbs.

Spinal Cord

Lateral Spinothalamic Tract

The lateral spinothalamic tract is a crossed tract, which carries the sensations of pain and temperature. It arises at all levels of the spinal cord namly cervical thoracic lumbar and sacral.

Lateral Spinothalamic Tract Origin, Course, and Termination

Lateral Spinothalamic Tract First-Order Neurons

These lie in the DRG of spinal nerves. The central process of the DRG cells then makes synaptic contacts in the neurons of substantia gelatinosa, whose main function is to modify the pain and temperature impulses.

Thus, the neurons of substantia gelatinosa only act as interneurons These interneurons then project on the neurons of second order.

Lateral Spinothalamic Tract Second-Order Neurons

These are situated in laminae 1, 4, and 5 (of the nucleus proprius). Their dendrites synapse with substantia gelatinosa cells and their axons form the lateral spinothalamic tract.

These axons soon cross over to the opposite side in ventral white commissure and ascend upwards in the lateral funiculus as the lateral spinothalamic tract. This tract, after ascending through the brainstem, terminates in the VPL nucleus of the thalamus.

There may be some awareness of pain and temperature at the level of the thalamus.

Lateral Spinothalamic Tract Third-Order Neurons

These are situated in the VPL nucleus of the thalamus. Their axons project to the primary sensory cortex ofthe cerebral hemisphere.

Lateral Spinothalamic Tract Functions

• This tract conveys pain and temperature (hot and cold) sensations from the limbs and trunk.
• Unilateral damage to the spinothalamic tract leads to loss of pain and temperature (hot and cold) sensations from the skin of the opposite side of the body, below the lesion.
• As the fibers of pain lie anterior and superficial to the fibers of temperature in this tract, the cutting of the anterolateral white column cuts the pain fibers.
• This surgical procedure is called cordotomy, which is usually performed at the cervical level to relieve the severe pain associated with malignancy (cancer) of the neck or thorax.

Ascending Tracts of the Ventral White Column

The important ascending tract present in the ventral white column is the anterior or ventral spinothalamic tract.

Spinal Cord

Anterior or Ventral Spinothalamic Tract

This tract conveys touch, pressure, and itch sensations. The first-order neurons, after entering the spinal cord, may ascend or descend for new segments in the dorsolateral tract of Lissauer before terminating into posterior horn cells. The details of this tract.

Descending Tracts

The descending tracts of the spinal cord begin from the brain and terminate at different levels in the spinal cord. These tracts are motor in nature. These tracts are grouped
as pyramidal and extrapyramidal tracts.

Pyramidal tracts: These tracts originate in the cerebral cortex and play a major role in controlling precise voluntary muscular movements.

Extrapyramidal tracts: These tracts originate mainly from the subcortical regions of the brain (corpus striatum, reticular nuclei, vestibular nuclei, olivary nuclei, etc.) and play an important role in integration for regulation of muscular movements and muscular tone.

The descending tracts are organized in the form of columns in the white matter of the spinal cord.

Pyramidal Tracts

The pyramidal tracts carry impulses that regulate precise and voluntary muscular movements. These tracts consist of fibers that originate from the motor cortex of the cerebrum.

The pyramidal tracts include the following:

• Corticonuclear, also called corticobulbar
• Corticospinal

These tracts form a part of the motor pathway that extends from the brain to the skeletal muscles.

The pathway consists of two sets of motor neurons that are referred to as upper motor neurons and lower motor neurons, respectively (see in the following text).

Upper and lower motor neurons

The upper motor neurons are situated in the cerebral cortex. These neurons send their axons to the different cranial nerve nuclei (motor neurons) in the brainstem, through corticobulbar/corticonuclear tracts.

The axons of the upper motor neurons also project to the anterior horn cells (motor neurons) at different levels of the spinal cord through corticospinal tracts.

The lower motor neurons are situated in the brainstem (cranial nerve nuclei) and the spinal cord (neurons located in the anterior horn).

The lower motor neurons receive impulses from the pyramidal tracts (corticonuclear and corticospinal), and these impulses are then transmitted along the axons of lower motor neurons to the skeletal muscles. Thus, the pyramidal system controls the voluntary movements of the body.

This system of descending fibers is known as the pyramidal system because the corticospinal fibers occupy the pyramid of the medulla.

It may be noted that though corticonuclear fibers are confined to the brainstem (i.e. they do not occupy the pyramid), however, they are also included in the category of the pyramidal system.

Corticonuclear or Corticobulbar Tracts

The fibers of these tracts arise from the neurons of the motor cortex, in the cerebrum, and end in the motor neurons of cranial nerve nuclei (located in the brainstem). However, before terminating in the cranial nerve nuclei these fibers cross to the opposite side.

Corticospinal Tract

The fibers of this tract arise from the cells of the cerebral cortex and end in the anterior horn cells of the spinal cord.

• The corticospinal tract consists of axons arising mainly from the cells of the precentral gyrus (motor area; area 4), from giant-sized Betz cells.
• These fibers subsequently descend through the posterior limb of the internal capsule, cerebral peduncle of the midbrain, pons, and medulla. In the upper part
  of the medulla, the corticospinal fibers form an elevated bundle known as a pyramid.
• Below the level of the pyramid, about 75-90% of corticospinal fibers cross towards the opposite side. This crossing forms the pyramidal decussation.
• The fibers that have crossed to the opposite side descend downwards in the lateral funiculus of the spinal cord as the lateral corticospinal tract.
• The rest of the fibers of the pyramid remain uncrossed and descend downwards in the anterior funiculus as the anterior or ventral corticospinal tract.

Lateral Corticospinal Tract

This tract is present at almost all the levels of the spinal cord (up to the fourth sacral segment).

As it gives fibers to the anterior horn cells ofthe same side at all levels, it progressively diminishes in size from above downwards.

Ventral Corticospinal Tract

This tract consists of only 10-25% of uncrossed fibers of the pyramidal tract. This tract is present in the cervical and upper thoracic regions of the spinal cord and is not found below the mid-thoracic spinal segments.

These fibers terminate on the anterior horn cells of the opposite side by crossing at various levels.

Ventral Corticospinal Tract Functions

The corticospinal tract is concerned with the contraction of skeletal muscles; hence, it controls the voluntary and skilled movement of the body.

Lesion of Corticospinal Tract

• The damage to the corticospinal tract is the damage to the upper motor neurons of the pyramidal system. This leads to paralysis (loss of motor power) of the skeletal muscles.
• The part of the body paralyzed will depend upon the site of the lesion.
• The lesion at the level of the internal capsule results in the paralysis of the opposite side of the body (hemiplegia). A lesion of the tract in the spinal cord will lead to paralysis of the skeletal muscle below the level of the lesion on the same side.

Extrapyramidal Descending Spinal Tracts

The extrapyramidal system is responsible for regulating muscle tone as well as posture and equilibrium of the body.

The major descending tracts of the spinal cord included in the extrapyramidal system are as follows:

• Tectospinal
• Rubrospinal
• Vestibulospinal
• Olivospinal
• Reticulospinal

Tectospinal Tract

• The first-order neurons are situated in the superior colliculus. Axons from these neurons cross to the opposite side in front of periaqueductal grey and form dorsal tegmental decussation.
• After decussation, the fibers descend downwards in the spinal cord in the anterior funiculus.
• They ultimately synapse with the motor neurons of the ventral horn (second-order neurons), which innervate muscles of the neck.

Tectospinal Tract Function

The tectospinal tract conveys impulses for reflex postural movements in response to visual and auditory stimuli.

Rubrospinal Tract

• The first-order neurons of this tract lie in the red nucleus of the midbrain. The axons soon decussate in the tegmentum of the midbrain (ventral tegmental decussation) and cross to the opposite side to descend downwards in the lateral funiculus.
• The fibers of this tract synapse with the second-order neurons (alpha motor neurons of the ventral horn).

Rubrospinal Tract Functions

The rubrospinal tract exerts facilitator}’ influence on flexor muscle tone and inhibitory influence on extensor muscle tone.

Vestibulospinal Tract

• The vestibulospinal tract is concerned with the maintenance of the body’s equilibrium.
• This tract conveys the vestibular and cerebellar influences to the spinal cord. The vestibular nuclei are situated at the junction of the pons and medulla.
• The vestibulospinal tracts originate from the lateral and medial vestibular nuclei.
• The fibers of the lateral vestibulospinal tract extend throughout the spinal cord. The tract is facilitatory to motor neurons supplying extensor muscles.
• The medial vestibulospinal tract only reaches the upper thoracic region. This tract is inhibitory to the muscles of the neck and back.

Olivospinal Tract

The olivary nucleus is situated deep to the olive in the medulla. This tract descends from the inferior olivary nucleus to neurons of the anterior horn in the spinal cord. The details of this tract are not known.

Reticulospinal Tract

• A reticular formation is a group of scattered nerve cells and nerve fibers that are present in the midbrain, pons, and medulla. This tract arises from the neurons situated in the reticular formation of the brainstem.
• The axons of this tract descend in the anterior funiculus of the spinal cord.
• This tract reaches all the levels of the spinal cord. The fibers of this tract terminate on the alpha and gamma motor neurons, either directly or indirectly through interneurons.

Reticulospinal Tract Functions

The reticulospinal tract is facilitatory to the muscles of the trunk and limbs and helps in the maintenance of body posture. It also modifies the transmission of pain impluses.

Pyramidal and Extrapyramidal Tracts: A Comparison

Spinal Cord Trauma

• Trauma to the spinal cord occurs as a result of vertebral fractures. Most of these injuries are due to automobile or motorcycle accidents, sports-related injuries, falls from a height gunshot, and stab wounds. The trauma may result in complete transection, incomplete transection, or hemisection of the spinal cord.
• Trauma is the most common cause affecting the spinal cord. It occurs as a result of a fracture of the vertebral column (spine).

Complete transection of the spinal cord: It results in complete loss of motor control (paralysis) and loss of sensations at and below the level of injury to the spinal cord.

Hemisection of the spinal cord (Brown-Sequard syndrome): In this lesion, the lateral half of the cord is damaged. Extensive paralysis of muscles on the same side below the level of injury occurs while extensive sensory loss occurs on the opposite side (normal side). This phenomenon is called the BrownSequard syndrome.

Syringomyelia: In this disease, dilatation of the central canal occurs, which leads to damage to decussating fibers of the spinothalamic tract in the cervical to upper thoracic segments of the cord.

Loss of pain and temperature sensations occurs in hands and arms bilaterally while the sense of touch is retained.

 

Complete Transection of the Spinal Cord

The complete transection of the spinal cord results in immediate loss of motor control (paralysis) and loss of sensation (anesthesia) at and below the level of injury to the spinal cord

Hemisection of the Spinal Cord (Brown-Sequard Syndrome)

In this lesson on the spinal cord, the lateral half of the cord is damaged (lateral hemisection;. This usually happens due to gunshot injury. The clinical features of this injury include minimal changes in autonomic functions but extensive changes in the somatic nervous system.

Changes below the level of hemisection of the spinal cord

1. On the same side of injury

• Proprioceptive sensations such as vibration, kinaesthetic, fine touch, and tactile localization are lost on the same side of the lesion due to damage to the fasciculi gracilis and cuneatus (dorsal white column).
• As the spinothalamic tract crosses to the opposite side, the sensations of pain, temperature, and crude touch remain unaffected.
• As the lateral corticospinal tract (crossed pyramidal tract) gets damaged, all the skeletal muscles below the level of injury are paralyzed. The paralysis is of the UMN type.
• However, the anterior corticospinal tract (uncrossed pyramidal tract) which runs on the opposite side escapes injury. Therefore, some muscles on the side of the injury are not paralyzed.

2. On the opposite side of injury

• There is no loss of proprioceptive sensation.
• Loss of pain, temperature, and crude touch below the level of injury because of the spinothalamic tract.
• Carrying these sensations crosses to the opposite side and gets damaged.
• Usually, there is no noticeable paralysis of muscles on this side.
• Damage to the direct pyramidal tract (anterior corticospinal tract) may lead to paralysis of a few muscles on this side.
• To summarise the changes in the preceding text, it may be said that, below the level of lesion, extensive paralysis of muscles occurs on the same side of injury, while extensive sensory loss occurs on the opposite side (normal side). This phenomenon is called the Brown-Sequard syndrome.

Degenerative And Demyelinating Disorders Of The Spinal Cord

Syringomyelia

• Syringomyelia is a chronic disease characterized by the formation of cavities in the central canal of the spinal cord.
• This results in the dilatation of the central canal and damage to the nervous tissue (spinothalamic tract) surrounding the central canal.
• At the beginning of the disease, loss of pain and thermal sensations in the hands and arms bilaterally with the preservation of touch sensation occurs.
• Touch is retained as it has a double pathway. Fine touch ascends in the dorsal white column.

Tabes Dorsalis

• Tabes dorsalis is a disease that leads to sensory disturbances, usually in the lower limbs. This disease is caused due to immunological reactions in patients suffering from syphilis.
• Because of the immunological reaction, degeneration in the central processes of spined ganglion cells occurs.
• The site of the lesion is located on the dorsal spinal nerve root, just lateral to its entrance into the spinal cord.

Ascending Descending Tracts Of The Spinal Cord Summary

• The ascending and descending fibers in the spinal cord are organized in the form of ‘tracts’.
• A tract is defined as ‘the bundle of fibers having the same origin, course, termination and function’.
• The ascending tracts of the spinal cord carry sensory information to the brain while the descending fibres carry motor information from the brain towards the peripheral tissues.
• These tracts are present in the anterior, lateral, and posterior funiculi of the spinal cord.

Ascending Tracts

• The sensory nerve impulses, in ascending tracts, are usually carried through three sensory neurons arranged in sequence: first-order, second-order, and third-order neurons.
• The first-order sensory neuron is located in the dorsal root ganglion of the spinal nerve and third-order neurons are located in the thalamus.
• As a general rule, the axons of the second-order sensory neurons cross to the opposite side before reaching the thalamus.

1. Ascending tracts in the dorsal white column (funiculus)

Fasciculi gracilis and cuneatus: These tracts carry the sensations of conscious proprioception—fine touch, vibration, and two-point discrimination.

The first-order sensory neurons lie in the dorsal root ganglion Their central processes ascend in the dorsal funiculus without relay in the posterior horn.

These fibers terminate on second-order sensory neurons present in gracile and cuneate nuclei Fibres of the second-order neurons (internal arcuate fibers) decussate in the midline to form medial lemniscus.

These fibers ascend through the medulla, pons, and midbrain to terminate on third-order neurons in the thalamus. The third-order neurons project on the cerebral cortex.

2. Ascending tracts in the lateral white column

Posterior spinocerebellar tract: It carries unconscious proprioceptive impulses, touch, and pressure sensation from the lower limb and caudal part ofthe body to the cerebellum. This pathway consists of only two neurons.

The first-order sensory neurons lie in the dorsal root ganglion Their central processes end in the nucleus dorsalis (Clarke’s column) of the dorsal grey horn of the spinal cord The axons of the second-order neurons form the dorsal spinocerebellar tract. This tract ascends on the same side tract ends in the vermis of the cerebellum.

Anterior spinocerebellar tract: It carries unconscious proprioceptive and exteroceptive impulses. There are two sets of neurons involved in this pathway.

The first-order neurons lie in the dorsal root ganglion Their central processes end in dorsal grey column cells The axons of the second-order neuron immediately cross to the opposite side and give origin to the ventral spinocerebellar tract.

This tract runs through the medulla, pons, midbrain, and superior cerebellar peduncle to terminate in the opposite vermis of the cerebellum Thus, fibers of the second-order neurons cross twice and reach the ipsilateral cerebellar hemisphere.

Lateral spinothalamic tract: It is a crossed tract, which carries sensations of pain and temperature. The first-order sensory neurons are located in the dorsal root ganglion The central process of these neurons makes synaptic contact with substantia gelatinosa The second-order neurons are situated in the nucleus proprius.

The axons of second-order neurons soon cross to the opposite side to form the lateral spinothalamic tract The tract after ascending through the brainstem terminates in the thalamus The third-order neurons of the thalamus project on the sensory cortex of the cerebrum.

3. Ascending tract in the ventral white column

Ventralspinothalamic tract: It conveys the sensations of touch, pressure, and itch.

The first-order neurons are in the dorsal root ganglion → The second-order neurons are located in the dorsal horn of the spinal cord →The axons of these cells cross to the opposite side to form the ventral spinothalamic tract The tract terminates in the thalamus from where third-order neurons send their axons to the cerebral cortex.

Descending Tracts

1. Pyramidal tracts: These tracts play an important role in the control of precise voluntary muscular movements. These tracts begin from the brain (cerebrum) and terminate at different levels of the spinal cord. The pyramidal pathway consists of two sets of neurons:

1. Upper motor neurons (humans) and
2. Lower motor neurons (lmns).

Corticonuclear tracts: The first-order neurons are located in the motor cortex which sends their axons to cranial nerve nuclei. These axons constitute the corticonuclear tract. The axons of second-order neurons innervate the skeletal nucleus.

Corticospinal tracts: They are concerned with the control of voluntary and skilled movements ofthe body.

The first-order motor neurons (UMN) are located in the cerebral cortex The axons of these neurons pass through the midbrain, pons, and medulla as the pyramidal tract and form a pyramid in the medulla Below the level of the pyramid, 75-90% fibers cross towards the opposite side and form the lateral corticospinal tract.

The rest of the fibers of the pyramid remain uncrossed and descend as the ventral corticospinal tract The second-order motor neurons (LMN) are located in the anterior horn of the spinal cord.

Fibers of both corticospinal tracts (ventral and lateral corticospinal) make synaptic contact with the anterior horn cells of the opposite side.

2. Extrapyramidal tracts: These descending tracts originate mainly from the subcortical regions of the brain and terminate at different levels of the spinal cord.

The extrapyramidal system is responsible for regulating muscle tone as well as posture and equilibrium of the body. These tracts are not involved in voluntary control over skeletal muscles.

Tectospinal tract: It is concerned with visuospinal reflexes. The first-order neurons are situated in the superior colliculus.

Axons of this tract cross to the opposite side in tegmental decussation and terminate on the motor neurons of the ventral horn (second-order neurons) of the cervical spinal cord Axons of motor neurons innervate muscles of the neck.

Rubrospinal tract: It is concerned with the regulation of tone in the flexor muscles. The first-order neurons are situated in the red nucleus Axons of these neurons cross to the opposite side in ventral tegmental decussation and form the rubrospinal tract The second-order neurons are located in the ventral horn cells of the spinal cord.

Vestibulospinal tract: It is concerned with the maintenance of body equilibrium. The first-order neurons are located in medial and lateral vestibular nuclei Axons of these neurons descend on the same side Fibres of the tract terminate on second-order neurons (ventral horn cells).

Olivospinal tract: The first-order neurons are situated in the inferior olivary nucleus The second-order neurons of this tract are situated in the anterior horn of the spinal cord.

Reticulospinal tract: It is concerned with regulating the muscle tone in antigravity muscles. The first-order neurons are situated in the reticular formation of the brainstem.

Axons of these neurons form two tracts:

Medial reticulospinal and lateral reticulospinal The second-order neurons are alpha and gamma motor neurons of the spinal cord.

 

Neuron

A neuron consists of a cell body or soma and neuronal processes. The neuronal processes that emerge from the cell body are called dendrites (usually multiple) and axons (single).

Nerve Cell Body Or Soma

The nerve cell body has a typical pale staining euchromatic nucleus with a prominent dark nucleolus. The cytoplasm of a neuron contains prominent basophilic material called Nissl bodies.

The cell body also contains the Golgi apparatus, mitochondria, lysosomes, and smooth endoplasmic reticulum.

Normally, neurons after birth are unable to replicate their DNA and thus do not undergo cell division.

Nerve Cell Processes

The elongated cytoplasmic processes take origin from the cell body. These processes may travel long distances from the neuron. They are of two types:

Dendrites and Axon. Usually, a neuron consists of a single axon and multiple dendrites.

Dendrites

Neurons usually have short, multiple dendrites, and each of these may branch extensively to form a ‘dendritic tree’. Dendrites are involved in receiving information from other cells.

The cytoplasm of dendrites contains Nissl bodies, microtubules, microfilaments, and other organelles.

The cytoskeleton of a neuron is formed by microtubules, neurofilaments, and microfilaments.

Axons

• Axons are the nerve cell processes that send information in the form of electrical signals away from the nerve cell body to other cells.
• Usually, there is a single, extremely long axon (in some neurons, the length may reach up to 1 m) for each neuron.
• It originates at a conical region of the cell body known as the axon hillock.

3 Types Of Neurons And Functions

Classification Of Neurons

Neurons are classified based on their structure, size, and function.

Classification Based On Structure of Neurons

The structure and shape of a cell body are dependent on the number and orientation of cell processes arising from it.

Depending on the number of processes emerging from the cell body, the neurons can be classified as follows:

• Multipolar
• Bipolar
• Unipolar or pseudo-unipolar

Multipolar neurons: Multipolar neurons have a single axon and many dendrites extending from the cell body. Most of the neurons present in the CNS are multipolar.

Bipolar neurons: Bipolar neurons have two processes emerging from the cell body (e.g. bipolar neurons of the retina and neurons of olfactory neuroepithelium).

3 Types Of Neurons And Functions

Unipolar or pseudo-unipolar neurons: Unipolar or pseudo-unipolar neurons are found in the dorsal root ganglion of spinal nerves (sensory ganglion) and sensory root ganglia of cranial nerves.

Such a neuron has a single process that extends from the cell body; this process then bifurcates to form a T-shaped process.

According to some authors, true unipolar neurons have only a single process extending from the cell body, usually the dendrite.

These kinds of neurons are present in the mesencephalic nucleus of the trigeminal nerve.

Classification Based On Size of Neurons

The neurons can also be classified based on their size.

According to this classification, the neurons are as follows:

• Golgi type1 neurons
• Golgi type2 neurons

Golgi typeI neurons: Golgi type1 neurons have a single, long axon, sometimes more than a meter in length.

The axons of these neurons (e.g. pyramidal cells, Purkinje cells) extend long distances within the CNS (as part of the tract) or in the PNS (as part of peripheral nerves, e.g. sciatic nerve;

Golgi type 2 neurons: Golgi type 2 neurons have short axons and these terminate close to the cell body (e.g. stellate and granule cells of the cerebellar cortex).

3 Types Of Neurons And Functions

Classification Based On Function of Neurons

Functionally, neurons are classified into sensory”, motor, and interneurons.

Sensory or afferent neurons: Sensory neurons are specialized cells capable of detecting various kinds of stimuli, i.e. pain, touch, temperature, light, pressure, and chemicals.

Motor (efferent) neurons: Motor neurons carry efferent to the sensory stimuli.

Motor neurons are of two types:

Somatic and visceral. These neurons send motor impulses to the muscles, which results in their contraction.

Somatic motor neurons send impulses to the skeletal muscles while visceral motor neurons stimulate the smooth and cardiac muscles.

The visceral motor neurons also send signals (secretomotor) to exocrine glands, which results in secretion from these glands.

Interneurons: Interneurons are located in between sensory and motor neurons. These neurons interconnect sensory and motor neurons. These neurons carry out the integrative function.

3 Types Of Neurons And Functions

Neuroglia

Besides neurons, the nervous tissue also consists of supporting cells, known as neuroglia (glial cells). These cells and their processes fill up the space between neurons.

Neuroglia provide insulation to neurons and their processes by completely enveloping them.

The size of neuroglial cells is much smaller than that of neurons. However, in the CNS, their number is 5-50 times more than that of neurons.

The neuroglial cells are capable of multiplying in mature nervous tissue. Brain tumors may have their origin in glial cells and are called gliomas.

In contrast to the nerve cells, neuroglial cells are unable to enerate or transmit the impulse.

Neuroglial cells are classified as follows:

1. Neuroglial cells of the CNS

• Ependymal cells
• Astrocytes
• Oligodendrocytes
• Microglia

2. Neuroglial cells of the PNS

• Schwann cells
• Satellite cells

Neuroglial Cells Of The CNS

Ependymal Cells

The ependymal cells are arranged in a single layer. They are cuboidal or columnar. The microvilli/cilia are present on their apical surface. Ependymal cells line the ventricles of the brain and the central canal of the spinal cord.

Function. Ependymal cells help in the formation and circulation of CSF.

Astrocytes

Astrocytes are so-called because they are star-shaped They have many star-like radiating processes.

Astrocytes are of two types: Protoplasmic astrocytes and Fibrous astrocytes.

Protoplasmic astrocytes have thick processes with abundant granular cytoplasm.

They are mainly found in grey matter. Some of these are attached to the neighboring blood capillaries by bulbous ‘end feet’ or ‘vascular feet’.

Fibrous astrocytes have long, thin, and straight processes These cells are present only in the white matter.

Astrocytes Functions. Astrocytes completely cover the brain surface and nonsynaptic regions of the neurons. The blood-brain barrier is formed by astrocytes.

Oligodendrocytes

• Oligodendrocytes are small, round, or oval cells with only a few cytoplasmic processes.
• These cells are only present in the CNS. Embryologically, oligodendrocytes develop from the neural tube.
• Functions Oligodendrocytes produce myelin sheaths around the axons of multiple neurons in the CNS.

Microglia

Microglial cells are involved in phagocytic activities within the CNS. These cells are small with few tortuous processes.

Neuroglial Cells Of The PNS

Schwann Cells

Schwann cells are flattened cells with a flattened nucleus that is surrounded by abundant cytoplasm. These cells are present in the PNS only.

Function. Schwann cells produce a myelin sheath along the axon of a neuron, in the PNS. They also participate in the regeneration of the PNS axon.

Satellite Cells

Satellite cells surround the nerve cells of the ganglia (spinal and with prominent nuclei They are present only in the PNS.

Function. Satellite cells insulate and support neurons of the ganglia (in the PNS).

The functions of glial cells.

Nervous Tissue Summary

• The nervous tissue is composed of neurons, nerve cell processes (axons and dendrites), and neuroglia.
• Neurons are highly specialized cells and carry electrical signals from one cell to another.
• These cells have a typical pale staining achromatic nucleus, dark nucleolus, and Nissl granules in the cytoplasm.
• The elongated cytoplasmic processes take origin from the nerve cell body. They are of two different types:
• Axon and Dendrites.
• Neurons usually have short, multiple, branching dendrites. They receive information from other cells and usually have only one extremely long axon.
• Axons send information away from the nerve cell body to other cells.
• The proteins (neurotransmitters) produced in the cell are transported to the distal region of the axon and back. This is known as axonal transport.
• The classification of neurons is based on their structure (multipolar, bipolar, etc.), their size (Golgi types 1 and 2), and their functions (sensory neurons, motor neurons, and interneurons).
• Neuroglia (glial cells) are supporting cells of nervous tissue. They also provide insulation to neurons (satellite cells)
  and their processes (oligodendrocytes and Schwann cells).
• Ependymal cells help in the formation and circulation of the cerebrospinal fluid. The blood-brain barrier is formed by astrocytes while microglia are involved in phagocytic activities within the CNS.

Multiple Choice Questions

Question 1. Which of the following statements is false?

• The neuron is specialized to carry information in the form of electrical signals
• Dendrites are multiple, short processes while an axon is a single, long process
• The cytoplasm of neurons contains prominent eosinophilic material called Nissl bodies
• The nucleus of a neuron is euchromatic

Answer: 3. Cytoplasm of neurons contains prominent eosinophilic material called Nissl bodies

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

1. Axon begins at Axon Hillock
2. The cytoplasm of the axon (axoplasm) contains microfilaments and microtubules
3. Axon sends electrical signals (impulses) in one direction only, i.e. away from the nerve cell body
4. Axonal cytoplasm (axoplasm) carries proteins from the cell body towards the distal end of the axon, i.e. in one direction only.

Answer: 4. Axonal cytoplasm (axoplasm) carries proteins from the cell body towards the distal end of the axon, i.e. in one direction only.

Question 3. Which ofthe following statements about Nissl substance is false?

1. It has an acidic component
2. Nissl bodies are aggregations of rough endoplasmic reticulum
3. Nissl bodies are also the aggregation of ribosomes
4. Axons are free ofNissl granules but dendrites contain Nissl granules
5. None of the above

Answer: 5. None of the above

Question 4. Which of the following statements about the transport of tetanus toxin is true?

1. By retrograde axonal transport
2. By anterograde axonal transport
3. By transport through endoneurial space
4. None of the above

Answer: 3. By transport through endoneurial space

Question 5. Which of the following statements about the location of bipolar neurons is false?

1. Neurons in the inferior ganglion of the vagus
2. Neurons in retina
3. Neurons ofvestibular and spinal ganglion
4. Neurons olfactory neuroepithelium

Answer: 1. Neurons in the inferior ganglion of the vagus

Question 6. Which kind of neurons is found in the dorsal root ganglion of spinal nerves?

1. Multipolar
2. Bipolar
3. Unipolar or pseudo-unipolar
4. Anaxonic

Answer: 3. Unipolar or pseudo-unipolar

Question 7. Which of the following cells is unable to propagate nerve impulses?

1. Sensory neurons
2. Motor neurons
3. Neuroglia
4. Interneurons
5. Golgi type2 neurons

Answer: 3. Neuroglia

Question 8. Which of the following neuroglial cells is not present in the CNS?

1. Ependymal cell
2. Fibrous astrocytes
3. Oligodendrocytes
4. Satellite cells
5. Microglial cells

Answer: 4. Microglial cells

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

They secrete growth stimulants

1. They cover the brain surface and the surface of neurons in the ganglia
2. They are metabolic neurotransmitters
3. They help in the formation of the blood-brain barrier
4. They form scar tissue after brain damage

Answer: 2. They cover the brain surface and the surface of neurons in the ganglia

Question 10. Which of the following statements about Schwann cells are true?

• They are present in the peripheral nerve system only
• Embryologically, they are derived from neural crest cells
• They form a neurilemmal sheath around axons in the PNS
• They help in the regeneration of nerve fibers
• All of the above

Answer: 5. All of the above

Spinal Cord

The spinal cord is a long, cylindrical structure present in the upper two-thirds of the vertebral canal.

Within the vertebral canal, the spinal cord is well protected by bony vertebrae, meninges, and cerebrospinal fluid (CSF).

External Structure Of The Spinal Cord

The spinal cord extends from the level of the upper border of the first cervical vertebra up to the lower border of the first lumbar vertebra.

The length of the cord varies from 42 to 45 cm and its weight is about 30 grains. The diameter of the spinal cord is minimum in the thoracic region, about 2 cm.

The gross anatomical features that are examined externally are as follows:

1. Enlargements
2. Conus medullaris
3. Filum terminal
4. Fissure and sulci
5. Spinal nerves
6. Spinal cord segments

Enlargements

The spinal cord shows the fusiform enlargement in the cervical and lumbosacral regions. These enlargements are due to the collection of a large number of spinal neurons for the nerve supply of the upper and lower limbs.

Therefore, these enlargements are also sometimes called limb enlargements, i.e. cervical and lumbosacral enlargements.

The cervical enlargement extends from C4 to T2 spinal cord segments.

The ventral rami of C5 to T1 spinal nerves form the brachial plexuses, which supply the upper limb.

The lumbosacral enlargement extends front LI to S3 spinal cord segments.

The ventral rami of the 1.2 to S3 spinal cord segment forms the lumbosacral plexus to supply the lower limb.

Conus Medullaris

Just below the lumbosacral enlargement, the lower end of the spinal cord is tapering and is known as conus medullaris.

Filum Terminale

The Filum terminal is a slender filament, which extends from the tip of the conus medullaris to the dorsum of the first coccygeal vertebra.

Filum terminale lies in the middle of cauda equina. The cauda equine is a bunch of spinal nerves around the filum terminale. Cauda equine consists of lumbar, sacral, and coccygeal spinal nerves.

Fissure and Sulci

On the anterior aspect of the spinal cord, there is an anterior median fissure, which extends throughout its length. On either side of this fissure, there is the presence of shallow anterolateral sulci.

Similarly, on the posterior aspect of the cord, a single median posteromedian sulcus and two posterolateral sulcus, one lying on each side of the posteromedian sulcus, are present. This fissure and sulci can be better appreciated in the transverse section of the spinal cord.

Spinal Nerves

Thirty-one pairs of spinal nerves are attached to the spinal cord. These are grouped into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

Spinal Cord Segments

Based on the origin of the 31 pairs of spinal nerves, the spinal cord is also divided into 31 spinal segments.

The spinal segment is the portion of the spinal cord to which a pair of dorsal and ventral spinal roots is attached.

The spinal cord is divided into the following segments; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Spinal Cord Segments and Vertebral Levels

Students should note that the spinal cord segments do not
correspond to the vertebral column segments in adults.

However, in early fetal age, there is a perfect correlation between spinal cord segments and vertebral segments. However, after the early fetal age, the vertebral column grows at a much faster rate as compared to the growth of the spinal cord.

Therefore, the lengthening of spinal nerve roots between the spinal cord and the intervertebral foramen occurs. This lengthening of spinal nerves is more marked in lumbar and sacral spinal roots.

The spinal nerves at the lower level (lower thoracic and lumbar) have to pass obliquely downwards for a considerable distance to get out of their respective foramina.

Thus, the nerve roots from the lumbosacral region (conus medullaris) emerge in the form of a bunch.

These nerve roots surround the filum terminale and are known as cauda equina (resembling the tail of the horse).

As there are seven cervical vertebrae and eight pairs of cervical spinal nerves, spinal nerves from Cl to C7 come out above the corresponding vertebrae while spinal nerve C8 comes out between C7 and T1 vertebrae.

The T1 spinal nerve and all others below this level come out below their corresponding vertebrae.

Internal Structure Of The Spinal Cord

By studying the transverse section of the spinal cord, one can learn the basics of its internal structure. The transverse section of the spinal cord shows that it is composed of grey and white matter.

Grey matter is placed centrally and white matter is on the periphery. Grey matter is composed of cell bodies (neurons) and their processes while white matter is composed of myelinated and nonmyelinated nerve fibers.

Grey Matter

The grey matter of the spinal cord, on each side, is subdivided into anterior (ventral), posterior (dorsal), and lateral grey columns (horns).

The anterior, posterior, and lateral grey columns (horns) lie anterior, posterior, and lateral to the transverse band of grey commissure.

Anterior (Ventral) Grey Columns (Horns)

• The anterior grey columns contain cell bodies of motor neurons (alpha and gamma) and Renshaw cells. The Renshaw cell is a type of interneuron.
• The cells of this column give origin to axons, which form the ventral root of the spinal nerve. These fibers carry nerve impulses for the contraction of skeletal muscles.

Spinal Cord Function And Structure

Posterior (Dorsal) Grey Columns

The posterior grey columns are directed backward and laterally toward the surface. This column contains somatic and autonomic sensory neurons and receives the central processes of pseudo-unipolar cells of the dorsal root ganglion.

The posterior grey column gives origin to the axons, which form ascending (sensory) tracts of the white matter. Conventionally, the dorsal horn is divided dorsoventrally into the apex, body, and base.

Lateral Grey Columns

The lateral grey columns are present between the anterior and the posterior grey columns on each side.

These columns extend from Tl to L2 and S2 to S4 spinal segments.

They contain the preganglionic autonomic motor neurons that carry nerve impulses for the regulation of the activities of the smooth muscle, cardiac muscle, and glands (see in the following text).

Nuclei in the Grey Matter

The H-shaped grey matter of the spinal cord consists of neurons of various sizes and shapes. These nerve cells are organized in different groups and each group extends longitudinally within the grey matter of the spinal cord.

It is due to this reason that each nerve cell group is called a column or nucleus.

Nuclei in the anterior grey column (ventral horn): The nerve cell groups in the anterior column or ventral horn are motor nuclei.

These are arranged in three groups:

1. Medial,
2. Lateral and
3. Central.

Medial group: This group is present throughout the length of the spinal cord.

Spinal Cord Function And Structure

It is subdivided into two groups:

Ventromedial and Dorsomedial. The ventromedial group supplies the muscles of the back while the dorsomedial group supplies anterolaterally situated trunk muscles.

Central group: This group is present in the cervical and lumbosacral segments of the spinal cord.

It consists of three nuclei:

• Phrenic (between C3 and C5 spinal segments),
• Spinal accessory (between Cl and C6 segments) and
• Lumbosacral nuclei (between L2 and SI spinal segments).

Lateral group: This group is present only in cervical and lumbar enlargements of the cord as it supplies the muscles of the upper and lower limbs.

It is subdivided into three subnuclei:

1. Ventrolateral,
2. Dorsolateral and
3. Retrodorsolateral

Nuclei in the lateral grey column: The lateral grey 3. column, or the intermediate nucleus, is present between the anterior and posterior grey columns on either side of the central canal.

• A distinct group of nerve cells is present in the lateral grey column between the T1 and L2 spinal segments.
• The axons of these cells give rise to preganglionic sympathetic motor fibers, which travel in the ventral spinal root.
• A similar less distinct group is also seen between S2 and S4 spinal segments. The axons of these cells give origin to preganglionic parasympathetic motor fibers.

Spinal Cord Function And Structure

Nuclei in the posterior grey column (dorsal horn): This group consists of sensory nerve cells.

The sensory neurons of the dorsal grey column are much smaller in size as compared to the motor neurons of the anterior grey column.

A brief description of these groups is arranged in the dorsoventral direction.

Marginal nucleus: It is a thin layer of small neurons situated dorsal to substantia gelatinosa at the apex of the dorsal grey column.

Substantia gelatinosa: Substantia gelatinosa cells are present near the apex of the posterior grey column.

This group extends throughout the spinal cord. Substantia gelatinosa is continuous above (in the medulla and lower pons) with the nucleus of the spinal tract of the trigeminal nerve.

Nucleus proprius: It (magnocellular nucleus) is a large nucleus of the posterior grey column. This nucleus extends throughout the spinal cord.

Dorsal nucleus (Clarke’s column): It (thoracic nucleus or Clarke’s column) extends from C8 to L4 spinal segments. It is present at the base of the posterior grey column.

Intermediomedial nucleus: It (visceral afferent) is situated ventral to the dorsal nucleus at the base of the dorsal grey column lateral to the central canal.

It extends throughout the entire spinal cord. Probably, it receives sensory impulses from the viscera.

Laminar Architecture of the Spinal Grey Matter

• The grey matter of the spinal cord may be divided into 10 laminae (layers) based on the cytoarchitectonic organization (i.e. shape, size, and packing density of neurons).
• Though Rexed studied these laminae in the spinal cord of cats, however, it is believed that a similar lamination exists in all mammals including man.
• These laminae are designated by Roman numerals I to X. Laminae I to IX are designated starting from the tip of the posterior horn and moving towards the ventral horn.
• Laminae I to 4 occupy the dorsal grey horn, lamina VII occupies the intermediate grey matter (between dorsal and ventral grey horns) while laminae 8 and 9 are present in the ventral grey horn. Lamina X is the grey matter surrounding the central canal.
• The nuclei of the ventral, dorsal, and lateral grey horns are summarised.

White Matter

The white matter of the spinal cord is organized into three broad areas called funiculi (or columns).

These are anterior (ventral) funiculus, lateral funiculus and dorsal (posterior) funiculus.

Each funiculus consists of distinct tracts (a tract is defined as bundles of nerve axons having a common origin, termination, and carrying similar impulses).

• In the white matter of the spinal cord, tracts are mainly of three types. These are sensory or ascending tracts, motor or descending tracts, and intersegmental tracts.
• Sensory (ascending) tracts carry sensory impulses from the spinal cord to the brain, for example spinothalamic tract.
• The descending (motor) tracts carry motor impulses from the brain to the spinal cord, for example, pyramidal and extrapyramidal tracts.

The intersegmental tracts run for short distances; that is, they remain confined within a few segments of the spinal cord.

They connect the neurons within segments or of different segmental levels.

Dorsal Funiculus

• The dorsal (posterior) funiculus is bounded medially by the posteromedian septum and laterally by the dorsal grey column. It consists of medially placed gracile and laterally placed cuneate fasciculi.
• The gracile fasciculus extends throughout the length of the spinal cord but the cuneate fasciculus is present only above the level of the midthoracic region. Both the gracile and the cuneate asciculi are sensory.

Lateral Funiculus

The lateral funiculus is present between dorsal and ventral grey columns (between ventral spinal roots and posterolateral sulcus). It consists of both motor (descending) and sensory (ascending) tracts.

Ventral Funiculus

The ventral (anterior) funiculus is present between the anterior median fissure and the lateral-most fibers of the ventral roots.

Axons decussate in the ventral white commissure. It also consists of both motor (descending) and sensory (ascending) tracts. Enumerate the ascending and descending tracts in various funiculi of the spinal cord.

Regional Differences In The Arrangement Of White And Grey Matter

• Some regional differences are observed when transverse sections of the spinal cord are examined in different regions.
• The amount of white matter gradually increases from the sacral to the cervical region (below upwards) of the spinal cord. The white matter is the maximum in the cervical part of the spinal cord.
• The amount of grey matter is much higher in the cervical and lumbosacral regions (enlargements). The neurons of these enlargements are responsible for the innervation of limbs.
• The grey matter is much less in the thoracic region than at each segmental level. It mainly innervates the intercostal space only.

Spinal Cord Function And Structure

Formation Of A Typical Spinal Nerve

A typical spinal nerve arises from the spinal cord in the form of two roots: Ventral and Dorsal.

Ventral Root

The ventral root (anterior root) contains bundles of efferent (motor) fibers, which arise from the motor neurons situated in the anterior grey column. These fibers supply the skeletal muscles.

Dorsal Root

• The dorsal root (posterior root) contains bundles of afferent (sensory) fibers.
• These fibers arise from the nerve cells present in the dorsal root ganglion. The dorsal root ganglion cells are pseudo-unipolar.
• ‘The cell body of these neurons gives a single process, which divides into central and peripheral processes.
• Hie peripheral process goes peripherally through the spinal nerve to supply the tissues while the central process passes through the dorsal root to the dorsal grey column of the spinal cord.

The trunk of the Spinal Nerve

The dorsal and ventral nerve roots unite to form the trunk of the spinal nerve. Within the trunk of the spinal nerve, both sensory and motor fibers are present.

Branches

The trunk of the spinal nerve immediately divides into two rami: dorsal and ventral. These rami soon come out of the intervertebral foramen. Both the rami contain mixed fibers (motor and sensory).

Spinal Cord Function And Structure

Ventral Ramus

• The ventral ramus and its branches supply motor and sensory nerve fibers to the anterior and lateral regions of the trunk and the upper and lower limbs. The ventral rami is larger than dorsal rami.
• They supply the limbs and anterolateral aspect of the trunk (thorax and abdomen). In the thoracic region, ventral rami are known as intercostal nerves.
• They run independently and retain their segmental innervations. However, the ventral rami of the cervical, lumbar, and sacral regions form nerve plexuses by uniting with each other. In the cervical region, they form cervical and brachial plexuses.
• The brachial plexus supplies the muscles of the upper limb. The lumbar and sacral ventral rami form lumbosacral plexuses which supply the muscles of the lower limb.

Dorsal Ramus

The dorsal ramus and its branches carry motor fibers to the deep muscles of the back and sensory fibers to the overlying skin. The dorsal rami of spinal nerves supply the dorsal aspect of the body.

They are usually smaller than the ventral rami. Similar to thoracic ventral rami (intercostal nerves), dorsal rami also have segmental distribution.

Spinal Cord Function And Structure

The dorsal rami do not form nerve plexuses. A dorsal ramus usually divides into medial and lateral branches and supplies muscles and skin of the posterior region of the neck and trunk.

Spinal Cord Summary

• The spinal cord is a long cylindrical structure present in the upper two-thirds of the vertebral canal. The vertebral canal is well protected by bony vertebrae, meninges (dura, arachnoid, and pia), and a cushion of cerebrospinal fluid.
• The upper end of the spinal cord lies at the level of the upper border of the first cervical vertebra while the lower end lies at the lower border of the LI vertebra. The length of the cord varies from 42 to 45 cm.
• On the external surface of the spinal cord, we get the following fissures and sulci: one anterior median fissure, two anterolateral sulci, one posteromedian sulcus, and two posterolateral sulci.
• The spinal cord shows the fusiform enlargement in the cervical and lumbosacral regions. Just below the lumbosacral enlargement, the lower end of the spinal cord is tapering and is known as conus medullaris.
• Filum terminate is a slender filament, which extends between the tip of the conus medullaris and the dorsum of the first coccygeal vertebra.
• Thirty-one pairs of spinal nerves are attached to the spinal cord, i.e. 8 cervical, 1 2 thoracics, 5 lumbar, 5 sacral, and 1 coccygeal.
• The transverse section of the spinal cord shows that it is composed of grey and white matter. The grey matter is placed centrally and the white matter is on the periphery.
• The grey matter of the spinal cord, on each side, is subdivided into anterior (ventral), posterior (dorsal), and lateral grey columns (horns).
• Various nerve cell nuclei in the spinal grey columns (horns) are presented,
• The white matter of the spinal cord is organized into three broad areas called funiculi (or columns). These are anterior (ventral) funiculus, lateral funiculus and dorsal (posterior) funiculus.
• Each funiculus consists of distinct tracts, i.e. sensory or ascending tract, motor or descending tract, and intersegmental tract.

Spinal Cord Multiple Choice Questions

Question 1. The spinal cord extends from the upper border of the atlas vertebra to

1. The lower border of S2
2. Lower border of
3. Lower border ofL3
4. Lower border ofL2

Answer: 2. Lower border of

Question 2. At birth, the spinal cord usually lies at the level of

1. The lower border of S2
2. The lower border of SI
3. The lower border of LI
4. The lower border of L3

Answer: 4. Lower border of L3

Question 3. Which ofthe following statements about the spinal cord is false?

1. The anteromedian sulcus is present in the midline anteriorly
2. Anterolateral sulci are present on either side of the midline
3. Posterolateral sulci are present on either side of the posteromedian sulcus
4. Posteromedian sulcus is present in the midline posteriorly

Answer: 1. Anteromedian sulcus is present in the midline anteriorly

Question 4. Which ofthe following statements about filum terminale is false?

1. It extends from the tip of the conus medullaris to the first coccygeal vertebra
2. It consists of pia mater surrounding the neuroglial element
3. It is a vestigial structure
4. It lies above Cauda equine

Answer: 4. It lies above cauda equine

Question 5. Which ofthe following statements about spinal nerves is false?

1. There are 33 pairs of spinal nerves
2. There are 7 pairs of cervical spinal nerves
3. There are 3 pairs of coccygeal nerves
4. All of the above

Answer: 4. All of the above

Question 6. All of the following statements are correct regarding the grey matter of the spinal cord except

1. The anterior grey horn contains cell bodies of motor neurons
2. The posterior grey horn contains cell bodies of sensory neurons
3. The lateral grey horn contains postganglionic autonomic motor neurons
4. The grey matter of the spinal cord contains interneuron

Answer: 3. Lateral grey horn contains postganglionic autonomic motor neurons

Question 7. Which nerve cell group is not present in a large part of the posterior grey column?

1. Marginal nucleus
2. Substantia gelatinosa
3. Nucleus properties
4. Clarke’s column
5. Interomediolateral nucleus

Answer: 5. Interomediolateral nucleus

Question 8. Which is not the major ascending tract of the spinal cord?

1. Ventral spinothalamic tract
2. Pyramidal tract
3. Fasciculus gracilis
4. Lateral spinothalamic

Answer: 2. Pyramidal tract

Question 9. Sensation of touch is carried by

1. Lateral spinothalamic tract
2. Anterior spinothalamic tract
3. Posterior column tracts
4. Anterior spinothalamic and posterior column tract

Answer: 4. Anterior spinothalamic and posterior column tract.

Central Nervous System

The central nervous system (CNS) consists of the brain and the spinal cord, which are made up of nervous tissues (nerve cell bodies, axons and dendrites, neuroglial cells, and blood vessels.

The surface of the brain is covered with a thin layer of grey matter.

This grey matter is called the cortex. However, many areas of grey matter are also present deep within the white matter of the brain. These areas are called ‘nuclei’.

In the spinal cord, grey matter is situated centrally and is ‘H’ shaped. The grey matter is surrounded by the white matter.

The grey matter shows two ventral (anterior) grey columns (horns) that contain motor neurons.

The dorsal grey columns (horns) contain sensory neurons. In the thoracic and upper lumbar regions, the grey matter also exhibits a lateral horn containing preganglionic autonomic neurons.

The white matter of the spinal cord is the collection of nerve fiber bundles, which are known as tracts. There are some ascending and descending tracts.

Central Nervous System Function

Both the brain and the spinal cord are surrounded by three membranes called meninges, namely the pia mater, arachnoid mater, and dura mater. Pia mater is

A thin delicate membrane, which intimately covers the outer surface of the brain and the spinal cord.

The space between the pia and the arachnoid mater is filled with cerebrospinal fluid (CSF). Blood vessels also run in subarachnoid space.

Central Nervous System Function

The dura mater is the outermost membrane that is very tough. It is fused with the bones of the cranial cavity (brain box).

Brain

The brain lies in the cranial cavity and is continuous with the spinal cord just below the level of the foramen magnum.

The brain, in the ascending order, is divided into three principal parts:

1. Hindbrain
2. Midbrain
3. Forebrain

The medulla, pons, and midbrain are called brainstems. The midbrain is a small part, which connects the forebrain to the hindbrain. Posterior to the brainstem lies the cerebellum. The forebrain lies superior to the brainstem.

Hindbrain

The hindbrain consists of the medulla oblongata, pons, and cerebellum.

Medulla Oblongata

• Medulla oblongata is also known as medulla. It is conical in shape and continuous below the spinal cord. (just below the foramen magnum) and above with the pons.
• On the ventral aspect, there is the presence of an anterior median fissure, which divides it into right and left halves.
• On either side of the fissure is a ridge called a pyramid. Posterolateral to the pyramid is an oval swelling, known as the olive.
• Posterolateral to olives are inferior cerebellar peduncles, which contain the white fibers and connect the medulla to the cerebellum.
• The lower part of the medulla contains the central canal, which is continuous below the central canal of the spinal cord.
• The central canal opens above in the fourth ventricle, which is situated posterior to the upper part of the medulla and pons.

Pons

• Pons lies above the medulla and below the midbrain. On the ventral aspect, it forms a bulge that is convex from side to side and above downwards.
• On each side, the pons are continuous with the middle cerebral peduncle, which extends posteriorly into the cerebellum.

Central Nervous System Function

Cerebellum

• The cerebellum overlaps the posterior surfaces of the brainstem, i.e. midbrain, pons, and medulla.
• It lies in posterior cranial fossa inferior to a horizontal fold of dura matter (tentorium cerebelli).
• The surface of the cerebellum is covered by grey matter which shows a large number of closely set transverse fissures.

Midbrain

• The midbrain is the short segment of the brainstem situated above the pons.
• It joins the forebrain to the hindbrain. On the ventral aspect, it exhibits two broad white bundles of nerve fibers, the crura cerebri.
• These crura cerebri form the posterolateral boundary of a fossa called the interpeduncular fossa containing a posterior perforated substance.
• The posterior surface of the midbrain is deeply situated and overlapped by the cerebellum. It consists of four small swellings, called the colliculi (two superior and two inferior
  colliculi).
• The midbrain is traversed by a narrow canal, the cerebral aqueduct, which is connected superiorly with the third ventricle and below with the fourth ventricle.

Forebrain

The forebrain consists of the diencephalon and cerebrum.

Diencephalon (Thalamus and Hypothalamus)

• The diencephalon is deeply situated in the forebrain. Only the inferior surface of the hypothalamus is visible in the roof of the interpeduncular fossa.
• The inferior surface of the hypothalamus is formed by the tuber cinereum and mammillary bodies.
• The thalamus and hypothalamus are seen on the lateral surface of the third ventricle in the median sagittal section of the brain.
• The thalamus is situated dorsal to the hypothalamus. The third ventricular cavity is situated between the right and the left diencephalons.

Cerebrum

• The cerebrum is the largest part of the brain. The cerebrum consists of two cerebral hemispheres which are separated from each other by a midline fissure, called the longitudinal fissure.
• At the bottom of this fissure, there is the presence of the corpus callosum. The corpus callosum is a thick band of nerve fibers that connects the corresponding areas of two cerebral hemispheres (right and left).
• Each cerebral hemisphere is covered by grey matter (cortex). The surface of each hemisphere shows extensive folding to increase the surface area of the cortex.
• Thus, the cortex presents a large number of grooves (sulci) and ridges (gyri).
• Deep to the grey matter of the cortex, each hemisphere has a central core of the white matter.
• Many large collections of grey matter are present within the subcortical white matter and are referred to as basal nuclei (e.g. caudate and lentiform nuclei.
• Each cerebral hemisphere contains a large cavity, called the lateral ventricle. Each cerebral hemisphere has three surfaces: superolateral, inferior, and medial.

Ventricular System of the Brain

• The brain has large cavity-like spaces that are referred to as ventricles. The ventricles are named lateral ventricles, third ventricles, and fourth ventricles.
• The lateral ventricle is a large cavity situated in each cerebral hemisphere.
• Both the lateral ventricular cavities communicate with a median narrow cavity (third ventricle), which is situated between the right and the left diencephalons.
• The third ventricle communicates below with the fourth ventricle through the cerebral aqueduct of the midbrain.
• The fourth ventricle is continuous below with the central canal situated in the lower medulla and spinal cord.
• The ventricular system is lined with epithelium known as ependyma and at places it contains choroid plexuses which secrete CSF.

Spinal Cord

• The spinal cord is about a 45-cm long, cylindrical structure situated within the vertebral canal of the vertebral column. In adults, it extends from the medulla to the superior border of the second lumbar vertebrae.
• Within the vertebral canal, the cord is well protected by three meninges (pia, arachnoid, and dura) and CSF.
• Meninges covering the spinal cord are continuous above with the meninges covering the brain.
• The spinal cord shows two enlargements—cervical and lumbar—which give origin to cervical and lumbosacral.

Peripheral Nervous System

• The peripheral nervous system (PNS) is that part of the nervous system that is present outside the CNS (brain and spinal cord).
• This part of the nervous system is responsible for carrying the impulses to and away from the CNS.
• The PNS consists of nerve fibers and cell bodies located outside the CNS.
• The collection of cell bodies outside the CNS is called a ganglion, for example, a collection of neurons in the dorsal root ganglion, ganglion of cranial nerve, and sympathetic and parasympathetic ganglia.

Peripheral nerves take their origin from the CNS (brain and spinal cord) and are classified into cranial and spinal nerves. Ganglia are associated with cranial and spinal nerves.

1. Cranial nerves
2. Spinal nerves
3. Ganglia

A typical spinal nerve is attached to the spinal cord through ventral nerve roots (containing motor axons or fibers) and dorsal nerve roots (containing sensory fibers).

The dorsal root contains a ganglion (dorsal root ganglion) in which cell bodies of sensory neurons are situated.

Cranial Nerves

• The 12 pairs of nerves that arise from the brain are called cranial nerves.
• All the cranial nerves come out from the cranial cavity through the foramina present at the base of the skull (cranium).
• Cranial nerves innervate the head and neck and to a lesser extent the thoracic and abdominal viscera.
• Out of 12 cranial nerves, the first two (olfactory and optic) are attached to the forebrain while the remaining 10 are attached to the brainstem.
• Some cranial nerves also carry autonomic (parasympathetic) fibers.

Ganglia

Ganglia are a collection of neurons outside the CNS and are a part of the PNS as they are present in association with cranial and spinal nerves. They are of two types: Sensory and Autonomic.

Some Important Terms

The following terms will be frequently used during discussion in the book.

Grey and White Matter

A section of the fresh brain or spinal cord shows some areas as white and others as grey.

• White matter is the aggregation of myelinated processes (fibers). As the myelin is white, the collection of fibers also looks white.
• On the other hand, grey matter of the nervous system is the collection of nerve cell bodies, dendrites, unmyelinated axons starting from or ending on the nerve cells (forming a synaptic junction), and neuroglial cells.
• The grey matter looks greyish as there is no myelin in these areas. Blood vessels are present in both grey and white matter.
• The grey and white matter of the brain and spinal cord.

Cortex and Nucleus

• The thin layer of grey matter on the surface of the cerebrum and cerebellum is called the cortex. Deep in the cortex is the presence of white matter.
• A nucleus is a mass of grey matter embedded within the white matter inside the CNS (brain and spinal cord).
• The nucleus consists of an aggregation of nerve cell bodies within the CNS, which share a common function, for example, the thalamus and caudate nucleus.

Fibers in a Peripheral Nerve

A peripheral nerve is a mixed nerve containing both afferent (sensory) and efferent (motor) fibers.

Afferent Fibres

The fibers that carry sensory impulses from various parts of the body (skin and deeper structures) to the CNS are called afferent fibers (sensory nerve fibers).

Efferent Fibres

The fibers which carry impulses from the CNS to various parts of the body are called efferent fibers. Many of these fibers are connected to muscles and are called motor nerve fibers.

Summary

• The CNS consists of the brain and the spinal cord.
• The brain consists of the forebrain, midbrain, and hindbrain.
• Both the brain and spinal cord are made up of grey and white matter.

The CNS is surrounded by three meninges:

1. Pia,
2. Arachnoid and
3. Dura mater.

• The hindbrain consists of the medulla, pons, and cerebellum. The forebrain consists of the diencephalon (thalamus and hypothalamus) and cerebrum. The midbrain joins the forebrain with the hindbrain.
• Each cerebral hemisphere presents three surfaces: Superolateral,
• Medial and
• Inferior.
• Each surface is covered by cortex which shows a large number of sulci and gyri.
• Each hemisphere has a central core of white matter. Many large collections of grey matter are present within the white matter and are referred to as nuclei.
• The brain contains many large cavities called ventricles, i.e. lateral ventricles, third ventricles, and fourth ventricles.
• The PNS consists of 1 2 pairs of cranial and 31 pairs of spinal nerves.
• Sensory ganglia are a collection of nerves in association with cranial and spinal nerves.
• Autonomic ganglia are classified into sympathetic and parasympathetic.
• Sympathetic ganglia are located in the sympathetic trunk while parasympathetic ganglia are located close to the organ they supply.

 Central And Peripheral Nervous Systems Multiple Choice Questions

Question 1. Which of the following statements about the spinal cord is true?

1. The ventral grey horn contains motor neurons
2. The dorsal grey horn contains sensory neurons
3. Lateral grey horn contains preganglionic autonomic
   neurons
4. All of the above

Answer: 4. All of the above

Question 2. Which of the following statements about the grey matter of the nervous system is true?

1. It contains nerve cell bodies
2. It contains neuroglial cells
3. There is no myelin in grey matter
4. It contains blood vessels
5. All of the above

Answer: 5. All of the above

Question 3. Which of the following ganglia is a collection of nerve cell bodies?

1. Spinal root ganglion
2. Sympathetic ganglion
3. Parasympathetic ganglion
4. All of the above

Answer: 5. All of the above

Question 4. Which of the following statements is false?

1. Nucleus is the mass of grey matter in the spinal cord
2. The nucleus is the mass of grey matter in the brain
3. The nucleus is a thin layer of grey matter on the surface of the cerebellum
4. The thalamus is an example of nucleus

Answer: 3. Nucleus is a thin layer of grey matter on the surface of the cerebellum

Question 5. The collection of white matter in the CNS is known as

1. Tract
2. Funiculus
3. Lemniscus
4. All of the above

Answer: 4. All of the above

Question 6. Which are not the afferent fibers?

1. They carry the sensory impulse
2. They carry the motor impulse
3. They carry both motor and sensory impulses
4. They carry impulses from the brain to various parts of the body

Answer: 2. They carry the motor impulse

Question 7. Hindbrain consists of

1. Medulla, pons, and cerebellum
2. Midbrain, pons, and medulla
3. The midbrain, pons, medulla, and cerebellum
4. Midbrain, pons, and cerebellum

Answer: 1. Medulla, pons, and cerebellum

Question 8. Which of the following statements is true?

1. Pons is connected to the cerebellum through middle cerebellar peduncles
2. The midbrain is connected to the cerebellum through superior cerebellar peduncles
3. The medulla is connected to the cerebellum through inferior cerebellar peduncles
4. All of the above

Answer: 4. All of the above

Question 9. Diencephalon consists of

1. Telencephalon
2. Thalamus
3. Hypothalamus
4. Corpus striatum

Answer: 2. Thalamus

Question 10. The peripheral nerve consists of

1. Cranial nerves
2. Spinal nerves
3. Nerve fibers and cell bodies outside the CNS
4. All of the above

Answer: 4. All of the above

Question 11. The ganglia are ofthe following type:

1. Sensory ganglia of cranial nerves
2. Autonomic ganglia
3. Dorsal root ganglia of spinal nerves
4. Ventral root ganglia of spinal nerves

Answer: 1. Sensory ganglia of cranial nerves.

• Introduction To The Nervous System Notes
• Nervous Tissue Notes
• Overview of The Central and Peripheral Nervous Systems Notes
• Spinal Cord Notes
• Ascending and Descending Tracts of the Spinal Cord Notes
• Brainstem:Medulla Oblongata Notes
• Brainstem: Pons Notes
• Brainstem: Midbrain Notes
• Cranial Nerves: Nuclei and Functional Aspects Notes
• Reticular Formation Notes
• Cerebellum Notes
• Diencephalon – I: Thalamus, Metathalamus and Epithalamus Notes
• Diencephalon -II: Subthalamus and Hypothalamus Notes
• Cerebral Hemispheres Notes
• Functional Areas of Cerebral Cortex Notes
• White Matter of Cerebrum Notes
• Basal Nuclei Notes
• Olfactory System Notes
• Limbic System Notes
• Visual System Notes
• Auditory and Vestibular Systems Notes
• Ventricles of the Brain Notes
• Menings and Cerebrospinal Fluid Notes
• Blood Supply of The Brain and Spinal Cord Notes
• Autonomic Nervous System Notes