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Cerebral Hemispheres

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

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

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

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

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

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

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

External Features Of The Cerebral Hemisphere

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

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

Surfaces

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

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

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

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

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

Borders

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

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

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

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

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

Poles

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

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

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

Lobes Of The Cerebral Hemisphere

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

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

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

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

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

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

Sulci and Gyri of the Cerebral Hemisphere

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

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

Superolateral Surface

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

Medial Surface

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

Inferior Surface

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

Cerebral Hemispheres Summary

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

Each cerebral hemisphere consists of four lobes:

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

Cerebral Hemispheres Multiple Choice Questions

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

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

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

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

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

Answer: 4. All of the above

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

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

Answer: 2. Orbital

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

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

Answer: 5. All of the above

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

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

Answer: 4. Interparietal

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

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

Answer: 5. All of the above

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

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

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

Thalamus, Metathalamus And Epithalamus

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

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

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

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Components Of Diencephalon

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

Dorsal Diencephalon

The dorsal diencephalon consists of:

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

Ventral Diencephalon

The ventral diencephalon consists of:

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

Thalamus

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

External Features

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

Ends

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

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

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

Surfaces

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

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

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

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

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

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

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

Internal Structure

The internal structure of the thalamus consists of

• White matter and
• Grey matter

White Matter

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

Grey Matter

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

Anterior part

1. Medial part
2. Lateral part

Thalamic Nuclei

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

Anterior Part

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

Medial Part

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

Lateral Part

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

White Matter

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

Grey Matter

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

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

Thalamic Nuclei

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

Anterior Part

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

Medial Part

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

Lateral Part

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

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

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

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

Other Nuclei

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

Intralaminar Nuclei

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

Medial and Lateral Geniculate Bodies

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

Connections of Thalamus: An Overview

The thalamus has extensive afferent and efferent connections.

Afferents

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

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

Efferents

The efferents from the thalamus project to the following structures:

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

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

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

Functions of Thalamus

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

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

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

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

The thalamus serves as an integrative centre for motor functions.

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

Thalamic Syndrome

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

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

Metathalamus

The metathalamus consists of medial and lateral geniculate bodies.

Medial Geniculate Body

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

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

Lateral Geniculate Body

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

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

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

Afferents

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

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

Efferents

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

Epithalamus

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

Pineal Gland

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

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

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

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

Functions

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

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

Habenular Nucleus, Stria Medullaris Thalami and Posterior Commissure

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

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

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

Thalamus, Metathalamus And Epithalamus Summary

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

Thalamus

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

Anterior part

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

Medial and lateral geniculate bodies

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

Epithalamus

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

Thalamus, Metathalamus And Epithalamus Multiple Choice Questions

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

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

Answer: 5. All of the above

Question 2. The diencephalon consists of the following subdivisions except

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

Answer: 4. Lateral thalamus

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

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

Answer: 4. Stria terminalis

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

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

Answer: 2. Posterior part

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

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

Answer: 3. Ventral posterolateral

Question 6. Which of the following statements is true?

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

Answer: 4. All of the above

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

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

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

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

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

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

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

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

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

Diencephalon-2 Subthalamus And Hypothalamus

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

Ventral Thalamus (Subthalamus)

The ventral thalamus was previously described as the subthalamus.

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

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

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1. Zona incerta
2. Reticular nucleus

Zona Incerta

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

Reticular Nucleus

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

Hypothalamus

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

Boundaries of Hypothalamus

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

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

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

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

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

Lateral Boundary: Lterally, The hypothalamus concerning the internal capsule

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

Subdivisions of Hypothalamus

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

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

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

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

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

The hypothalamus is divided into four regions:

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

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

In the medial zone, it has three nuclei:

1. Paraventricular,
2. Anterior and
3. Suprachiasmatic

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

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

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

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

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

Nervous Connection of Hypothalamus

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

Afferent Connections

The hypothalamus receives the afferent connections from the following structure

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

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

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

Afferent from the retina: To suprachiasmatic nuclei.

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

Efferent Connections

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

Efferents to hippocampal formation: Through the medial forebrain bundle

Efferents to the amygdaloid nuclear complex: Through stria terminalis.

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

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

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

Hypothalamic Control Of The Pituitary Gland

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

Hypothalama-Hypophyseal Portal System

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

Connection of Hypothalamus with Neurohypophysis (Posterior Pituitary)

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

Functions Of Hypothalamus

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

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

Diabetes Insipidus

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

Subthalamus And Hypothalamus Summary

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

Anteroposteriorly, it is divided into four regions:

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

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

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

Multiple Choice Questions

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

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

Answer: 5. None of the above

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

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

Answer: 1. Lamina terminalis

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

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

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

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

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

Answer: 3. Infundibulotuberal

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

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

Answer: 6. All of the above

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

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

Answer: 3. Mammillothalamic tract

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

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

Answer: 2. Supraoptic nucleus

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

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

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

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

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

Answer: 5. None of the above

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

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

Answer: Control of coordinated muscular movement during walking

Cerebellum

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

External Features Of Cerebellum

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

Surfaces of Cerebellum

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

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Fissures and Lobes of Cerebellum

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

1. Primary fissure and
2. Posterolateral fissure.

Posterolateral fissure.

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

Horizontal Fissure

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

Other Fissures and Lobules

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

Cerebellar Tonsils

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

Relation of the Cerebellum To The Fourth Ventricle

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

Cerebellar Cortex

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

Cerebellar Peduncles

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

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

Phylogenetic Classification Of Cerebellum

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

Archicerebelium

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

Paleocerebellum

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

Neocerebellum

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

Functional Division Of Cerebellum

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

Internal Structure Of Cerebellum

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

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

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

Cerebellar Cortex

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

The cortex is composed of three layers:

• Molecular,
• Purkinje and
• Granular layers.

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

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

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

Cerebellar Nuclei

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

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

• Fastigial
• Globose
• Emboliform
• Dentate

The connections and functions of these nuclei.

White Matter of the Cerebellum

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

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

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

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

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

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

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

Functions Of Cerebellum

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

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

This is achieved by:

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

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

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

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

Patients show the following disorders:

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

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

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

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

Neocerebeliar Syndrome

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

This syndrome shows the following signs:

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

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

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

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

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

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

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

Nystagmus: It is present if the vermis is also involved

Cerebellum Summary

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

Multiple Choice Questions

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

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

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

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

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

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

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

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

Answer: 4. All of the above

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

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

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

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

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

Answer: 2. Vestibular

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

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

Answer: 4. Trigeminocerebellar

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

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

Answer: 4. Olivocerebellum

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

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

Answer: 2. Coordination of exteroceptive sensory impulses

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

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

Answer: 4. Purkinje cells

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

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

Answer: 2. Vertebral and basilar arteries

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

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

Answer: 2. Granule cells

Reticular Formation

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

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

Extent And Location Of Reticular Formation

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

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

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Composition Of Reticular Formation

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

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

Nuclei Of Reticular Formation

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

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

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

Some important nuclei of all three zones are given in

Connections Of Reticular Formation

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

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

Chemoarchitectonic Organisation Of Reticular Formation

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

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

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

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

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

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

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

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

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

Functions Of Reticular Formation

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

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

Specifically, the functions of reticular formation are as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Sleep and wakefulness

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

Wakefulness:

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

ARAS:

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

Coma

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

Summary

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

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

1. Median,
2. Medial and Lateral.

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

Reticular Formation Multiple Choice Questions

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

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

Answer: 5. None of the above

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

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

Answer: 5. None of the above

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

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

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

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

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

Answer: 4. Posterolateral zone

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

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

Answer: 3. Nucleus raphe magnus

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

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

Answer: 4. All of the above

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

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

Answer: 5. All of the above

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

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

Answer: 3. Dorsomedial reticulospinal tract

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

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

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

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.

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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

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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.

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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.

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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.

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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.

Posterior Spinocerebellar Tract 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.

Posterior Spinocerebellar Tract First-Order Neurons

These are located in the DRG of the spinal nerves

Posterior Spinocerebellar Tract 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.