Textbook of Clinical Neuroanatomy, 2 ed.

11. Diencephalon and Third Ventricle

Diencephalon

The diencephalon is the part of brain between the cerebrum and the brainstem. The cavity within it is termed third ventricle.

The diencephalon comprises two major subdivisions: pars dorsalis and pars ventralis. These subdivisions are seen on midsagittal view of the brain, and are separated from each other by a shallow groove, the hypothalamic sulcus which extends from interventricular foramen to the rostral end of the cerebral aqueduct of midbrain (Fig. 11.10).

• Pars dorsalis lies above (dorsal) to the hypothalamic sulcus and consists of: (a) thalamus, (b) metathalamus which includes the medial and lateral geniculate bodies, and (c) epithalamus which consists of pineal body (gland), habenular nuclei and commissure, posterior commissure and the stria medullaris thalami.

• Pars ventralis lies below (ventral) to the hypothalamic sulcus and consists of: (a) subthalamus, and (b) hypothalamus.

The main divisions and subdivisions of diencephalon are listed in Table 11.1.

Table 11.1

Divisions and subdivisions of the diencephalon

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Thalamus

Anatomically the thalamus is a large ovoid mass of grey matter laying above the midbrain, from which it is separated by a small amount of neural tissue, the subthalamus. There are two thalami situated one on each side of a slitlike cavity, the third ventricle (Fig. 11.1).

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FIG. 11.1 The thalami and third ventricle as seen from above, after removal of overlying tela choroidea. (AC = anterior commissure, PC = posterior commissure.)

Each thalamus is 3.5 cm in length and 1.5 cm in breadth.

The long axes of the thalami are set obliquely running backwards and laterally. The pointed anterior ends are nearer to the median plane whereas the wider posterior ends are separated from each other by pineal body, superior colliculi and habenular triangles. The thalami are usually attached across the median plane by a narrow interthalamic connexus of grey matter (also called interthalamic adhesion). Each thalamus forms most of the lateral wall of the third ventricle and floor of the central part of the lateral ventricle.

Functionally, the thalamus is generally considered as the greatsensory gateway to the cerebral cortex. It receives impulses from the opposite half of the body and transmits most of them to the sensory area of the cerebral cortex (Brodmann areas 3, 2, and 1).

External features

Each thalamus has two ends and four surfaces.

Ends

• The anterior end is narrow and constitutes the tubercle of thalamus. It forms the posterior boundary of the interventricular foramen.

• The posterior end is expanded and is known as pulvinar. It overhangs the medial and lateral geniculate bodies, and superior colliculi with their brachia.

Surfaces (Fig. 11.2)

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FIG. 11.2 A coronal section of the brain showing relations of the thalamus.

• Superior surface. Its lateral part forms the floor of the central part of the lateral ventricle and its medial part is covered by the tela choroidea of the third ventricle.

• Inferior surface. Its anterior part is fused with the sub-thalamus while its posterior part is free, forming the inferior aspect of the pulvinar.

• Medial surface. It forms the greater part of the lateral wall of the third ventricle.

• Lateral surface. It forms the medial boundary of the posterior limb of internal capsule.

N.B. A linear ridge of white fibres along the junction of medial and super surfaces is termed stria medullaris thai-ami or habenular stria.

A line of reflection of ependyma of third ventricle from its medial wall to its roof is termed taenia thalami.

Internal structure (Fig. 11.3)

The thalamus consists mainly of grey matter and only a small amount of white matter.

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FIG. 11.3 Horizontal section of the thalamus (schematic) to show the location of various thalamic nuclei. The diagram in the inset is the coronal section of thalamus passing in front of pulvinar showing ventral posteromedial (VPM), ventral posterolateral (VPL) nuclei, and centromedian nucleus. (P = pulvinar, LD = lateral dorsal nucleus, LP = lateral posterior nucleus, VA = ventral anterior nucleus, VL = ventral lateral nucleus, VPL = ventral posterolateral nucleus, MN = Medio- dorsal nucleus.)

White matter

The lateral surface of the thalamus is covered by a thin layer of white matter called external medullary lamina and its superior surface by a similar layer of white matter called stratum zonale.

A vertical Y-shaped sheet of white matter within the thalamus is called internal medullary lamina.

Grey matter

The thalamic grey matter consists of number of nuclei (Fig. 11.3).

The grey matter of the thalamus is traversed anteropos-teriorly by a vertical sheet of white fibres, the internal medullary lamina which bifurcates anteriorly to assume a Y-shaped configuration. This Y-shaped internal medullary lamina divides the thalamus into three main parts: anterior, medial, and lateral.

The anterior part includes the anterior tubercle and lies between the ‘limbs’ of the Y, the medial and lateral parts lie on either side of the ‘stem’ of the Y. Each of these parts consists of number of nuclei.

Thalamic nuclei (Fig. 11.3)

Nuclei in the anterior part

The nuclei in this part are collectively referred to as anterior nucleus.

Nuclei in the medial part

Nuclei in medial part consist of a large medial dorsal nucleus and a small medial ventral nucleus.

Nuclei in the lateral part

Nuclei in the lateral part are divided into dorsal and ventral parts.

The dorsal part is subdivided craniocaudally into 3 nuclei: (1) lateral dorsal (LD), (2) lateral posterior (LP), and (3) a large caudal nuclear mass, the pulvinar (P). These nuclei are termed as dorsal tier of nuclei.

The ventral part is also subdivided craniocaudally into 3 nuclei: (1) ventral anterior (VA), (2) ventral lateral (VL) or ventral intermediate (VI), and (3) ventral posterior (VP). These nuclei are termed as ventral tier of nuclei.

The ventral posterior nucleus (VP) is further subdivided into a lateral part, the ventral posterolateral nucleus (VPL) and a medial part, the ventral posteromedial nucleus (VPM).

Other thalamic nuclei

In addition to the above mentioned nuclei, the thalamus consists of following other nuclei:

• Intralaminar nuclei. They are several in numbers and are embedded in the internal medullary lamina. The largest and most important of these is termed centrome-dian nucleus.

• Midline (paraventricular) nuclei. They consist of scattered cells that lie between the medial part of the thala-mus and the ependyma of the third ventricle.

• Reticular nucleus. It is a thin curved sheet of grey matter on the lateral aspect of the thalamus from which it is separated by the external medullary lamina.

• Medial and lateral geniculate bodies. These are located posteroventral to the pulvinar. Conventionally these nuclei are described under metathalamus, but nowadays they are considered as thalamic nuclei.

The thalamic nuclei are summarized in Table 11.2.

Table 11.2

Nuclei in different parts of the thalamus

Part

Nuclei

Anterior part

Anterior nucleus

Medial part

Medial dorsal nucleus, medial ventral nucleus

Lateral part

 

• Dorsal tier nuclei

Lateral dorsal, lateral posterior, pulvinar

• Ventral tier nuclei

Ventral anterior (VA), ventral lateral (VL), ventral posterior—(a) ventral posterolateral (VPL), (b) ventral posteromedial (VPM)

Other parts

Intralaminar nuclei, reticular nucleus, medial and lateral geniculate bodies

The thalamic nuclei are classified into three main functional groups: specific, nonspecific, and reticular.

Connections of thalamic nuclei (Figs 11.411.5)

Connections of the specific nuclei

These nuclei receive input from certain ascending tracts and project it to the specific (primary) cortical areas. Nuclei of this group comprise ventral tier nuclei and medial and lateral geniculate bodies. Their connections are enumerated in Table 11.3.

Table 11.3

Connections of the specific thalamic nuclei

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FIG. 11.4 Main connections of the thalamus. The afferent fibres are shown on the left side and the efferent, fibres on the right side. (NG = nucleus gracilis, NC = nucleus cuneatus.)

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FIG. 11.5 Scheme to show connections of ventral and lateral groups of thalamic nuclei. The figure inset on left upper corner shows the dorsolateral view of thalamus and its major subdivisions. (LD = lateral dorsal nucleus, LP = lateral posterior nucleus, VA = ventral anterior nucleus, VI = ventral intermediate N, VPL = ventral posterolateral n. VPM = ventral posteromedial n.)

Clinical Correlation

• From a clinical point of view the connections of ventral posterior nucleus are most important because its smaller medial portion, the ventral posteromedial nucleus (VPM) receives general sensory modalities from the head and face through trigeminal lemniscus and taste sensations from taste buds through solitariothalamic tract; and its larger lateral portion, the ventral posterolateral nucleus (VPL) receives exteroceptive sensations (pain, touch and temperature) through spinal lemniscus and proprioceptive sensations (muscle and joint sense, vibration, two point discrimination) through medial lemniscus, from rest of the body except face and head.

  All the sensations reaching the ventral posterior nucleus are carried to the primary sensory area of the cerebral cortex by fibres passing through the posterior limb of the internal capsule (superior thalamic radiation). The vascular lesions involving posterior limb of internal capsule, which are not uncommon cause impairment of all forms of sensibility on the opposite side of the body.

• The integrity of anterior nucleus and its connections is necessary for attention and recent memory, therefore a lesion involving them can lead to loss of recent memory.

• Since the medial dorsal nucleus is associated with “moods” (“feeling tone”) and emotional balance, depending on the nature of the present sensory input and past experience, the mood may be that of well-being or malaise, or of euphoria or mild depression.

Connections of the nonspecific nuclei

Nonspecific nuclei do not receive afferents from ascending tracts, but have abundant connections with other diencephalic nuclei. They mostly project to the cortical ‘association areas’ in the frontal and parietal lobes. Nuclei of this group comprise anterior nucleus, dorsal medial nucleus and dorsal tier nuclei of thalamus. Their connections are enumerated in Table 11.4.

Table 11.4

Connections of the nonspecific thalamic nuclei

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Connections of the reticular nuclei

The reticular nuclei of thalamus include reticular nucleus, intralaminar nuclei and median nuclei. These nuclei are connected with the reticular formation. There connections are given in Table 11.5.

Table 11.5

Connections of the reticular nuclei of thalamus

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Functions of thalamus

• It is a sensory integration and relay station of all the sensory pathways except for the olfactory pathway, which is projected directly to the cerebral cortex without being relayed in the thalamus.

• It is capable of recognition, though poorly of the pain, thermal and some tactile sensations at its own level.

• It influences voluntary movements by receiving impulses from basal ganglia and cerebellum and relaying them to the motor cortex, which in turn influences lower motor neurons through corticonuclear and corticospi-nal pathways.

• Through ascending reticular activating system, the thal-amic reticular component participates in the maintenance of the state of wakefulness and alertness.

• By receiving impulses from hypothalamus and projecting them to the prefrontal and cingulate gyri, it participates in affective reactions, viz. determination of mood.

• It is thought to have role in recent memory and emotions.

• It influences the electrical activity of the cerebral cortex, i.e. it plays a role in synchronization or desynchroniza-tion of EEG waves.

Clinical Correlation

• Thalamic syndrome

  It usually occurs subsequent to a vascular lesion of the thalamus (viz. thrombosis of thalamogenicu-late artery). when the patient is recovering from a thalamic infarct.

  Characteristic features

  The threshold for pain, touch and temperature is decreased on the opposite side of the body (thalamic overreaction) but, when the threshold is reached, the sensations are exaggerated, perverted and disagreeable. For example, the prick of a pin may be felt as a severe burning sensation, and even music that is ordinarily pleasing may be disagreeable. Sometimes even light touch may produce excruciating pain. The spontaneous pain may occur in some instances that may become intractable and fail to respond to powerful analgesics (pain relieving) drugs.

  There may be emotional instability, with spontaneous (or forced) laughing and crying.

• Thalamic hand

  It is sometimes seen in thalamic lesions. The opposite hand is held in an abnormal posture. The forearm is pronated, wrist flexed, metacarpophalangeal joints flexed and interphalangeal joints extended. The fingers can be moved but slowly, due to altered muscle tone in the different muscle groups.

Metathalamus

The metathalamus consists of the medial and lateral geniculate bodies (Fig. 11.6). These are small rounded elevations on the inferior aspect of the posterior part of thalamus, lateral to each side of the midbrain. The medial and lateral geniculate bodies are relay stations for the auditory and visual pathways respectively.

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FIG. 11.6 Schematic diagram showing the location of the medial and lateral geniculate bodies.

Medial geniculate body

Medial geniculate body is an oval elevation on the inferior aspect of the pulvinar of the thalamus, lateral to the superior colliculus. It is more prominent than the lateral geniculate body. The inferior brachium runs upwards, laterally and forwards from inferior colliculus to the medial geniculate body.

Connections

Afferents: Auditory pathways through lateral lemniscus either directly or after relay in the inferior colliculus. These fibres pass through brachium of inferior colliculus (inferior brachium).

Efferents: Geniculocortical fibres project as auditory radiation through sublentiform part of internal capsule to the primary auditory area in the temporal lobe (area 41, 42).

Lateral geniculate body

Lateral geniculate body is a small ovoid prominence visible at the terminal end of the optic tract. It is situated on the inferior surface of the pulvinar, anterolateral to the medial geniculate body. It is smaller than the medial geniculate body and connected to the superior colliculus by the superior brachium.

Structure

The grey matter of lateral geniculate body consists of six layers (or laminae) separated by nerve fibres. These layers are numbered 1 to 6 from ventral to dorsal side. Laminae 1, 4, and 6 receive fibres from the retina of opposite side and laminae 2, 3, and 5 from the retina of the same side (Fig. 11.7).

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FIG. 11.7 Six layers (laminae) of the lateral geniculate body and their afferent connections.

Connections

Afferents: Lateral root of the optic tract consisting of most of the retinal fibres of both the eyes (temporal fibres of the same side and nasal fibres of the opposite side).

Efferents: Geniculocalcarine fibres project as the optic radiation through retrolentiform part of internal capsule to the visual cortex of the occipital lobe (areas 17, 18, and 19).

Functions

The lateral geniculate body is the last relay station on the visual pathway.

N.B. Not all retinal fibres synapse in the lateral geniculate body. These uninterrupted retinal fibres terminate in the pretectal area and superior colliculus of midbrain to serve light reflexes.

The comparison between the medial and lateral geniculate bodies is provided in Table 11.6.

Table 11.6

Comparison between the medial and lateral geniculate bodies

Medial geniculate body

Lateral geniculate body

Oval-shaped collection of grey matter on the inferior aspect of the pulvinar

Bean-shaped collection of grey matter on the inferior aspect of inferior aspect of the pulvinar

Hilum absent

Hilum present

No lamination

Consists of 6 laminae, numbered 1 to 6 from ventral surface to dorsal surface

Destruction of medial geniculate on one side has little or no effect on hearing

Destruction of lateral geniculate body on one side produces blindness in the opposite half of the field of vision

Last relay station on the auditory pathway

Last relay station on the optic pathway

Sends auditory impulse through auditory radiation to the auditory area of the temporal lobe

Sends visual impulses through optic radiation to the visual radiation to the cortex of the occipital lobe

Epithalamus

Pineal gland (epiphysis cerebri)

Pineal gland is a midline cone-shaped reddish grey structure (only 3 mm × 5 mm in size) occupying the vertical groove between the two superior colliculi below the splenium of corpus callosum. It has a stalk which divides into two laminae. The ventral (or inferior) lamina is continuous with the posterior commissure, and the dorsal (or superior) lamina is continuous with the habenular commissure (Fig. 11.8). The extension of the cavity of third ventricle between the two laminae is termed pineal recess.

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FIG. 11.8 The pineal gland in relation to the third ventricle and habenular and posterior commissures.

The pineal gland is supplied by a nerve called nervus con-arii, which consists of postganglionic sympathetic fibres arising from superior cervical sympathetic ganglion.

Structure

The pineal gland is a neuroendocrine gland and consists of parenchymal cells, called pinealocytes, and neuroglial cells. The pinealocytes secrete a hormone called melatonin. The calcium phosphates and carbonates are deposited in the gland with age in the form of multilaminar corpuscles called corpora arenacea (or brain sand).

Functions

Descartes described the pineal gland as the seat of soul. It is not for sure, a functionless vestigial organ representing dorsal third eye (found in some types of fishes and amphibians) as was assumed in the recent past. At present, it is considered to be the most highly evolved gland of the body and has following known functions:

• It has a neuroendocrine activity in regulation and modulation of the pituitary and all other endocrine organs, mostly inhibitory.

• It acts as biological clock for physiological and behavioural control.

• It secretes a hormone, melatonin which inhibits secretion of gonadotrophins (GnRH) from hypothalamus. Thus, it has inhibitory effect on the reproductive system (sexual maturity). The melatonin is produced at night and its production falls during day time. Melatonin probably holds back the reproductive development until a suitable age has reached by inhibiting the secretion of gonadotro-phic hormones.

  The neural pathway for pineal secretion is as follows: Retina → optic tract → brainstem → superior cervical sympathetic ganglion → nervus conarii → pineal gland.

Unique features

• Pineal gland is the only part of the brain, which has no nerve cells in it.

• It is the only part of the brain which is supplied by a nerve (nervus conarii) which arises from outside the brain.

Clinical Correlation

• The lesions of the pineal gland are associated with precocious puberty.

• The calcification of the pineal gland is demonstrable radiologically in more than 50% of normal adults. It lies in midline or midsagittal plane of the skull, and is about 5 cm above the external auditory meatus, in lateral view of the X-ray skull (However, sometimes, the shadow of pineal gland is seen slightly to the left, as at this level the right cerebral hemisphere is slightly wider than the left). It may serve as a useful landmark to detect any shift of the brain from the midline due to some space occupying lesions within the brain. The displacement of pineal shadow indicates towards the site of the intracranial space occupying lesion.

Habenular Nucleus

Habenular nucleus lies beneath the habenular triangle, which is a small triangular area above the superior colliculus and medial to the pulvinar of thalamus. Medially the triangle is bounded by stria medullaris thalami and stalk of pituitary gland (Fig. 11.1).

Habenular nucleus together with its connections forms the part of the limbic system.

Habenular Commissure

Habenular commissure connects the habenular nuclei of the two sides and crosses the midline by passing through the superior lamina of the stalk of pineal gland (Fig. 11.8).

Posterior Commissure

Posterior commissure is composite bundle of fibres which connect the medial longitudinal fasciculi, interstitial nuclei, superior colliculi, pretectal nuclei and posterior thalamic nuclei of the two sides. It crosses the midline by passing through the inferior lamina of the stalk of the pineal gland (Fig. 11.8).

Subthalamus

Subthalamus is described in detail on page 158.

Hypothalamus

The hypothalamus is a part of diencephalon which lies below the thalamus. It forms the floor and the lower parts of lateral walls of the third ventricle. Anatomically the hypothalamus is small in size weighing only 4 g and forming only 0.3% of the total brain mass but physiologically there is hardly any activity in the body that is not influenced by it. Thus, the functional significance of the hypothalamus is disproportionate to its size. The hypothalamus controls three systems: (a) autonomic nervous system, (b) endocrine system, and (c) limbic system. The hypothalamus helps to maintain the homeostasis.

N.B. Being the principal autonomic centre of the brain, it has been regarded as the head ganglion of the autonomic nervous system by Sherrington.

Boundaries of the hypothalamus

Strategically the hypothalamus is placed close to the limbic system, thalamus and hypophysis cerebri. Its boundaries are as follows:

Anteriorly: Lamina terminalis (lamina terminalis extends from the optic chiasma to the anterior commissure). Posteriorly: Subthalamus.

lnferiorly: Structures in the floor of third ventricle, viz. tuber cinereum, infundibulum, and mammillary bodies. (These structures are actually the parts of hypothalamus.)

Superiorly: Thalamus.

Laterally: Internal capsule.

Medially: Cavity of third ventricle.

Subdivisions of the hypothalamus

For the sake of convenience of description, the hypothala-mus is divided into number of regions/zones.

The hypothalamus is divided into two lateral halves by the cavity of third ventricle (above) and an imaginary median plane (below). As per conventional teaching, the hypothalamus is considered as a single structure but strictly speaking, it is a bilateral structure.

The anterior column of fornix traverses the hypothalamus to reach the mammillary body and serves as a point of reference for a sagittal plane that divides the hypothalamus into medial and lateral zones. The mammillothalamic tract and fasciculus retroflexus also lie in this plane (Fig. 11.9).

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FIG. 11.9 Schematic diagram to show medial and lateral zones of the hypothalamus with fornix and mammillothalamic tract lying in the plane between the two zones.

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FIG. 11.10 Different nuclei of hypothalamus in sagittal section. The lateral nucleus of the hypothalamus is not shown.

The medial zone is again divided into a thin subependymal or periventricular zone and a thicker intermediate zone.

Thus, the hypothalamus is divided, from medial to lateral side into the following three zones:

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The hypothalamus is also subdivided anteroposteriorly into the following four regions:

1. Preoptic region adjoining the lamina terminalis.

2. Supraoptic region above the optic chiasma.

3. Tuberal region includes the tuber cinereum, infundibu-lum and area around it.

4. Mammillary region includes the mammillary bodies and area around it.

The preoptic region lies anterior to the hypothalamus between the optic chiasma and anterior commissure. Anatomically it belongs to the telencephalon but functionally to the hypothalamus.

The tuber cinereum is the region bounded, caudally by mammillary bodies and rostrally by optic chiasma. The infundibulum connects the posterior lobe of the hypophysis cerebri with the tuber cinereum. The tuber cinereum around the base of the infundibulum is raised to form a median eminence.

Hypothalamic nuclei

The hypothalamus is made up of numerous small nuclear masses, called hypothalamic nuclei. The nuclei present in different regions of the hypothalamus are listed in Table 11.7 and shown in Figure 11.10.

Table 11.7

Hypothalamic regions and nuclei in them

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In general, the hypothalamic nuclei are divided into four groups – preoptic, supraoptic, tuberal, and mammillary.

N.B. The large nerve cells throughout the lateral zone are relatively sparse and collectively constitute the lateral nucleus. Since this nucleus occupies the whole anteropos-terior extent of the hypothalamus, it could not be included in above-mentioned four groups.

The major hypothalamic nuclei and their functions are enumerated in Table 11.8.

Table 11.8

Major hypothalamic nuclei and their functions

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Connections of the hypothalamus

The connections of hypothalamus (afferents and efferents) are numerous and complex, therefore, only main connections are described here.

Afferent connections

• Fornix connects the hippocampus to the mammillary bodies.

• Stria terminalis connects the amygdaloid body to the preoptic and anterior hypothalamic nuclei.

• Mammillary peduncle conveys sensory impulses from the spinal cord and the brainstem to the lateral hypotha-lamic nucleus.

• Medial forebrain bundle connects the autonomic and limbic structures of the forebrain to the hypothalamus.

• Thalamohypothalamic, pallidohypothalamic and sub-thalamohypothalamic pathways connect the thalamus, corpus striatum and subthalamus to the hypothalamus respectively.

• Direct physical and chemical receptors. The circulating blood is constantly monitored by the hypothalamic cells, which function as thermoreceptors, osmoreceptors or chemoreceptors.

Efferent connections

• Mammillothalamic tract connects the mammillary body to the anterior nucleus of the thalamus, which in turn projects to the cingulate gyrus.

• Mammillotegmental tract connects the mammillary body to the reticular formation of the brainstem.

• Descending fibres to the brainstem and spinal cord influence the peripheral neurons of the autonomic nervous system.

Through reticular formation, the hypothalamus is connected to the parasympathetic nuclei of oculomotor, facial, glossopharyngeal, and vagus nerves in the brainstem.

Similarly, the hypothalamus is also connected to the preganglionic sympathetic neurons in the lateral horns of T1 to L2 spinal segments and to the preganglionic para-sympathetic neurons in the lateral horns of S2, 3, and 4 spinal segments.

• Hypothalamus is connected to the neurohypophysis and adenohypophysis of the hypophysis cerebri through supraopticohypophyseal and tuberoinfundibular tracts respectively.

In view of their greater clinical significance, the connections of hypothalamus with the hypophysis cerebri are discussed in detail in the following text.

Connections of the hypothalamus with the hypophysis cerebri (pituitary gland)

Connections with the neurohypophysis (Fig. 11.11)

The fibres arising from supraoptic and paraventricular nuclei project to the posterior lobe of pituitary gland (neuro-hypophysis) as the hypothalamo-hypophyseal tract (also called supraoptico-hypophyseal tract). The hormones vasopressin and oxytocin are synthesized in the nerve cells of the supraoptic and paraventricular nuclei, respectively and are transported to the posterior pituitary along the fibres of this tract (by axoplasmic flow). The neuro-secretory axons terminate as small expansions on the capillary blood vessels in the neurohypophysis. Here these hormones are absorbed into the blood stream in the capillaries.

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FIG. 11.11 The origin and distribution of the fibres of supra-opticohypophyseal (hypothalamohypophyseal) tract.

The vasopressin (antidiuretic hormone) is vasoconstrictor and causes an increased absorption of water in the distal convoluted tubules and collecting tubules of the kidney (antidiuretic effect).

The oxytocin causes contraction of the uterine muscle and myoepithelial cells that surround the alveoli of the mammary gland.

Clinical Correlation

Diabetes insipidus

It develops due to impaired secretion of antidiuretic hormone (vasopressin) following lesions of— supraoptic and paraventricular nuclei or supraoptico-hypophyseal tract. The characteristic features are: (a) polyuria (urine volume is very large with low specific gravity), and (b) polydipsia (increased water intake). It is the best known hypothalamic syndrome.

N.B. The absence of glycosuria differentiates it from diabetes mellitus.

Connections with adenohypophysis (Fig. 11.12)

The releasing hormones and release-inhibiting hormones are produced in the cells of tuberal and infundibular nuclei and are transported to the median eminence along the tubero-infundibular tract. From here, these hormones are carried by the hypothalamohypophyseal portal system to the secretory cells of the anterior lobe of the hypophysis cerebri (adenohypophysis).

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FIG. 11.12 The connections of hypothalamus with adenohypo-physis through tuberoinfundibular tract and hypothalamo-hypophyseal portal system.

The releasing hormones stimulate the production and release of adrenocorticotropic hormone (ACTH), follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), and growth hormone (GH).

The release-inhibiting hormones inhibit the release of the melanocyte stimulating hormone (MSH) and luteotropic hormone (LTH). The LTH is also called lactogenic hormone or prolactin.

The hypophyseal portal system is formed by the hypophy-seal branches of the internal carotid artery. After entering the median eminence these branches divide into tuft of capillaries drain into descending vessels (hypophyseal portal veins) that end in the anterior lobe of the hypophysis by dividing into vascular sinusoids between the secretory cells of the anterior lobe and then drained by a hypophyseal vein.

Functions of hypothalamus

• Autonomic control. The anterior part of the hypothala-mus controls the parasympathetic nervous system while posterior part controls the sympathetic nervous system.

• Endocrine control. The hypothalamic centres are sensitive to circulating hormonal levels, providing negative or positive feedback. The hypothalamus regulates the hormonal secretion of anterior pituitary by forming the releasing factors or release inhibiting factors, which in turn controls the endocrine activities of the body.

• Neurosecretion. The hypothalamus secretes oxytocin and vasopressin.

• Regulation of food and water intake. The lateral part of the hypothalamus acts as a hunger centre while the medial part acts as a satiety centre. A thirst centre in the lateral part regulates the water intake.

• Emotional expression. The autonomic emotions like laughing, crying, sweating or blushing are expressed by the integrated activity of the ANS and somatic efferent system.

• Regulates the sexual behaviour and reproduction. This is done by influencing the secretion of gonadotrophic hormones by the pituitary gland.

• Temperature regulation. The cold and heat sensors located in the hypothalamus respond appropriately to maintain the body temperature at optimum level in diverse conditions. The anterior portion of the hypothalamus prevents the rise in body temperature while posterior portion promotes heat conservation and heat production.

• Biological clock. It regulates the cyclic activities of the body (circadian rhythm), viz. sleeping and waking cycle but itself affected by diurnal rhythms. The circadian rhythm for many body functions is of about 24 hours.

Clinical Correlation

Craniopharyngioma

It is congenital tumour which develops from remnants of the Rathke's pouch. It is the most common supraten-torial tumour in children and is the most common cause of hypopituitarism in them. It is often a begnin tumour which is often cystic and/or calcifies (Fig. 11.13). It compresses optic chiasma and hypothalamus. Clinically it presents as: (a) bitemporal hemianopia, due to pressure on optic chiasma, and (b) hypotha-lamic syndrome (i.e. diabetes insipidus, adiposites, relentless weight gain, disturbance of temperature regulation, etc.) due to pressure on the hypothalamus.

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FIG. 11.13 The craniopharyngioma. (A) Pathology specimen showing cystic changes and calcification. (B) MRI—midsagittal section of the brainstem and diencephalon (arrows). Source: Haslet Christopher, Chilvers, Edwin R, Boon, Nicholas, A, et al. Davidson's Principles and Practice of Medicine. 19th edition. Oxford: Elsevier Science Ltd., 2002

Third Ventricle

The third ventricle is the cavity of diencephalon. It is a midline slit-like cavity situated between the two thalami and the part of hypothalamus. It extends from the lamina terminalis anteriorly to the superior end of the cerebral aqueduct ol midbrain posteriorly. The cavity of third ventricle is lined by a ciliated columnar epithelium, the ependyma, and traversec by a mass of grey matter, the interthalamic adhesion, connecting the two thalami. The outline of the cavity is very irreg ular due to the presence of several diverticula or recesses.

Communications

Anteriorly on each side, the third ventricle communicates with the lateral ventricle through interventricular foramen (ofMonro), and posteriorly with the fourth ventricle through cerebral aqueduct (of Sylvius). It receives cerebrospinal fluid (CSF) from the lateral ventricles through interventric-ular foramina and transports it to the fourth ventricle through cerebral aqueduct.

Boundaries (Fig. 11.14)

The third ventricle has anterior wall, posterior wall, roof, floor, and two lateral walls.

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FIG. 11.14 Boundaries and recesses of third ventricle as seen in sagittal section. (HS = hypothalamic sulcus, I = interthalamic adhesion, 1. infundibular recess, 2. optic recess, 3. anterior recess, 4. suprapineal recess, 5. pineal recess.)

Anterior wall is formed from above downwards by,

– anterior column of fornix,

– anterior commissure, and

– lamina terminalis.

Posterior wall is formed from above downwards by,

– pineal gland,

– posterior commissure, and

– commencement of cerebral aqueduct.

Roof is formed by the ependyma that stretches across the upper limits of two thalami. Exactly speaking the ependyma is reflected from one thalamus to the other at the site of stria medullaris thalami. Anteroposteriorly the roof extends from interventricular foramen to the habenular commissure.

Floor is formed from before backwards by,

– optic chiasma,

– tuber cinereum and infundibulum (pituitary stalk),

– mammillary bodies,

– posterior perforated substance, and

– tegmentum of the midbrain.

N.B. All above structures are the structures of interpe-duncular fossa except the optic chiasma and tegmentum of the midbrain.

Lateral wall is marked by a curved sulcus, the hypotha-lamic sulcus extending from the interventricular foramen to the cerebral aqueduct. The sulcus divides the lateral wall into a larger upper part and smaller lower part.

• The larger upper part of the lateral wall is formed by the medial surface of anterior two-third of the thalamus.

• The smaller lower part of the lateral wall is formed by the hypothalamus and it is continuous with the ventricular floor.

N.B. The two lateral walls of the third ventricle are normally closely approximated, hence in coronal section of the brain, the cavity of the third ventricle appears as a median vertical slit.

Recesses of the Ventricle (Fig. 11.14)

The cavity of third ventricle extends into the surrounding structures as a pocket-like protrusion called recesses. These are as follows:

• Infundibular recess. It is a deep tunnel-shaped recess extending downwards through the tuber cinereum into the infundibulum, i.e. the stalk of the pituitary gland.

• Optic (or chiasmatic) recess. It is angular recess situated at the junction the anterior wall and the floor of the ventricle just above the optic chiasma.

• Anterior recess (vulva of ventricle). It is a triangular recess which extends anteriorly in front of interventricular foramen and behind anterior commissure between the diverging anterior columns of the fornix.

• Suprapineal recess. It is a fairly capacious blind diverticulum which extends posteriorly above the stalk of pineal gland and below the tela choroidea.

• Pineal recess. It is a small diverticulum, which extends posteriorly between the superior and inferior laminae of the stalk of the pineal gland.

Choroid Plexus and Tela Choroidea of the Third Ventricle

The tela choroidea in the roof of the third ventricle is triangular in shape. The choroid plexus of the third ventricle hangs downwards from the tela choroidea as two longitudinal anteroposterior vascular fringes. The blood vessels contributing to its formation are derived from the anterior choroidal arteries.

Clinical Correlation

• The third ventricle being a narrow slit-like space is easily obstructed by the local brain tumours or congenital defects. The obstruction results in excessive accumulation of CSF inside the brain resulting in an increased intracranial pressure in adults and in a hydrocephalus in children.

  The site of obstruction can be found out by ventriculography.

• Tumours in the floor of third ventricle give rise to hypothalamic syndrome, consisting of diabetes insipidus, obesity, etc.

• The narrow cavity of the ventricle and its recesses are important in localizing lesions in the central parts of the hemispheres since they manifest as slight deviations from the midline.

• It is important not to interpret the interthalamic adhesion (connexus) as an abnormal mass in transverse CT scans.

Clinical Problems

1. A 55-year-old patient was admitted in the hospital with the complaint of loss of sensation on the left side of the body. A few days later, the patient appeared to be improving and there was an evidence of return of sensation to the left side of his body, but he suddenly started to complain of agonizing pain in his left arm and leg. The pain would start spontaneously or be initiated even by the light touch of the bed sheet or by little exposure to cold. The pain failed to respond even to powerful analgesic drugs. What is the diagnosis and give the cause of the symptoms?

2. A 60-year-old man was admitted in the hospital as a suspected case of cerebral tumour. The neurosurgeon advised X-ray skull AP and lateral views to see for any lateral displacement of the brain within the skull. Name the radiological finding that would indicate the neurosurgeon in detecting lateral displacement.

3. Explain, why the lesions which destroy the pineal gland, lead to precocious puberty.

4. What is the basic difference between diabetes insipidus and diabetes mellitus and mention the causative factors for these two clinical conditions.

5. Explain, how a tumour or pressure from third ventricle can produce a central temporal hemianopic scotoma.

Clinical Problem Solving

1. This patient is suffering from what is called thalamic syndrome. It occurs as the patient is recovering from infarction of the thalamus secondary to thrombosis of thalamogeniculate branch of the right posterior cerebral artery. It is characterized by thalamic overreaction particularly to tactile stimuli (also see Clinical correlation on page 130).

2. The lateral displacement of the shadow of calcified pineal gland will indicate the shift in the position of the brain (for details see Clinical correlation on page 133).

3. The pineal gland secretes a hormone called melatonin which regulates the onset of puberty. It probably holds back the development of gonads until a suitable age has been reached by inhibiting the release of gonadotrophic hormones or their releasing factors.

  For this reason, the lesions which destroy the pineal gland, lead to precocious puberty.

4. Both in diabetes insipidus and diabetes mellitus there is polyuria (that the patient passes excess volume of urine). The basic difference between these two clinical conditions is that, in diabetes mellitus there is glycosuria (i.e. presence of glucose in the urine).

  The diabetes insipidus occurs due to impaired secretion of ADH (vasopressin) and diabetes mellitus due to impaired secretion of insulin.

5. Since the decussating macular fibres in the optic chiasma are located just below the supraoptic recess of third ventricle. Therefore, a tumour or pressure from the ventricle pressing on the optic chiasma will produce central temporal hemianopic scotomas.