Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

15. Diencephalon

15.1 Diencephalon, Overview and Development

A The diencephalon in situ

Midsagittal section of the right hemisphere viewed from the medial side. The diencephalon is located below the corpus callosum, part of the telencephalon, and above the mesencephalon (midbrain). The lateral wall of the third ventricle, visible here, forms the medial boundary of the diencephalon. The thalamus makes up four-fifths of the entire diencephalon, but the only parts of the diencephalon that can be seen externally are the hypothalamus (visible from the basal aspect) and portions of the epithalamus (pineal, visible from the occipital aspect). The differentiation of these structures from the embryonic diencephalon is shown in D, and their functions are listed on page 214 (A). In the adult brain, the diencephalon is involved in endocrine functioning and autonomic coordination of the pineal, posterior pituitary lobe and hypothalamus. It also acts as a relay station for sensory information and somatic motor control (via the thalamus). Units 5.2 and 5.3 deal with the external and internal structure of the diencephalon as a whole. Later units examine the individual parts of the diencephalon, devoting the most attention to the clinically important thalamus (5.4, 5.5) and hypothalamus (5.6, 5.7). The final unit covers the epithalamus and subthalamus (5.8).

В Development of the diencephalon from the cranial neural tube Anterior view. To understand the location and extent of the diencephalon in the adult brain, it is necessary to know how it develops from the neural tube. The diencephalon and telencephalon both develop from the prosencephalon, or telencephalic vesicle (see p. 183). As development proceeds, the two hemispheres of the telencephalic vesicle (red) expand, overgrowing the diencephalic vesicle (blue). This process shifts the boundary between the telencephalon and diencephalon until only a small area of the diencephalon can be seen at the base of the adult brain (see A).

C Posterior telodiencephalic boundary

Coronal sections.

a Embryonic brain. The development of the telencephalon (red) has progressed considerably in relation to B. The lateral ventricles containing the choroid plexus have already completely overgrown the diencephalon (blue) from behind. The medial wall of the lateral ventricles is very thin and has not yet fused to the diencephalon. Between the telencephalon and diencephalon is a vascularized sheet of connective tissue, the tela choroidea.

b Adult brain. By the adult stage, the tela choroidea and the medial wall of the lateral ventricle have become fused to the diencephalon. Removing the choroid plexus and the thin tela choroidea affords a direct view of the posteromedial boundary of the diencephalon (see B, p. 212).

D Organization of the diencephalon during embryonic development

Coronal section of an embryonic brain (left) and an adult brain (right) demonstrating the parts of the diencephalon.

Because the diencephalon of the adult brain lies between the telencephalon and mesencephalon, the ascending and descending axons must penetrate this part of the brain during development, forming the internal capsule. As development proceeds, the axon bundles that form the internal capsule migrate through the subthalamus (black arrows), displacing the greater portion of it laterally. This laterally displaced part of the subthalamus is called the globus pallidus. Although the globus pallidus is displaced anatomically into the telencephalon and is considered part of the telencephalon in a topographical sense, it still retains close functional ties with the subthalamus, as both are part of the extrapyramidal motor system. The medial part of the subthalamus remains in the diencephalon as the true subthalamus (not visible in this plane of section). As a result, the internal capsule of the telencephalon forms the lateral boundary of the diencephalon. The different parts of the diencephalon grow to reach different definitive sizes. The thalamus grows disproportionately and eventually occupies four-fifths of the mature diencephalon.

15.2 Diencephalon, External Structure

A The diencephalon and brainstem

Left lateral view. The telencephalon has been removed from around the thalamus, and the cerebellum has also been removed. The parts of the diencephalon visible in this dissection are the thalamus, the lateral geniculate body, and the optic tract. The lateral geniculate body and optic tract are components of the visual pathway.

Note: the retina and associated optic nerve form an anterior extension of the diencephalon. Departing from the convention of yellow for nerves, we have colored the optic nerve blue to emphasize this relationship.

В Arrangement of the diencephalon around the third ventricle

Posterior view of an oblique transverse section through the telencephalon with the corpus callosum, fornix, and choroid plexus removed. Removal of the choroid plexus leaves behind its line of attachment, the taenia choroidea. The thin wall of the third ventricle has been removed with the choroid plexus to expose the thalamic surface medial to the boundary line of the taenia choroidea. The thin ventricular wall has been left on the thalamus lateral to the taenia choroidea. This thin layer of telencephalon, called the lamina affixa, is colored brown in the drawing and covers the thalamus (part of the diencephalon), shown in blue. Because the thalamostriate vein marks this boundary between the diencephalon and telencephalon, it is featured prominently in the drawing. Lateral to the vein is the caudate nucleus, which is part of the telencephalon (compare with C, p-211).

C The diencephalon and brainstem

a Anterior view, b posterior view with the cerebellum and telencephalon removed.

a The optic tract marks the lateral boundary of the diencephalon. It winds around the cerebral peduncles (crura cerebri), which are part of the adjacent midbrain (mesencephalon).

b The epithalamus, which is formed by the pineal and the two habenulae (“reins”), is well displayed in this posterior view. The lateral geniculate body is an important relay station in the visual pathway, just as the medial geniculate body is an important relay station in the auditory pathway. Both are counted among the thalamic nuclei, and together they constitute the metathalamus, an extension of the thalamus proper. The pulvinar (“pillow"), which encompasses the posterior thalamic nuclei, is seen particularly well in this section.

D Location of the diencephalon in the adult brain

Basal view of the brain (the brainstem has been sectioned at the level of the mesencephalon). The structures that can be identified in this view represent the parts of the diencephalon situated on the basal surface of the brain. This view also demonstrates how the optic tract, which is part of the diencephalon, winds around the cerebral peduncles of the mesencephalon (see Ca). Due to the expansion of the telencephalon, only a few structures of the diencephalon can be seen on the undersurface of the brain:

 Optic nerve

 Optic chiasm

 Optic tract

 Tuber cinereum with the infundibulum

 Mammillary bodies

 Medial geniculate body (see Cb)

 Lateral geniculate body

 Posterior lobe of the pituitary gland (neurohypophysis, see p. 222)

5.3 Diencephalon, Internal Structure

A The four parts of the diencephalon

Part

Boundary line

Structure

Function

Epithalamus

 

• Pineal gland

• Habenulae

Regulation of circadian rhythms; linking of olfactory system to brainstem

       

Thalamus

 

• Thalamus

Relayof sensory information; assistance in regulation of motor function

       

Subthalamus

 

• Subthalamic nucleus

• Zona incerta

• Globus pallidus (see E, p. 225)

Relay of sensory information (somatomotor zone of diencephalon)

       

Hypothalamus

 

• Optic chiasm, optic tract

• Tubercinereum, neurohypophysis

• Mammillary bodies

Coordination of autonomic nervous system with endocrine system; participation in visual pathway

* This is the only sulcus shown in A.

В Coronal sections through the diencephalon at three different levels

a Level of the optic chiasm: Portions of the diencephalon and telencephalon appear in this section, which clearly shows the position of the diencephalon on both sides of the third ventricle. An outpouching of the third ventricle, the preoptic recess, is located above the optic chiasm. Its connection to the third ventricle lies outside this plane of section.

b Level of the tuber cinereum, just behind the interventricular foramen: The boundary between the diencephalon and telencephalon is clearly defined only in the region about the ventricles; the underlying nuclear areas blend together with no apparent boundary. Along the lateral ventricles, the boundary between the diencephalon and telencephalon is marked by the lamina affixa, a narrow strip of telencephalon that overlies the thalamus. It can be seen that layers of gray matter permeate the internal capsule in its dorsal portion.

c Level of the mammillary bodies: This section displays the thalamic nuclei. More than 120 separate nuclei may be counted, depending on the system of nomenclature used. Most of these nuclei cannot be grossly identified in anatomical specimens. Their classification is reviewed on p.216 (after Kahle and Frotscher, quoted from Villinger and Ludwig).

15.4 Thalamus: Thalamic Nuclei

A Functional organization of the thalamus

Almost all of the sensory pathways are relayed via the thalamus and project to the cerebral cortex (see G. thalamic radiation). Consequently, a lesion of the thalamus or its cortical projection fibers caused by a stroke or other disease leads to sensory disturbances. Although a diffuse kind of sensory perception may take place at the thalamic level (especially pain perception), cortical processing (by the telencephalon) is necessary in order to transform unconscious perception into conscious perception. The olfactory system is an exception to this rule, although its olfactory bulb is still an extension of the telencephalon.

Note: Major descending motor tracts from the cerebral cortex generally bypass the thalamus.

В Spatial arrangement of the thalamic nuclear groups

Left thalamus viewed from the lateral and occipital aspect, slightly rotated relative to the views on p. 212. The thalamus is a collection of approximately 120 nuclei that process sensory information. They are broadly classified as specific or nonspecific:

• Specific nuclei and the fibers arising from them (thalamic radiation, see G) have direct connections with specific areas of the cerebral cortex. The specific thalamic nuclei are subdivided into four groups:

 Anterior nuclei (yellow)

 Medial nuclei (red)

 Ventrolateral nuclei (green)

 Dorsal nuclei (blue).

The dorsal nuclei are in contact with the the medial and lateral geniculate bodies. Located beneath the pulvinar, these two nuclear bodies contain the nuclei of the medial and lateral geniculate bodies, and are collectively called the metathalamus. Like the pulvinar, they belong to the category of specific thalamic nuclei.

• Nonspecific nuclei have no direct connections with the cerebral cortex. Part of a general arousal system, they are connected directly to the brainstem. The only nonspecific nuclei shown in this diagram (orange, see F for further details) are the centromedian nucleus and the intralaminar nuclei.

C Nomenclature of the thalamic nuclei

Name

Alternative name

Properties

Specific thalamic nuclei (cortically dependent)

Pa 11 iothalamus

Project to the cerebral cortex (pallium)

Nonspecific thalamic nuclei (cortically independent)

Truncothalamus

Project to the brainstem, diencephalon, and corpus striatum

Integration nuclei

 

Project to other nuclei within the thalamus (classified as nonspecific thalamic nuclei)

Intralaminar nuclei

 

Nuclei in the white matter of the internal medullary lamina (classified as nonspecific thalamic nuclei)

D Division of the thalamic nuclei by the medullary laminae

Coronal section at the level of the mammillary bodies. Several groups of thalamic nuclei are grossly separated into larger nuclear complexes by fibrous sheets called medullary laminae. The following laminae are shown in the diagram:

 Internal medullary lamina between the medial and ventrolateral thalamic nuclei

 External medullary lamina between the lateral nuclei and the reticular nucleus of the thalamus

E Somatotopic organization of the specific thalamic nuclei

Transverse section. The specific thalamic nuclei (defined in В, C) are topographically arranged according to their functional relation to specific regions of the body. Afferent fibers from the spinal cord, brainstem and cerebellum are localized to specific areas of the thalamus, where the corresponding thalamic nuclei are clustered. This pattern of somatotopic arrangement, a recurring theme in neural organization, is here illustrated for the ventrolateral thalamic nuclei (green in B, D, E). Axons from the crossed superior cerebellar peduncle terminate in the ventral lateral nucleus of the thalamus (2); information on body position, coordination and muscle tone travels by this pathway to the motor cortex, which also shows a pattern of somatotopic organization (seep. 339). The lateral part of the ventral lateral nucleus relays impulses from the extremities, while the medial part relays impulses from the head. The ventral intermediate nucleus (3) receives afferent input from the vestibular nuclei concerning the coordination of gaze toward the ipsilateral side. The large sensory pathways of the spinal cord (the tracts of the posterior funiculus) are relayed to the nuclei cuneatus and gracilus, which send their axons through the medial lemniscus to terminate in the ventral posterolateral nucleus (4), while the trigeminal sensory pathways from the head terminate in the ventral posteromedial nucleus (5, trigeminal lemniscus, seep.275). Topographical localization according to function is a basic principle of neural organization.

F Nonspecific thalamic nuclei

Coronal sections presented in an oral-to-caudal series. The nonspecific thalamic nuclei project to the brainstem, to other nuclei in the diencephalon (including other thalamic nuclei), and to the corpus striatum. They have no direct connections with the cerebral cortex, acting only indirectly on the cortex. The medial nonspecific thalamic nuclei are subdivided into two groups:

 Nuclei of the central thalamic gray matter (median nucleus): small groups of cells distributed along the wall of the third ventricle

 Intralaminar nuclei, located in the internal medullary lamina. The largest nucleus of this group is the centromedian nucleus.

The lateral specific thalamic nucleus shown in the diagram is the reticular nucleus of the thalamus, which is situated lateral to the other specific thalamic nuclei. The reticular nucleus is the source of the electrical impulses recorded in an electroencephalogram (EEG).

C Thalamic radiations

Lateral ventricle of the left hemisphere. The axons of the specific thalamic nuclei (so called because their fibers project to specific cortical areas) are collected into tracts that form the thalamic radiations. The arrangement of the fibers shows that the specific thalamic nuclei have connections with all areas of the cortex. The anterior thalamic radiation projects to the frontal lobe, the central thalamic radiation to the parietal lobe, the posterior thalamic radiation to the occipital lobe, and the inferiorthalamic radiation to the temporal lobe.

15.5 Thalamus: Projections of the Thalamic Nuclei

A Ventrolateral thalamic nuclei: afferent and efferent connections

The ventral posterolateral nucleus (VPL) and ventral posteromedial nucleus (VPM) are the majorthalamic relay centers for somatosensory information.

 The medial lemniscus ends in the VPL. It contains sensory fibers for position sense, vibration, pressure, discrimination, and touch that are relayed from the nucleus gracilis and nucleus cuneatus.

 Pain and temperature fibers from the trunk and limbs travel through the lateral spinothalamic tract to lateral portions of the VPL. These sensations are relayed from this nucleus to the somatosensory cortex.

 Pain and temperature information from the head region is conveyed by the trigeminal system (= trigeminothalamic tract) to the VPM. As in the VL, they synapse with third- order thalamic neurons that project to the postcentral gyrus.

A lesion of the VPL leads to contralateral disturbances of superficial and deep sensation with dysesthesia and an abnormal feeling of heaviness in the limbs (lesion of the medial lemniscus). Because the pain fibers of the spinothalamic tract terminate in the basal portions of the VPL, lesions in that region may additionally cause severe pain (“thalamic pain”). The ventral lateral nucleus (VL) projects to somatomotor cortical areas (6aot and бар). The VL nuclei form a feedback loop with the motor areas of the cortex, and so lesions of these nuclei are characterized by motor deficits.

В Anterior nucleus and centromedian nucleus: afferent and efferent connections

The anterior nucleus receives afferent fibers from the mammillary body by way of the mammillothalamic fasciculus (bundle of Vicq-d’Azyr). The anterior nucleus establishes both afferent and efferent connections with the cingulate gyrus of the telencephalon. The largest nonspecific thalamic nucleus is the centromedian nucleus, which is one of the intralaminar nuclei. It receives afferent fibers from the cerebellum, reticular formation, and medial pallidus. Its efferent fibers project to the head of the caudate nucleus and the putamen. The centromedian nucleus is an important component of the ascending reticular activation system (ARAS, arousal system). Essential for maintaining the waking state, the ARAS begins in the reticular formation of the brainstem and is relayed in the centromedian nucleus.

C Medial, dorsal, and lateral thalamic nuclei: afferent and efferent connections The medial thalamic nuclei receive their afferent input from ventral and intralaminar thalamic nuclei (not shown), the hypothalamus, the mesencephalon, and the globus/pallidus. Their efferent fibers project to the frontal lobe and premotor cortex, and afferent fibers from these regions return to the nuclei. The destruction of these tracts leads to frontal lobe syndrome, which is characterized by a loss of self-control (episodes of childish jocularity alternating with suspicion and petulance). The dorsal nuclei are formed by the pulvinar, which is the largest nuclear complex of the thalamus. The pulvinar receives afferent fibers from other thalamic nuclei, particularly the intralaminar nuclei (not shown). Its efferent fibers terminate in the association areas of the parietal and occipital lobes, which have reciprocal connections with the pulvinar. The lateral geniculate body (part of the visual pathway) projects to the visual cortex, while the medial geniculate body (part of the auditory pathway) projects to the auditory cortex. The lateral nuclei consists of the lateral dorsal nucleus and lateral posterior nucleus. They represent the dorsal portion of the ventrolateral group and receive their input from other thalamic nuclei (hence the term “integration nuclei," see p.216). Their efferent fibers terminate in the parietal lobe of the brain.

D Synopsis of some clinically important connections of the specific thalamic nuclei

The specific thalamic nuclei project to the cerebral cortex. The table below lists the origins of the tracts that terminate in the nuclei, the nuclei themselves, and the sites to which their afferent fibers project.

Thalamic afferents (Structures that project to the thalamus)

Thalamic nucleus

(abbreviation)

Thalamic efferents (Structure to which the thalamus projects)

Mammillary body (mammillothalamic fasciclus)

Anterior nucleus (NA)

Cingulate gyrus (limbic system)

Cerebellum, red nucleus

Ventral lateral nucleus (VL)

Premotor cortex (areas 6aa and 6a0)

Posterior funiculus, lateral funiculus (somatosensory input, limbs, trunk)

Ventral posterolateral nucleus (VPL)

Postcentral gyrus (sensory cortex) = somatosensory cortex (see A)

Trigeminothalamic tract (somatosensory input, head)

Ventral posteromedial nucleus (VPM)

Postcentral gyrus (sensory cortex) = somatosensory cortex (see A)

Inferior brachium

(part of the auditory pathway)

Medial geniculate nucleus (body) (MCB)

Transverse temporal gyri (auditory cortex)

Optic tract

(part of the visual pathway)

Lateral geniculate nucleus (body) (LCB)

Striate area (visual cortex)

15.6 Hypothalamus

A Location of the hypothalamus

Coronal section. The hypothalamus is the lowest level of the diencephalon, situated below the thalamus. It is the only externally visible portion of the diencephalon (see D, p. 213). Located on either side of the third ventricle, its size is most clearly appreciated in a midsagittal section that bisects the third ventricle (see Ba).

В Nuclei in the right hypothalamus

a Midsagittal section of the right hemisphereviewed from the medial side. b,c Coronal sections. The hypothalamus is a small nuclear complex located ventral to the thalamus and separated from it by the hypothalamic sulcus. Despite its small size, the hypothalamus is the command center for all autonomic functions in the body. The Terminologia Anatomica lists over 30 hypothalamic nuclei located in the lateral wall and floor of the third ventricle. Only a few of the larger, more clinically important nuclei are mentioned in this unit. Three groups of nuclei are listed below in an oral-to-caudal sequence, and their functions are briefly described:

 The anterior (rostral) group of nuclei (green) synthesizes the hormones released from the posterior lobe of the pituitary gland, and consists of the:

- preoptic nucleus,

- paraventricular nucleus, and

- supraoptic nucleus.

 The middle (tuberal) group of nuclei (blue) controls hormone release from the anterior lobe of the pituitary gland, and consists of the:

- dorsomedial nucleus,

- ventromedial nucleus, and

- tuberal nuclei.

• The posterior (mammillary) group of nuclei (red) activates the sympathetic nervous system when stimulated. It consists of the:

- posterior nucleus and

- mammillary nuclei located in the mammillary bodies.

The coronal section (c) shows the further subdivision of the hypothalamus by the fornix into lateral and medial zones. The three nuclear groups described above are part of the medial zone, whereas the nuclei in the lateral zone are not subdivided into specific groups (e.g., the area lateralis takes the place of a nucleus; the course of the fornix is described on p.205). Bilateral lesions of the mammillary bodies and their nuclei are manifested by Korsakoff syndrome, which is frequently associated with alcoholism (cause: vitamin Вт [thiamine] deficiency). The memory impairment that occurs in this syndrome mainly affects shortterm memory, and the patient may fill in the memory gaps with fabricated information. A major neuropathological finding is the presence of hemorrhages in the mammillary bodies, which are sectioned at autopsy to confirm the diagnosis.

C Important afferent and efferent connections of the hypothalamus

Midsaggital section of the right hemisphere viewed from the medial side. Because the hypothalamus coordinates all the autonomic functions in the body, it establishes afferent (blue) and efferent (red) connections with many brain regions. The following are particularly important:

a Afferent connections (to the hypothalamus):

 The fornix conveys afferent fibers from the hippocampus; it is an important fiber tract of the limbic system.

 The medial forebrain bundle transmits afferent fibers from the olfactory areas to the preoptic nuclei.

 The stria terminate conveys afferent fibers from the amygdala.

 The peduncle of the mammillary bodies transmits visceral afferent fibers and impulses from erogenous zones (nipples, genitalia).

b Efferent connections (from the hypothalamus):

 The dorsal longitudinal fasciculus passes to the brainstem where it is relayed several times before reaching the parasympathetic nuclei.

 The mammillotegmental tract distributes efferent fibers to the tegmentum of the midbrain; these are then relayed to the reticular formation. The fibers of this tract mediate the exchange of autonomic information between the hypothalamus, cranial nerve nuclei, and spinal cord.

 The mammillothalamic fasciculus (bundle of Vicq d’Azyr) conveys efferent fibers to the anterior thalamic nucleus, which is connected to the cingulated gyrus. This is part of the limbic system (see p. 374).

 The hypothalamic-hypophyseal and tuberohypophyseaI tracts are efferent tracts to the pituitary gland (see p. 222).

Region or nucleus

Function

Anterior preoptic region

Maintain constant body temperature;

Lesion: central hypothermia

Posterior region

Respond to temperature changes, e.g., sweating;

Lesion: hypothermia

Midanterior and posterior regions

Activate sympathetic nervous system

Paraventricular and anterior regions

Activate parasympathetic nervous system

Supraoptic and paraventricular nuclei

Regulate water balance; Lesion: Diabetes insipidus, also lack of thirst response resulting in hyponatremia

Anterior nuclei

Regulate appetite and food intake

• Medial part

• Lesion: Obesity

• Lateral part

• Lesion: Anorexia and emaciation

D Functions of the hypothalamus

The hypothalamus is the coordinating center of the autonomic nervous system. There is no specific sympathetic or parasympathatic control center. Certain functions can be assigned to specific regions or nuclei in the hypothalamus, and these relationships are outlined in the table. Not all of the regions or nuclei listed in the table are shown in the drawings.

15.7 Pituitary Gland (Hypophysis)

A Divisions of the pituitary gland

Midsagittal sections: a Schematic representation, b Histological appearance. The pituitary gland (hypophysis) consists of two lobes:

 Anterior lobe (adenohypophysis), the hormone-produc/ng part (see Dand E), and

 Posterior lobe (neurohypophysis), the horm о ne-re/eosing part (see B).

While the posterior pituitary lobe is an extension of the diencephalon, the anterior pituitary lobe is derived from the epithelium of the roof of the pharynx. The two lobes establish contact during embryonic development. The pituitary stalk (infundibulum) attaches both lobes of the gland to the hypothalamus, which contains the cell bodies of the neurosecretory neurons. The pituitary gland is surrounded by a fibrous capsule and lies in the sella turcica over the sphenoid sinus, which provides a route of surgical access to pituitary tumors.

В Connections of the hypothalamic nuclei to the posterior lobe of the pituitary gland

a Hypothalamic-(neuro)pituitary axis, b Neurosecretory neuron in the hypothalamic nucleus.

Pituitary hormones are not synthesized in the posterior pituitary lobe (neurohypophysis) but in neurons located in the paraventricular nucleus and supraoptic nucleus of the hypothalamus. They are then transported by axons of the hypothalamic-hypophyseal tract to the neurohypophysis, where they are released as needed. Terminals of the paraventricular and supraoptic hypothalamic nuclei release two hormones in the posterior pituitary lobe:

 Oxytocin from the neurons of the paraventricular nucleus.

 Antidiuretic hormone (ADH) or vasopressin from the neurons of the supraoptic nucleus.

The axons from both nuclei pass through the pituitary stalk to the posterior lobe of the pituitary gland. The peptide hormones are stored in vesicles (aggregated into large “Herring bodies”) in the cell bodies of the neurosecretory neurons and are carried to the posterior lobe by antegrade axoplasmic transport.

C Hypophyseal portal circulation and connections of the hypothalamic nuclei to the anterior pituitary lobe

The superior hypophyseal arteries from each side of the body form a vascular plexus around the infundibulum (pituitary stalk). The axons from neurons of the hypothalamic nuclei (dark red and dark blue arrows) terminate at this plexus and secrete hormones that have been produced in smaller (parvocellular) neurons of the hypothalamus. The secreted hypothalamic hormones are of two types:

 Releasing factors which stimulate hormone release from cells of the anterior pituitary lobe, and

 Release-inhibiting factors which inhibit release from these cells.

These hormones are carried by the hypophyseal portal venous system (named after the portal circulation of the liver) to capillaries in the anterior lobe, establishing communication between the hypothalamus and endocrine cells of the anterior pituitary.

D Histology of the anterior pituitary gland

Three types of cell can be distinguished in the anterior pituitary gland using classic histologic methods: acidophilic cells, basophilic cells, and chromophobic cells. The latter have already released their hormones, and are therefore negative in immunohistochemical tests that specifically detect peptide hormones: they are not listed in E. The acidophilic (a) cells secrete hormones that act directly on target cells (non-glandotropic hormones) while the basophilic (b) cells stimulate subordinate endocrine cells (glandotropic hormones).

E Hormones of the anterior pituitary lobe (adenohypophysis)

Hormones and synonyms

Cell designation*

Hormone actions

Somatotropin (STH) Growth hormone (GH) Somatotropic hormone

Somatotropic (a)

Stimulates longitudinal growth; acts on carbohydrate and lipid metabolism

Prolactin (PRLorLTH)

Luteotropic hormone

Mammotropic hormone

Mammotropic (a)

Stimulates lactation and proliferation of glandular breast tissue

Follitropin (FSH)

Follicle-stimulating hormone

Gonadotropic (b)

Acts on the gonads; stimulates follicular maturation, spermatogenesis, estrogen production, expression of lutropin receptors and proliferation of granulosa cells

Lutropin (LH)

Interstitial cell stimulating hormone - ICSH

Luteinizing hormone

Gonadotropic (b)

Triggers ovulation; stimulates proliferation of follicular epithelial cells, production of testosterone in interstitial Leydig cells of the testis, and synthesis of progesterone; has general anabolic activity

Thyrotropin (TSH)

Thyroid stimulating hormone

Thyrotropic hormone

Thyrotropic (b)

Stimulates thyroid gland activity; increases O2 consumption and protein synthesis; influences carbohydrate and lipid metabolism

Corticotropin (ACTH)

Adrenocorticotropic hormone

Adrenotropic (b)

Stimulates hormone production in adrenal cortex; influences water and electrolyte balance; acts on carbohydrate formation in liver

Alpha/beta Melanotropin (MSH)

Melanotropic(b)

Aids in melanin formation and skin pigmentation; protects against UV radiation**

* Cells are classified as either acidophilic (a) or basophilic (b).

* * In humans, melanotropin serves as a neurotransmitter in various brain regions.

15.8 Epithalamus and Subthalamus

A Location of the epithalamus and subthalamus

Coronal section. The appropriateness of the term “epithalamus” can be appreciated inthis plane of section, which shows the epithalamus riding upon the thalamus (epi = “upon”). The epithalamus (green) consists of the following structures:

 Pineal gland (epiphysis), see B.

 Habenulae with the habenular nuclei, see D.

 Habenular commissure, see C.

 Stria medullaris, see D.

 Epithalamic commissure (posterior), see Ca.

The region of the subthalamus (orange), formerly called the ventral thalamus, initially lies directly below the thalamus, but during embryonic development is displaced laterally into the telencephalon by fibers of the internal capsule, forming the globus pallidas (see D, p. 211). The subthalamus contains nuclei of the medial motor system (motor zones of the diencephalon), and has connections with the motor nuclei of the tegmentum. In fact, the subthalamus can be considered the cranial extension of the tegmentum.

В Location of the pineal

a Posterior view, b Midsagittal section of the right hemisphere viewed from the medial side.

The pineal resembles a pine cone when viewed from behind. It is connected to the diencephalon by the habenula, which contains both afferent and efferent tracts. Its topographical relationship to the third ventricle is seen particularly well in midsagittal section (pineal recess). In reptiles, the calvaria over the pineal is thinned so that it is receptive to light stimuli. This is not the case in humans, although retinal afferents still communicate with the pineal through relay stations in the hypothalamus and the superior cervical (sympathetic) ganglion, helping to regulate circadian rhythms.

C Structure of the pineal gland

a Gross midsagittal tissue section, b Histological section.

a In the gross tissue section, the habenular commissure can be identified at the oral end of the pineal. Below it is the posterior (epithalamic) commissure. Between the two commissures is the CSF- filled pineal recess of the third ventricle. Calcifications (corpora arenacea, “brain sand") are frequently present and may be visible on radiographs: they have no pathological significance.

b The histological section demonstrates the specific cells of the pineal, the pinealocytes, which are embedded in a connective-tissue stroma and are surrounded by astrocytes. The pinealocytes produce melatonin, which plays a role in the regulation of circadian rhythms; it may be taken prophylactically, for example, to moderate the effects of jet lag. If the pineal ceases to function during childhood, the individual may undergo precocious puberty, as the pineal has significant, mostly inhibitory, effects on various endocrine systems.

D Habenular nuclei and their fiber connections

Midsagittal section of the right hemisphere viewed from the medial side. The habenula (“reins”) and their nuclei function as a relay station for afferent olfactory impulses. After their relay in the habenular nuclei, their efferent fibers are distributed to the salivatory and motor nuclei (mastication) in the brainstem.

Afferent connections (blue): Afferent impulses from the anterior perforate substance (olfactory area), septal nuclei, and preoptic region are transmitted by the stria medullaris to the habenular nuclei. These nuclei also receive impulses from the amygdala via the stria terminalis.

Efferent connections (red): Efferent fibers from the habenular nuclei are projected to the midbrain along three tracts:

 Habenulotectal tract: terminates in the roof of the mesencephalon, the quadrigeminal plate, supplying it with olfactory impulses.

 Habenulotegmental tract: terminates in the dorsal tegmental nucleus, establishing connections with the dorsal longitudinal fasciculus and with the salivatory and motor cranial nerve nuclei. (The smell of food stimulates salivation and gastric acid secretion: e.g., Pavlovian response).

 Habenulointerpeduncular tract: terminates in the interpeduncular nucleus, which then connects with the reticular formation.

E Subthalamic nuclei with their afferent (blue) and efferent (red) connections

The principal nucleus of the subthalamus is the globus pallidas, which is displaced laterally during development into the telencephalon by the internal capsule. A lamina of white matter subdivides the globus pallidus into a medial (internal) and lateral (external) segment. Certain small nuclei are exempt from this migration and remain near the midline: these are the zona incerta and subthalamic nucleus. The subthalamic nucleus, substantia nigra, and putamen send afferent fibers to the globus pal lîdus. The globus pa Hid us in turn distributes efferent fibers to these regions and also to the thalamus through a tract called the lenticular fasciculus. Functionally, these nuclei are classified as portions of the basal ganglia. Lesions of these nuclei lead to a movement disorder called contralateral hemiballism (the functional role of the subthalamus is described on p.340).

A Terms of location and direction in the brainstem

These terms are based on the nearly vertical Meynert brainstem axis (compare with the horizontal Forel axis, which was used as a reference line in previous units). Just dorsal to the brainstem is the cerebellum, which will be described in chapter 7.

В Relationship of the brainstem to the cerebral and cerebellar hemispheres

Basal view. The brainstem is a midline structure flanked by the cerebrum and cerebellum. Its anatomical subdivisions are best appreciated in the midsagittal section (see C). The third through twelfth pairs of cranial nerves (CN III—XII) enter or emerge from the brainstem (see Ea).

C Division of the brainstem into levels

Midsagittal section. The brainstem is divided macroscopically into three levels, with the bulge of the pons marking the boundary lines between the parts:

 Mesencephalon (midbrain)

 Pons

 Medulla oblongata

The location and contents of these parts are summarized in D. The three levels are easily distinguished from one another by gross visual inspection, although they are not differentiated in a functional sense. The functional organization of the brainstem (see D) is determined chiefly by the arrangement of the cranial nerve nuclei. Given the close proximity of nuclei and large fiber tracts in this region, even a small lesion of the brainstem (e.g., hemorrhage, tumor) may lead to extensive and complex alterations of sensorimotor function.

D Overview of the brainstem

Topographical organization

 Craniocaudal direction:

- Mesencephalon (midbrain)

- Pons

- Medulla oblongata

 Anteroposterior direction:

- Base (mesencephalon: cerebral peduncles; pons: basal part; medulla oblongata: pyramids)

- Tegmentum (present as such in all three parts)

- Section of ventricular system (upper part: cerebral aqueduct, fourth ventricle, central canal)

- Tectum (“roof”; present only in the mesencephalon; quadrigeminal plate)

 The cerebellum adjoins the brainstem dorsally.

Functional organization

 Mediolaterally into four iongitudinal nuclear columns:

- Somatic efferent (motor) column

- Visceral efferent (motor) column

- Visceral afferent (sensory) column

- Somatic afferent (sensory) column

 Organization into different structures:

- Nuclei of cranial nerves III—XII

- Red nucleus, substantia nigra (motor coordination centers)

- Reticular formation (diffuse nuclear aggregations for autonomic functions)

- Ascending and descending tracts (see p. 232)

- Dorsal column nuclei (nucleus gracilis and nucleus cuneatus)

- Pontine nuclei