The Diencephalon

The auditory system | Main Anatomy Index | The hypothalamus

• The diencephalon are forebrain structures derived from the embryological diencephalic vesicles.
• It is surrounded by cerebral cortex.

• This is conventionally divided into 4 parts:

1. The epithalamus, which included the pineal gland and nearby structures;
2. The thalamus (G. inner chamber);
3. The subthalamus;
4. The hypothalamus.

• Rostral: plane through the optic chiasm and anterior commissure.
• Caudal: plane through the posterior commissure and the caudal edge of the mammillary bodies.

• The rostral and caudal boundaries are approximate and semiarbitrary.
• Functionally continuous tissues extend through both boundaries.

• Medial: wall of third ventricle, stria medullaris thalami and massa intermedia.
• Lateral: the internal capsule, tail of caudate and stria terminalis.

• Dorsal: the fornix and floor of the lateral ventricles.

• Because of the cephalic flexure, the axis of the diencephalon is inclined about 100 degrees with respect to the axis of the midbrain.

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• This includes the pineal gland and the habenular nuclei and their connections.

• This is a midline, unpaired structure.
• It is situated just rostral to the superior colliculi.

• It resembles a pinecone in shape and hence its name.

• The pineal gland consists of pinealocytes (collection of secretory cells), some glial cells and a rich vascular network.
• It receives light-regulated input via a circuitous pathway:

• In fish, amphibians, and many reptiles, it is involved in circadian and circannual rhythms.
• The mammalian pineal is an endocrine organ involved with reproductive cycles.
• It has, however, no known neural output.

• It secretes an antigonadotropic hormone called melatonin at relatively high rates during darkness.
• Therefore, an increase in day lengths leads to increased gonadal function.

• By the age of 17, calcareous concretions accrue in it (brain sand).
• This makes it opaque to x-rays, hence making it a useful radiological landmark.

• This is a small swelling on each side, rostral to the stalk of the pineal gland.
• Underlying each habenula are the habenular nuclei.
• The 2 habenula are interconnected by the small habenular commissure.

• Each habenula receives one major input bundle, the stria medullaris thalami.
• Each also gives rise to one major output bundle, the habenulointerpeduncular tract (fasciculus retroflexus).

• This extends from the habenula to the interpeduncular nucleus.
• This is located between the cerebral peduncles in the reticular formation of the rostral midbrain.

• This underlies a horizontal ridge on the dorsomedial surface of the thalamus.
• To this surface, the roof of the third ventricle is attached.

• The fibres of this originate in various limbic structures.
• The pathway through the habenula is one route the limbic system can influence the brainstem reticular formation.

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• The midbrain tegmentum continues into the diencephalon as the subthalamus.
• This area is completely surrounded by neural tissue.

• It is located:

1. Inferior to the thalamus;
2. Lateral to the hypothalamus;
3. And medial to the basis pedunculi and internal capsule.

• The subthalamus contains rostral potions of the red nucleus and substantia nigra.
• It is traversed by somatosensory pathways on their way to the thalamus, as well as several pathways involving the cerebellum and basal ganglia.

• In addition, the subthalamus contains the subthalamic nuclei.

• This is a lens-shaped, bi-convex structure.
• It is located medial and superior to portions of the basis pedunculi and internal capsule.

• This nucleus is interconnected with the basal ganglia.

• This is a small mass of grey matter intervening between the subthalamic nuclei and the thalamus.
• It appears to be a continuation of the midbrain reticular formation.
• It has widespread connections, including direct projections to the cerebral cortex.

• Its function is largely unknown.

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• This is a large, egg-shaped nuclear mass.
• It makes up about 80% of the mass of the diencephalon.

• It extends:

1. Anteriorly to the interventricular foramen;
2. Superiorly to the transverse cerebral fissure;
3. Inferiorly to the hypothalamic sulcus;
4. And posteriorly it overlaps the midbrain.

• The thalamus is part of a remarkably large number of pathways.
• All sensory pathways relay in the thalamus.
• Many anatomical loops comprising cerebellar, basal ganglionic, and limbic pathways also involve thalamic relays.
• The various systems utilise separate portions of the thalamus.
• The thalamus is therefore subdivided into a series of nuclei.

• Each thalamic nucleus receives inputs from one or more subcortical sources.
• They also receive feedback inputs from the cortical areas to which the nucleus projects.

• These feedback inputs appear to provide a means by which the cerebral cortex can control its own inputs.

• The thalamic nuclei can be distinguished from each other by:

1. Their topographical locations in the thalamus;
2. And the pattern of their connections with the cerebral cortex.

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Click here for a schematic diagram of the thalamus.

• This is a thin, curved sheet of myelinated fibres.
• It divides most of the thalamus into medial and lateral groups of nuclei.

• Anteriorly, the internal medullary lamina bifurcates and encloses the anterior nucleus (AN).

• This group contains only one large nucleus, the dorsomedial nucleus (DM).

• This group comprises the bulk of the thalamus.
• It is further subdivided into a ventral and dorsal tier.

• The dorsal tier consists of:

1. The large pulvinar;
2. The lateral posterior nucleus (LP);
3. And the lateral dorsal nucleus (LD).

• The lateral posterior nucleus is continuous with the pulvinar.
• Both nuclei have somewhat similar connections so the two are sometimes referred together as the pulvinar/LP complex.

• The ventral tier consists of 3 nuclei arranged along an anterior-posterior line:

1. The ventral anterior nucleus (VA);
2. The ventral lateral nucleus (VL);
3. And the ventral posterior nucleus (VP).

• The ventral posterior nucleus is further subdivided into the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei.
• The VPL is the somatosensory relay of the body.
• The VPM is the somatosensory relay of the head.

• Some nuclei do not fit into the above framework:

1. The lateral geniculate nucleus (visual system) and medial geniculate system (auditory system) are located posterior and a bit ventral to the VPL/VPM.
• They protrude posteriorly alongside the midbrain.
• These 2 nuclei are conveniently considered as a posterior extension of the ventral tier.
2. The internal medullary lamina, a certain locations, splits and encloses groups of cells.
• These nuclei are collectively called the intralaminar nuclei.
• The 2 largest are the centromedian (CM) and parafascicular (PF) nuclei.
• The centromedian nucleus is a large, round nucleus located medial to the VPL/VPM.
• The VPM conforms to the rounded shape of the CM.
• The parafascicular nucleus is located medial to the CM.
• The habenulointerpeduncular tract (fasciculus retroflexus) passes through it.
3. The external medullary lamina is a 2nd curved sheet of myelinated fibres that cover the lateral surface of the thalamus.
• The thin sheet of cells that intervenes between the external medullary lamina and the internal capsule is the reticular nucleus.
• The reticular nucleus looks continuous with the zona incerta.
• This, however, appears to be of no functional significance.
4. The midline nuclei of the thalamus are a thin layer of cells that covers part of the medial surface of the thalamus.
• They are essentially rostral continuations of parts of the periaqueductal grey.
• The midline nuclei of the 2 sides fuse in the interthalamic adhesion (when it is present).

• These receive well-defined bundles of fibres and projects specifically to particular functional areas of the cerebral cortex.

• The sensory relay nuclei include:
1. The VPL/VPM;
2. And the geniculate nuclei.

• The motor relay nuclei receive superior cerebellar peduncle and various outputs from the basal ganglia and project to the motor and premotor cortex.
• They include:
1. The VA;
2. And the VL.

• The AN is the principle relay nucleus of the limbic system.
• It receives the mammillothalamic tract and projects to the cingulate gyrus.

• These nuclei receive their inputs from a variety of places and project to fairly broad areas of association cortex.
• The 2 great areas of association cortex are:
1. The prefrontal cortex anterior to the motor areas of the frontal lobe;
2. And the parietal-occipital-temporal association cortex, occupying the area surrounded by the primary somatosensory, visual, and auditory cortices.

• Corresponding to these 2 large areas are 2 large association nuclei.

• The dorsomedial nucleus is reciprocally interconnected with the prefrontal cortex.
• It is involved with prefrontal functions such as affect and foresight.

• Bilateral damage to the DM or its connections has effects similar to those of a prefrontal lobotomy.

• Major inputs to the DM, in addition to those of the prefrontal cortex, come from various elements of the limbic system, e.g., the amygdala.

• The pulvinar/LP complex is reciprocally interconnected with the parietal-occipital-temporal association cortex.
• The major inputs to this complex, apart from those arising in the association cortex, come from parts of the visual system.
• The role of the pulvinar and the LP is, however, almost entirely unknown.

• These nuclei project to extremely widespread areas of the cortex.

• The inputs to these nuclei are derived from:

1. The basal ganglia;
2. The cerebellum;
3. The brainstem reticular formation;
4. And the spinothalamic and spinoreticulothalamic fibres (carrying information about dull, aching pain).

• The major output projections are to noncortical targets such as the basal ganglia.
• These efferents, however, also send collaterals to the cerebral cortex.

• The broad extent of the cortical connections of the non-specific nuclei make them suitable for general roles such as regulation the level of cortical excitability.
• They are considered to be one route through which the ascending reticular activating system affects the cortex.

• Although the centromedian nucleus is one of the intralaminar nuclei, it has a special relationship with the basal ganglia.
• It probably has a different role to that of the other non-specific nuclei.

• This nucleus has no projections to the cerebral cortex at all.

• The output of each portion of the reticular nucleus goes to that thalamic nucleus from which it receives its input.

• The reticular nucleus controls thalamic output.
• There are some indicators that it may have a major role in modulating transmissions from the thalamus to the cortex during different phases of the sleep-wake cycle.

Table derived from Nolte (1993) The Human Brain 3rd Ed. Table 7, p. 259 and Dr. Robinson UNSW Neuroanatomy (ANAT 2007) Lecture Handout, 31 August 1998.

• This is mostly from branches (ganglionic or perforating) of the posterior cerebral artery.
• Specific branches of the posterior choroidal artery supply some dorsomedial regions.

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• This is a compact bundle of fibres through which the large collections of fibres pass, including:

1. The thalamocortical and corticothalamic fibres;
2. The corticopontine, corticobulbar and corticospinal fibres pass through the internal capsule and then the cerebral peduncles;
3. And the fibres projection from the cerebral cortex to the various nuclei of the extrapyramidal system (e.g., the putamen and caudate nucleus).

• It is a continuous sheet of fibres that forms the medial boundary of the lenticular nucleus.
• It continues around posteriorly and inferiorly to partially envelop this nucleus.

• Inferiorly, many of the fibres of the internal capsule funnel into the cerebral peduncles.
• Superiorly, the fibres fan out into the corona radiata.
• Here, they travel in the cerebral white matter to reach their cortical origins or destinations.

• The entire fibre system is shaped like a trumpet with a large notch cut out of its bell.
• The notch corresponds to the location of where this sheet of fibres is interrupted by the lateral sulcus.
• The lenticular nucleus sits where a mute would sit in a trumpet.

• The internal capsule is divided into 5 regions:

1. The anterior limb is the portion between the lenticular nucleus and the head of the caudate nucleus;
2. The posterior limb is the portion between the lenticular nucleus and the thalamus;
3. The genu the portion at the junction of the above 2 parts and is adjacent to the interventricular foramen;
4. The retrolenticular part is the portion posterior to the lenticular nucleus;
5. The sublenticular part is the portion inferior to the lenticular nucleus.

This table was contributed by David Boshell.

• Lesions to the internal capsule are called subcortical lesions.

Click here to go to "The Hypothalamus".

• This is a small portion of the diencephalon (4 grams).
• It is, however, an important in the pathways concerned with autonomic, endocrine, emotional, and somatic functions.

• The connections of the hypothalamus are widespread and complex but they fall into 3 general categories:

1. Interconnections with various components of the limbic system;
2. Outputs which influence the pituitary gland;
3. Interconnections with various visceral and somatic nuclei.

• The inferior surface is directly exposed to the subarachnoid space.
• It is bounded by the optic chiasm, the optic tracts, and the posterior edge of the mammillary bodies.

• The medial surface extends anteriorly to the lamina terminalis.
• It extends superiorly to the hypothalamic sulcus.
• It extends posteriorly to the caudal edge of the diencephalon.

• It is subdivided longitudinally into 3 regions:

1. The anterior region, is above the optic chiasm;
2. The tuberal region, is above the tuber cinereum;
3. And the posterior region is above and including the mammillary bodies.

• Furthermore, the entire hypothalamus on each side is divided into medial and lateral regions by a parasagittal plane through the fornix.

Nolte (1993) The Human Brain, An Introduction to its Functional Anatomy 3rd Ed. Mosby-Year Book, Inc., St. Louis, Missouri, USA. Table 8, p. 265.

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