Diencephalon

Borders

The third ventricle is a narrow vertical midline cleft between and below the two lateral ventricles and in between left and right thalami. The lateral ventricles communicate with the third ventricle via the interventricular foramen of Monro. It also communicates with the fourth ventricle posteroinferiorly via the cerebral aqueduct of Sylvius.

It possesses a roof, a floor and four walls: anterior, posterior and two lateral.

• The roof is formed by the thin tela choroidea, which is a combination of two membranes, the ependyma and pia mater. Within the tela choroidea are two plexuses of blood vessels (one on either side of the middle line) that bulge downwards into the cavity of the third ventricle. These are the choroid plexuses of the third ventricle which functions as a point of production of the cerebrospinal fluid (CSF).  
• The floor is made up of the optic chiasm, the tuber cinereum and infundibulum, the mammillary bodies, the posterior perforated substance and the uppermost part of the mesencephalic tegmentum.
• The anterior wall is the delicate lamina terminalis, as well as the anterior commissure and anterior column of the fornix..
• The short posterior wall is formed by the stalk of the pineal gland, posterior commissure and the Habenular commissures.
• The lateral walls are of the cavity are formed by the medial walls of each thalami. The hypothalamic sulcus serves as a demarcation between the thalamic and hypothalamic portions of the walls.

Blood supply

The diencephalon is richly supplied by several blood vessels, notably the thalamogeniculate branches of the posterior cerebral artery and thalamoperforating branches of the posterior cerebral artery and posterior communicating artery.

Nuclei

The thalamus also comprises several nuclei, which are divided into 5 groups: anterior, medial, lateral, intralaminar and reticular nuclei.

Thalamic nuclei are in charge for many important functions, such as relaying of sensory and motor signals, and the regulation of consciousness, sleep, and alertness.

Note that the location of the thalamic reticular nucleus is often disputable. For now, it is listed as the part of the ventral thalamus that forms a capsule around the thalamus laterally. However, recent researches question this statement and define it as a part of the dorsal thalamus.

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Surfaces

The anterior pole of the thalamus lies just behind the interventricular foramen of Monro. Its posterior pole makes up the pulvinar, and lies just superior and lateral to the superior colliculus. The medial and lateral geniculate bodies are connected to the inferior and superior colliculi by the inferior and superior brachium quadrigeminum, respectively.

The medial surface forms the greater part of the lateral wall of the third ventricle, and is lined by ependyma. A mass of grey matter called the interthalamic adhesion (connexus) is found attached to the medial surfaces of the two thalami and this connexus interconnects the left and right thalami. Inferiorly, the medial surface is separated from the hypothalamus by the hypothalamic sulcus.

The internal capsule is related to the lateral surface of the thalamus and separates this surface from the lentiform nucleus (globus pallidus and putamen) of the basal ganglia. The thalamostriate vein and a bundle of fibres called the stria terminalis are in close apposition to the dorsal or superior surface of the thalamus, and these structures separate the thalamus from the caudate nucleus of the basal ganglia. The inferior surface of the thalamus is related to the hypothalamus.

The thalamus and caudate nucleus form the floor of the central part of the lateral ventricles. However, the medial part of the superior surface of the thalamus is separated from these ventricles by the fornix, and by a fold of pia mater called the tela choroidea. At the junction of the medial and lateral surfaces of the thalamus the ependyma of the third ventricle is reflected from the lateral wall to the roof. The line of reflection is marked by a line called the taenia thalami, underneath which there is a narrow bundle of fibres called the stria medullaris thalami.

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Characteristics and components

The subthalamus refers to the part of the diencephalon that lies below the posterior part of the thalamus just behind and lateral to the hypothalamus. It includes nuclei and grey matter such as the zona incerta, reticular nucleus, and the perigeniculate nucleus. The zona incerta provides gamma-aminobutyric acid stimulation (GABAergic) to the thalamus; while the reticular nucleus provides GABAergic regulation to the thalamocortical pathway. Collectively, the pre-geniculate nucleus and the lateral geniculate nucleus form the lateral geniculate complex.

The subthalamus is continuous with the upper ends of the red nucleus and substantia nigra of the tegmentum of the midbrain inferiorly. Laterally it reaches the lowest part of the internal capsule.

Before going any further in your quest to learn the diencephalon, make sure you've mastered the basics. Our free quizzes on the parts of the brain are a great place to start.

Connections

The pre-geniculate nucleus has connections similar to the lateral geniculate nucleus and includes fibres from the retina, pretectal region and superior colliculus, thus playing a role in vision and eye movements. The zona incerta has strong connections with the reticular nucleus of the thalamus which it connects as a thin lamina of grey matter. It is believed to be involved in the regulation of visceral activities such as sexuality, hydration and food intake as well as cardiovascular activity.

Fibres emerging from the dorsal thalamus (thalamus proper) traverse the ventral thalamus through the reticular nucleus. These fibres give the nucleus a reticulated appearance, hence the name. The reticular nucleus is made of a thin layer of neurons covering the lateral aspect of the dorsal thalamus, and is also related to the internal capsule. Inferiorly it becomes partially continuous with the zona incerta. The main efferent fibers of the reticular nucleus pass to the dorsal thalamus; however afferents reach it from the nucleus cuneiformis (in the reticular formation of the midbrain).

Other connections of the subthalamus include afferents from the cerebral cortex, putamen, globus pallidus, trigeminal nuclei, cerebellar nuclei and pretectal region. These afferents reach the reticular nucleus and zona incerta through which it sends efferent fibers to those structures and the spinal cord. The ventral thalamus also has connections with the dentate nucleus, cerebral peduncle, fasciculus lenticularis, ansa lenticularis, pre-rubral field and thalamic fasciculus.

Medial geniculate body

The medial geniculate body receives tonotopically (arrangement of sound based on frequency on the brain) organized auditory information from the inferior colliculus of the quadrigemini, via the brachium of the inferior colliculus. Processed information from the medial geniculate body is then relayed via the auditory radiation to the primary auditory cortex on the transverse gyrus of Heschel. The medial geniculate body consists of three major divisions:

• the dorsal division, consisting of the posterior part of the parvocellular nucleus
• the medial division, or magnocellular nucleus
• the ventral division, consisting of the ventral part of the parvocellular nucleus.

Lateral geniculate body

The lateral geniculate body receives retinotopic (mapping of visual input on the brain) input from the contralateral visual field via the optic tract. Information is relayed from the lateral geniculate in a topographic manner via part of the optic radiation (called Meyer’s loop) to the primary visual cortex. The lateral geniculate body consists of layers of neurons, and the visual information received is divided among the various layers.

Relations

As the name suggests, the hypothalamus is the part of the diencephalon that lies below the thalamus. Like the thalamus, it comprises several subdivisions and nuclei, including the periventricular zone, the medial zone and the lateral zone. On the medial side, it forms the wall of the third ventricle below the level of the hypothalamic sulcus. Posteriorly, the hypothalamus merges with the ventral thalamus, and through it with the tegmentum of the midbrain.

Anteriorly, it extends up to the lamina terminalis, and merges with certain olfactory structures in the region of the anterior perforated substance. Inferiorly, the hypothalamus is related to structures in the floor of the third ventricle. These are the tuber cinereum, the infundibulum, and the mammillary bodies, which are considered part of the hypothalamus. Via infundibulum, the hypothalamus connects to pituitary gland.

Learn the hypothalamus anatomy and function with our articles, video tutorials, quizzes and labeled diagrams.

Nuclei and areas

From medial to lateral, hypothalamus is divided into three zones: periventricular, medial and lateral. Anterior to posterior, there are three regions of hypothalamus; anterior, middle and posterior.

The hypothalamic nuclei are located within these regions, and functionally, there are 4 groups of nuclei: preoptic, supraoptic (chiasmatic), infundibular (tuberal) and mammillary. 

Functionally the hypothalamus is involved in the control of several cognitive behaviours, mainly owing to its connections with anatomical areas responsible for or involved in these activities. However the main functions attributed to the hypothalamus include the regulation of eating and drinking behaviour, regulation of sexual activity and reproduction, and the control of autonomic activity.

Other activities of the hypothalamus include regulation of emotional behaviour, control of several  endocrine activities, response to stress, temperature regulation, as well as the control of circadian rhythms.

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