The diencephalon is a division of the forebrain (embryonic prosencephalon), and is situated between the telencephalon and the midbrain (embryonic mesencephalon). It consists of structures that are on either side of the third ventricle, including the thalamus, the hypothalamus, the epithalamus and the subthalamus.

The diencephalon is one of the main vesicles of the brain formed during embryogenesis. During the third week of development a neural tube is created from the ectoderm, one of the three primary germ layers. The tube forms three main vesicles during the third week of development: the prosencephalon, the mesencephalon and the rhombencephalon. The prosencephlon gradually divides into the telencephalon and the diencephalon.

Diencephalon small
PrecursorProsencephalon, derived from the neural tube
Part ofHuman brain
PartsThalamus, the hypothalamus, the epithalamus and the subthalamus
NeuroLex IDbirnlex_1503
Anatomical terms of neuroanatomy


The diencephalon consists of the following structures:


The optic nerve (CNII) attaches to the diencephalon. The optic nerve is a sensory (afferent) nerve responsible for vision; it runs from the eye through the optic canal in the skull and attaches to the diencephalon. The retina itself is derived from the optic cup, a part of the embryonic diencephalon.


The diencephalon is the region of the embryonic vertebrate neural tube that gives rise to anterior forebrain structures including the thalamus, hypothalamus, posterior portion of the pituitary gland, and the pineal gland. The diencephalon encloses a cavity called the third ventricle. The thalamus serves as a relay centre for sensory and motor impulses between the spinal cord and medulla oblongata, and the cerebrum. It recognizes sensory impulses of heat, cold, pain, pressure etc. The floor of the third ventricle is called the hypothalamus. It has control centres for control of eye movement and hearing responses.

Additional images


Diagram depicting the main subdivisions of the embryonic vertebrate brain. These regions will later differentiate into forebrain, midbrain and hindbrain structures.


Reconstruction of peripheral nerves of a human embryo of 10.2 mm. (Label for Diencephalon is at left.)

See also


This article incorporates text in the public domain from page 807 of the 20th edition of Gray's Anatomy (1918)

External links


The brainstem (or brain stem) is the posterior part of the brain, continuous with the spinal cord. In the human brain the brainstem includes the midbrain, and the pons and medulla oblongata of the hindbrain. Sometimes the diencephalon, the caudal part of the forebrain, is included.The brainstem provides the main motor and sensory nerve supply to the face and neck via the cranial nerves. Of the thirteen pairs of cranial nerves, ten pairs (or twelve, if the diencephalon is included in the brainstem) come from the brainstem. The brainstem is an extremely important part of the brain as the nerve connections of the motor and sensory systems from the main part of the brain to the rest of the body pass through the brainstem. This includes the corticospinal tract (motor), the dorsal column-medial lemniscus pathway (fine touch, vibration sensation, and proprioception), and the spinothalamic tract (pain, temperature, itch, and crude touch).

The brainstem also plays an important role in the regulation of cardiac and respiratory function. It also regulates the central nervous system, and is pivotal in maintaining consciousness and regulating the sleep cycle. The brainstem has many basic functions including heart rate, breathing, sleeping, and eating.

Central nervous system

The central nervous system (CNS) is the part of the nervous system consisting of the brain and spinal cord. The central nervous system is so named because it integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish—and it contains the majority of the nervous system. Many consider the retina and the optic nerve (cranial nerve II), as well as the olfactory nerves (cranial nerve I) and olfactory epithelium as parts of the CNS, synapsing directly on brain tissue without intermediate ganglia. As such, the olfactory epithelium is the only central nervous tissue in direct contact with the environment, which opens up for therapeutic treatments.

The CNS is contained within the dorsal body cavity, with the brain housed in the cranial cavity and the spinal cord in the spinal canal. In vertebrates, the brain is protected by the skull, while the spinal cord is protected by the vertebrae. The brain and spinal cord are both enclosed in the meninges. In central nervous systems, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglial cells.

Cerebral aqueduct

The cerebral aqueduct, also known as the aqueductus mesencephali, mesencephalic duct, sylvian aqueduct, or aqueduct of Sylvius, is within the mesencephalon (or midbrain), contains cerebrospinal fluid (CSF), and connects the third ventricle in the diencephalon to the fourth ventricle within the region of the mesencephalon and metencephalon, located dorsal to the pons and ventral to the cerebellum.

Dentatothalamic tract

The dentatothalamic tract (or dentatorubrothalamic tract) is a tract which connects the dentate nucleus and the thalamus while sending collaterals to the red nucleus.The term "dentatorubrothalamocortical" is sometimes used to emphasize termination in the cerebral cortex.


The epithalamus is a (dorsal) posterior segment of the diencephalon. The diencephalon is a part of the forebrain that also contains the thalamus, the hypothalamus and pituitary gland.

The epithalamus includes the habenula and their interconnecting fibers, the habenular commissure, the stria medullaris and the pineal gland.

Fields of Forel

The fields of Forel are areas in a deep part of the brain known as the diencephalon. They are below the thalamus and consist of three defined, white matter areas of the subthalamus. These three regions are also named "H fields":

Field H1, is the thalamic fasciculus, a horizontal white matter tract composed of the ansa lenticularis, lenticular fasciculus, and cerebellothalamic tracts between the subthalamus and the thalamus. These fibers are projections to the ventral anterior and ventral lateral thalamus from the basal ganglia (globus pallidus) and the cerebellum. H1 is separated from H2 by the zona incerta.

Field H2 (synonymous with lenticular fasciculus) is also made up of projections from the pallidum to the thalamus, but these course the subthalamic nucleus (dorsal).

Field H (sometimes called field H3) is a large zone of mixed grey and white matter from the pallidothalamic tracts of the lenticular fasciculus and the ansa lenticularis which combine in an area just in front of the red nucleus. The grey matter from this field is said to form a prerubral nucleus.


In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three primary brain vesicles during the early development of the nervous system. The forebrain controls body temperature, reproductive functions, eating, sleeping, and the display of emotions.

At the five-vesicle stage, the forebrain separates into the diencephalon (thalamus, hypothalamus, subthalamus, epithalamus, and pretectum) and the telencephalon which develops into the cerebrum. The cerebrum consists of the cerebral cortex, underlying white matter, and the basal ganglia.

By 5 weeks in utero, it is visible as a single portion toward the front of the fetus. At 8 weeks in utero, the forebrain splits into the left and right cerebral hemispheres.

When the embryonic forebrain fails to divide the brain into two lobes, it results in a condition known as holoprosencephaly.

Habenular commissure

The habenular commissure, is a brain commissure (a band of nerve fibers) situated in front of the pineal gland that connects the habenular nuclei on both sides of the diencephalon.

The habenular commissure is part of the habenular trigone (a small depressed triangular area situated in front of the superior colliculus and on the lateral aspect of the posterior part of the tænia thalami). The trigonum habenulæ also contains groups of nerve cells termed the ganglion habenulæ. Fibers enter the trigonum habenulæ from the stalk of the pineal gland, and the habenular commissure. Most of the trigonum habenulæ's fibers are, however, directed downward and form a bundle, the fasciculus retroflexus of Meynert, which passes medial to the red nucleus, and, after decussating with the corresponding fasciculus of the opposite side, ends in the interpeduncular nucleus.

Hypothalamic sulcus

The hypothalamic sulcus (sulcus of Monro) is a groove in the lateral wall of the third ventricle, marking the boundary between the thalamus and hypothalamus. The upper and lower portions of the lateral wall of the third ventricle correspond to the alar lamina and basal lamina, respectively, of the lateral wall of the fore-brain vesicle and are separated from each other by a furrow, the hypothalamic sulcus, which extends from the interventricular foramen to the cerebral aqueduct.

Lemniscus (anatomy)

A lemniscus (Greek for ribbon or band) is a bundle of secondary sensory fibres in the brainstem. The medial lemniscus and lateral lemniscus terminate in specific relay nuclei of the diencephalon. The trigeminal lemniscus is sometimes considered as the cephalic part of the medial lemniscus.

Lenticular fasciculus

The lenticular fasciculus is a tract connecting the globus pallidus to the thalamic fasciculus. It is synonymous with field H2 of Forel. The thalamic fasciculus is (composed of the lenticular fasciculus and ansa lenticularis) runs into the thalamus.

It connects the globus pallidus to the thalamus.

List of regions in the human brain

The human brain anatomical regions are ordered following standard neuroanatomy hierarchies. Functional, connective, and developmental regions are listed in parentheses where appropriate.

Mammillary body

The mammillary bodies are a pair of small round bodies, located on the undersurface of the brain that, as part of the diencephalon, form part of the limbic system. They are located at the ends of the anterior arches of the fornix. They consist of two groups of nuclei, the medial mammillary nuclei and the lateral mammillary nuclei.Neuroanatomists have often categorized the mammillary bodies as part of the hypothalamus.


The midbrain or mesencephalon (UK: , US: ; from Greek mesos 'middle', and enkephalos 'brain') is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation.

Optic cup (embryology)

During embryonic development of the eye, the outer wall of the bulb of the optic vesicles becomes thickened and invaginated, and the bulb is thus converted into a cup, the optic cup (or ophthalmic cup), consisting of two strata of cells. These two strata are continuous with each other at the cup margin, which ultimately overlaps the front of the lens and reaches as far forward as the future aperture of the pupil.

The optic cup is part of the diencephalon and gives rise to the retina of the eye.


The subthalamus or prethalamus is a part of the diencephalon. Its most prominent structure is the subthalamic nucleus. The subthalamus connects to the globus pallidus, a basal nucleus of the telencephalon.


The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter in the dorsal part of the diencephalon of the brain with several functions such as relaying of sensory signals, including motor signals to the cerebral cortex, and the regulation of consciousness, sleep, and alertness.It is a midline symmetrical structure of two halves, within the vertebrate brain, situated between the cerebral cortex and the midbrain.

It is the main product of the embryonic diencephalon, as first assigned by Wilhelm His Sr. in 1893.

Third ventricle

The third ventricle is one of four connected fluid-filled cavities comprising the ventricular system within the mammalian brain. It is a median cleft in the diencephalon between the two thalami, and is filled with cerebrospinal fluid (CSF).

It is in the midline, between the left and right lateral ventricles. Running through the third ventricle is the interthalamic adhesion, which contains thalamic neurons and fibers that may connect the two thalami.

Zona limitans intrathalamica

The zona limitans intrathalamica (ZLI) is a lineage-restriction compartment and primary developmental boundary in the vertebrate forebrain (which is analogous to the human cerebrum) that serves as a signaling center and a restrictive border between the thalamus (also known as the dorsal thalamus) and the prethalamus (ventral thalamus).

Sonic hedgehog (shh) signaling from the ZLI is crucial in the development of the diencephalon, which develops into the thalamus, the pretectum, and the anterior tegmental structures. The ZLI together with the prethalamus and thalamus make up the mid-diencephalic territory (MDT).

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