Diencephalon: Definition, Structures, Pineal Gland and Functions

It consists of structures that are on either side of the third ventricle, and includes the thalamus, hypothalamus , epithalamus, and subthalamus.

The diencephalon is one of the main vesicles in the brain that forms 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 forebrain, midbrain, and rhombencephalon. The prosencephalon gradually divides into the telencephalon and the diencephalon. The diencephalon is made up of the following structures:


The thalamus (from the Greek θάλαμος, “chamber”) is the large mass of gray matter in the dorsal part of the diencephalon of the brain with various functions such as the transmission of sensory signals, including motor signals, to the cerebral cortex, and the regulation of consciousness, sleep and alertness.

It is a symmetrical midline structure of two halves, within the vertebrate brain, located between the cerebral cortex and the midbrain. It is the main product of the embryonic diencephalon, as first assigned by Wilhelm Hi s, Sr. in 1893.

The thalamus is located in the forebrain, which is superior to the midbrain, near the center of the brain, with nerve fibers projecting into the cerebral cortex in all directions.

The medial surface of the thalamus constitutes the upper part of the lateral wall of the third ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, the interthalamic adhesion.

The thalamus has multiple functions, it is generally believed to act as a relay station, or hub, transmitting information between different subcortical areas and the cerebral cortex.

In particular, each sensory system (with the exception of the olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area.

For the visual system, for example, inputs from the retina are sent to the lateral geniculate nucleus of the thalamus, which in turn projects to the visual cortex in the occipital lobe.

The thalamus is believed to process both sensory information and transmit it: each of the primary areas of the sensory relay receives strong feedback connections from the cerebral cortex.

Similarly, the medial geniculate nucleus acts as a key auditory relay between the lower colliculus of the midbrain and the primary auditory cortex.

The ventral posterior nucleus is a key somatosensory relay, sending tactile and proprioceptive information to the primary somatosensory cortex.

Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamic-corticothalamic circuits that are believed to be involved with consciousness. Damage to the thalamus can lead to a permanent coma.

The role of the thalamus in the most anterior pale and nigral territories in alterations of the basal ganglia system is recognized but not well understood.

The contribution of the thalamus to vestibular or tectal functions is almost ignored. The thalamus has been thought of as a “relay” that simply sends signals to the cerebral cortex.

More recent research suggests that thalamic function is more selective. Many different functions are tied to various regions of the thalamus.

This is the case for many of the sensory systems (except the olfactory system), such as the auditory, somatic, visceral, taste, and visual systems, where localized lesions cause specific sensory deficits.

An important role of the thalamus is in supporting motor and language systems, and much of the circuits involved for these systems are shared.

The thalamus is functionally connected to the hippocampus as part of the extended hippocampal system in the thalamic anterior nuclei with respect to spatial memory and spatial sensory data, they are crucial for human episodic memory and rodent event memory.

There is support for the hypothesis that the connection of the thalamic regions to particular parts of the mesiotemporal lobe provides a differentiation of the functioning of memory of recall and familiarity.

The neural information processes necessary for motor control were proposed as a network involving the thalamus as a subcortical motor center.

Through investigations of primate brain anatomy, the nature of the interconnected tissues of the cerebellum to the multiple motor cortices suggested that the thalamus plays a key role in providing the specific channels from the basal ganglia and cerebellum to the motor areas. cortical.

In an investigation of the saccadic and antisaccadic motor response in three monkeys, it was found that the thalamic regions were involved in the generation of antisaccadic eye movement (that is, the ability to inhibit the reflexive shaking movement of the eyes in the direction of a presented stimulus).

Recent research suggests that the mediodorsal thalamus may play a broader role in cognition. Specifically, the mediodorsal thalamus can:

‘Amplify the connectivity (signal strength) of only the circuits in the cortex appropriate to the current context and thus contribute to the flexibility (of the mammalian brain) to make complex decisions by wiring the many associations on which they depend decisions in weakly connected cortical circuits’.

The researchers found that “enhancing DM activity increased the mice’s ability to ‘think’, reducing their error rate by more than 25 percent when deciding which contradictory sensory stimuli to follow to find the reward.”

Hypothalamus, including posterior pituitary / posterior pituitary


The hypothalamus (from the Greek ὑπό, “under” and θάλαμος, thalamus) is a portion of the brain that contains a series of small nuclei with a variety of functions.

The hypothalamus is located below the thalamus and is part of the limbic system . In neuroanatomy terminology, it forms the ventral part of the diencephalon.

It synthesizes and secretes certain neurohormones, called releasing hormones or hypothalamic hormones, and these in turn stimulate or inhibit the secretion of pituitary hormones.

In mammals, the magnocellular neurosecretory cells of the paraventricular nucleus and the supraoptic nucleus of the hypothalamus produce neurohypophyseal hormones, oxytocin, and vasopressin.

These hormones are released into the blood in the posterior pituitary. Much smaller parvocellular neurosecretory cells, neurons in the paraventricular nucleus, release corticotropin-releasing hormone and other hormones into the pituitary portal system, where these hormones diffuse to the anterior pituitary.

Posterior pituitary / posterior pituitary

The posterior pituitary is not glandular like the anterior pituitary. The posterior pituitary is the neural tissue and consists solely of the distal axons of the hypothalamic magnocellular neurons that make up the neurohypophysis.

The perikarya ( cell bodies ) of these axons are located in paraventricular and supraoptic nuclei of the hypothalamus.

During embryogenesis, the neuroepithelial cells of the lining of the third ventricle mature into magnocellular neurons as they migrate laterally to and above the optic chiasm to form the supraoptic nuclei and to the walls of the third ventricle to form the paraventricular nuclei.

In the posterior pituitary, the axon terminals of magnocellular neurons contain neurosecretory granules, membrane-bound bundles of hormones stored for later release.

The blood supply for the anterior pituitary gland is through the hypothalamic / pituitary portal system, but the posterior pituitary gland is supplied directly from the inferior pituitary arteries, which are branches of the posterior and internal communicating arteries of the carotid.

Drainage is done in the cavernous sinus and the internal jugular vein. The posterior pituitary hormones, oxytocin and vasopressin, are synthesized for the most part in hormone-specific magnocellular neurons, although a small number of neurons (approximately 3%) express both peptides.

The supraoptic nucleus is relatively simple, with 80% to 90% of neurons producing vasopressin3 and practically all axons projecting towards the posterior pituitary. The organization of the paraventricular nuclei, however, is much more complex and varies between species.

In humans, there are five sub-nuclei and parvocellular divisions (smaller cells) that synthesize other peptides, such as corticotropin-releasing hormone, thyrotropin-releasing hormone, somatostatin, and opiates.

Parvocellular neurons project to the median eminence, brain stem, and spinal cord, where they play a role in a variety of autonomic neuroendocrine functions.

The suprachiasmatic nucleus, which lies in the basal midline and anterior to the third ventricle, also synthesizes vasopressin and controls circadian rhythms as well as seasonal rhythms.

The main stimulating neurotransmitter in the neurohypophysis is glutamate with stimulating noradrenergic inputs that act by stimulation of glutamate. Glutamate receptors represent 25% of synapses in magnocellular neurons.

The main inhibitory input is γ-aminobutyric acid, which represents between 20% and 40% of the synaptic input to magnocellular neurons.

Phasic activation of vasopressin neurons is the most efficient activity pattern for vasopressin release from axon terminals.

Phasic activity is controlled by glutamate stimulation and opiate inhibition.

Dynorphin is synthesized in vasopressin neurons and is co-released with vasopressin from dendrites at the somatic level, where it acts autocrine to inhibit the activity of vasopressin neurons, contributing to the phasic firing pattern.

One of the most remarkable aspects of the magnocellular system is the plasticity of the system in response to prolonged stimulation. This plasticity is of greatest importance in humans during labor and lactation.


The epithalamus comprises the thalamus, the hypothalamus, and also the pituitary gland. It is found in the dorsal aspect of the diencephalon. It also includes the habenula, the pineal gland, and also the stria medullaris.

The thalamus and hypothalamus have been described previously. The following tables highlight these different structures, their components, and also the functions of each of the areas.

The epithalamus is involved in many functions, including motor control, the sleep-wake cycle, and responses to stress.

The habenula is highly conserved in vertebrates and acts as a critical node connecting the forearm brain with the midbrain and the hindbrain receiving input from the limbic system and basal ganglia and projecting to the monoaminergic nuclei.

The pineal gland is critical for the regulation of circadian rhythms due to its production of melatonin, and the choroid plexus synthesizes cerebrospinal fluid and many growth factors.

Including fibroblasts and insulin-like and platelet-derived growth factors, and it performs important functions, such as providing a pathway for nutrients and removing by-products of metabolism.

Epithalamus dysfunction has been reported to be related to mood disorders such as major depression, schizophrenia, and sleep disorders. However, knowledge regarding the epithalamus development process is limited.

During early development, the parent domain in the diencephalon divides into three prosomers (p), ie, p1, p2, and p3, along the anterior-posterior axis. P1 and p3 give rise to the pretectum and prethalamus, respectively.

The most dorsal region of p2 produces the epithalamus, and the other part generates the thalamus. In the presumed epithelial progenitor domain, the most anterior area containing the roof plate becomes the third ventricular choroid plexus, while the adjacent part generates the habenular commissure, paired habenules, and the pineal gland.

Previously, a member of the fibroblast growth factor family, fibroblast growth factor, has been reported to regulate habenule and pineal gland development in a dose-dependent manner.

In zebrafish, fibroblast growth factor signaling also controls the specification of the pineal complex. However, the molecular and cellular mechanisms underlying the development of the epithalamus remain largely unknown.

Foxg1 encodes a winged helix transcriptional repressor and has been reported to play a critical role during telencephalic development. Patients with Foxg1 mutations have been reported to suffer from mental retardation, poor social interactions, and severe anxiety.

In particular, severe sleep disturbances, third ventricular deformation, and choroid plexus cysts have also been reported. Therefore, Foxg1 may also be involved in the regulation of capital development.

Habenular nuclei

Conserved throughout vertebrates, habenular nuclei are a pair of small symmetrical structures in the epithalamus.

The nuclei functionally connect the forebrain and midbrain by receiving information and projecting to various brain regions. Each habenular nucleus is made up of two larger asymmetric subnuclei, the medial habenula and the lateral habenula.

These sub-nuclei are associated with different physiological processes and disorders, such as depression , nicotine addiction, and encoding of expected aversive stimuli or omission of rewarding stimuli. Clarifying the functions of habenular nuclei at the molecular level requires knowledge of their neuropeptide complement.

Although it was predominantly studied for its demonstration of asymmetric brain development and function, in recent years many scientists have begun to examine the role of habenular nuclei in motivation and behavior in understanding the physiology of addiction. .

The medial habenula receives connections from the posterior septum pellucidum and Broca’s diagonal band; the lateral habenula receives afferents from the latera hypothalamus, nucleus accumbens, internal globus pallidus, ventral pallidum, and Broca’s diagonal band.

Thalamus medullary stria

The medullary stria is part of the epithalamus. It is a bundle of fibers that contains afferent fibers from the septal nuclei, the lateral preoptic-hypothalamic region, and the thalamic nuclei anterior to the habenula.

It forms a horizontal ridge on the medial surface of the thalamus, and lies on the border between the dorsal and medial surfaces of the thalamus. Superior and lateral trigone to habenular.

It projects to the habenular nuclei, from the perforated anterior substance and hypothalamus, to the habenular trigone, to the habenular commissure, to the habenular nucleus.

Posterior commissure

The posterior commissure (also known as the epithelial commissure) is a rounded band of white fibers that traverse the midline on the dorsal aspect of the upper end of the cerebral aqueduct. It is important in bilateral pupillary light reflex.

Its fibers acquire their spinal sheaths early, but their connections have not been definitively determined.

Most of them have their origin in a nucleus, the nucleus of the posterior commissure (nucleus of Darkschewitsch), which is located in the central gray matter of the upper end of the cerebral aqueduct, in front of the oculomotor nucleus.

Some are probably derived from the posterior part of the thalamus and superior colliculus, while others are believed to continue down into the median longitudinal fascicle. The posterior commissure interconnects the pretectal nuclei, mediating the consensual reflection of the pupillary light.

Pineal gland

The most important exception is a primitive vertebrate, the Myxini. Even in the Myxini, however, there may be a “pineal equivalent” structure in the dorsal diencephalon.

The lamprey (another primitive vertebrate), however, does possess one. Some more developed vertebrates lost pineal glands in the course of their evolution. René Descartes believed that the pineal gland was the “main seat of the soul.”

Unlike most of the mammalian brain, the pineal gland is not isolated from the body by the blood-brain barrier system; it has a profuse blood flow, surpassed only by the kidney, supplied by the choroid branches of the posterior cerebral artery.

Parasympathetic innervation of the pterygopalatine and otic ganglia is also present. In addition, some nerve fibers enter the pineal gland through the pineal stem (central innervation).

In addition, neurons in the trigeminal ganglion innervate the gland with nerve fibers that contain the neuropeptide pituitary adenylate cycle activator polypeptide (also known as PACAP).

The pineal body consists of human beings of a lobular parenchyma of pine nuts surrounded by spaces of connective tissue. The surface of the gland is covered by a pial capsule.

The pineal gland consists mainly of pinealocytes, but four other types of cells have been identified. Since it is quite cellular (relative to the cortex and white matter), it can be mistaken for a neoplasm.

In some parts of the brain, and particularly in the pineal gland, there are calcium structures, the number of which increases with age, called corpus arenacea (or “acervuli” or “cerebral sand”).

The human pineal gland grows in size until approximately 1-2 years of age, remaining stable thereafter, although its weight gradually increases after puberty.

High levels of melatonin in boys are believed to inhibit sexual development, and pineal tumors have been linked to precocious puberty. When puberty hits, the production of melatonin is reduced.

In zebrafish, the pineal gland does not extend on either side of the midline, but instead shows a skew to the left. In humans, the functional brain domain is accompanied by subtle anatomical asymmetry.

Melatonin has several functions in the central nervous system, the most important of which is to help modulate sleep patterns.

Light-sensitive nerve cells in the retina detect light and send this signal to the suprachiasmatic nucleus, synchronizing the suprachiasmatic nucleus with the day-night cycle.

Nerve fibers then transmit daylight information from the suprachiasmatic nucleus to the paraventricular nuclei (PVN), then to the spinal cord and through the sympathetic system to the upper cervical ganglia, and from there to the pineal gland.

The compound pinolin is also claimed to be produced in the pineal gland; it is one of the beta-carbolines. This claim is subject to some controversy.

Rodent studies suggest that the pineal gland influences the secretion of the pituitary gland’s sex hormones, follicle-stimulating hormone, and luteinizing hormone.

Pinealectomy performed in rodents did not produce changes in pituitary weight, but did cause an increase in the concentration of follicle-stimulating hormone and luteinizing hormone within the gland.

Melatonin administration did not return follicle-stimulating hormone concentrations to normal levels, suggesting that the pineal gland influences the pituitary gland’s secretion of follicle-stimulating hormone and luteinizing hormone through a non-transmission molecule. described.

The pineal gland contains receptors for the regulatory neuropeptide, endothelin-1, which, when injected in picomolar amounts into the lateral cerebral ventricle, causes a calcium-mediated increase in pineal glucose metabolism.

Studies in rodents suggest that the pineal gland may influence the actions of recreational drugs, such as cocaine, and antidepressants, such as fluoxetine (Prozac), and that its hormone melatonin may protect against neurodegeneration.

Studies in mice suggest that pineal-derived melatonin regulates new bone deposition. Pine-derived melatonin mediates its action on bone cells through MT2 receptors.

This pathway could be a potential new target for the treatment of osteoporosis, as the study shows the curative effect of oral melatonin treatment in a mouse model for postmenopausal osteoporosis.


The subthalamus or prethalamus is a part of the diencephalon. The subthalamus connects with the globus pallidus, a basal nucleus of the telencephalon.

The subthalamus is located ventral to the thalamus, medial to the internal capsule, and lateral to the hypothalamus. It is a region formed by several gray matter nuclei and their associated white matter structures, namely:

The subthalamic nucleus, whose neurons contain glutamate and have excitatory effects on the neurons of the globus pallus and the substantia nigra.

The uncertain zone, located between the Forel H1 and H2 fields. It is continuous with the reticular thalamic nucleus and receives input from the precentral cortex.

Caudally, the subthalamus or prethalamus is separated from the thalamus by the intrathalamic zona Limitns.

Postnatally, the subthalamus lies below the thalamus, hence ‘sub’ (meaning below) ‘thalamus’. It is also dorsolateral to the hypothalamus.


The optic nerve (CNII) joins 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 joins 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 the anterior structures of the brain, including the thalamus, the hypothalamus, the posterior portion of the pituitary gland, and the pineal gland.

The hypothalamus performs numerous vital functions, most of which are directly or indirectly related to the regulation of visceral activities through other brain regions and the autonomic nervous system .