DIENCEPHALON: CHARACTERISTICS, PARTS AND FUNCTIONS - GEOGRAFÍA - 2025

The diencephalon is one of the main regions of the brain. It is located just below the telencephalon (uppermost region of the brain) and just above the midbrain or midbrain. This part of the brain stands out for containing some of the most important structures of the human brain, such as the thalamus or the hypothalamus.

This brain region has a central location within the brain, is located between the cerebral hemispheres and the brain stem, and most of the fibers that go to the cerebral cortex travel through it.

Diencephalon (red frame)

Anatomically, the diencephalon stands out for representing only 2% of the total weight of the central nervous system. However, the connections made by this brain structure are vital for the performance of a wide variety of brain functions.

The most important seem to be the establishment of sensory pathways and motor pathways, which is why the diencephalon is a basic structure when it comes to connecting the higher structures with the lower structures of the brain.

Likewise, the diencephalon plays a major role in the limbic system of the brain, and it seems to be also involved in the visceral pathways and the endocrine system.

Characteristics and location of the diencephalon

The diencephalon is a set of nuclei of gray matter. In other words, it constitutes a series of brain structures that are characterized by containing nuclei of neurons within them.

When we speak of diencephalon we are not referring to a single brain structure, but rather to a region of the brain that includes a large number of different nuclei and structures.

On the other hand, the diencephalon also contains bundles of white matter that are responsible for establishing multiple connections with different regions of the brain.

For this reason, it is a region that is directly related to practically all brain structures. The most important are: the cerebral cortex, the brain nuclei, the brainstem, the spinal cord and the pituitary gland.

Anatomy

Anatomically, the diencephalon is characterized by having six main structures inside. From top to bottom these are: hypothalamus, epithalamus, thalamus, subthalamus, thalamus, and third ventricle.

Unlike the brain stem, which is more isolated and can be seen from the outside in almost its entire extension, the diencephalon is embedded between the two cerebral hemispheres so, without making cuts to the brain, only the posteroinferior aspect and the vertex can be observed. belongs to the hypothalamus.

Principal functions

The main functions of the diencephalon are related to the control of emotional life, due to its high involvement in the limbic system, as well as the transmission and processing of instinctive information (captured by instincts) and vegetative (generated in the body itself).

This region of the brain is the continuation of the midbrain (midbrain) as it is located just above it. And it establishes the connection between the lowest structures of the brain (metencephalon and myelncephalon) with the most superior ones (cerebral cortex).

Gross anatomy of the diencephalon

Diencephalon marked in red. Side view

The diencephalon is a large region of the brain that contains a wide variety of structures and regions within it. When defining their anatomical properties, different organizations and structural divisions can be carried out.

The external macroscopic configuration of the diencephalon (without taking into account the microscopic structures) is characterized mainly by the presence of the optic chiasm and the interpeduncular space of the midbrain.

Diencephalon

Specifically, the vertex or infundibulum of this brain region is related to the pituitary and the optic chiasm. On the other hand, in its posteroinferior aspect, the diencephalon is connected with the interpeduncular space of the midbrain.

In between these two connections are two important diencephalic structures: the mammillary bodies and the tuber cinereum. This last structure is in charge of prolonging inferiorly with the infundibulum, which in turn continues with the pituitary stalk and the pituitary.

Mamillary bodies

Through a frontal cut in the structure, it is observed that the lateral face is limited by a thick sheet of white matter known as the internal capsule. This capsule extends from the peduncles of the midbrain and, from the thalamus, reaches the cerebral cortex.

The internal capsule of the diencephalon is an important structure, since it contains the descending and ascending pathways that come from the brainstem and the thalamocortical pathways.

Medially, the diencephalon presents the ependymal cavity, the third ventricle and the continuation of the aqueduct of Silvio (referring to the midbrain).

In the superior region, the diencephalon is limited by the lateral ventricles of the cerebral hemispheres. The third ventricle empties into these ventricles through the orifices of Monro.

Finally, by means of a sagittal section, the path through which the third ventricle passes and the medial face is observed, which is covered with ependymal epithelium. In the horizontal section, on the other hand, the upper face is observed in its entirety, with the third ventricle in its middle part.

Third ventricle

Third ventricle and other structures

The third ventricle is one of the most important structures in the diencephalon. It is a cavity with a triangular shape that is mainly responsible for cushioning any type of trauma on all the structures of this region of the brain.

The third ventricle has a very marked lower vertex called the infundibular recess. The lateral wall of the ventricle, on the other hand, is more extensive and contains the hypothalamic or limiting sulcus, as well as the interthalamic adhesion, which crosses the ventricle from one side to the other.

Regarding its posteroinferior wall, there is an edge that originates the mouth of the aqueduct of Silvio, the peduncles of the midbrain, the mammillary tubercles and the tuber cinereum.

The posterior wall of the third ventricle is also very narrow and contains the anterior white commissure, a bundle of fibers that connects both hemispheres of the brain. Inside it, the terminal lamina is also observed, which is related to the optic chiasm and the medial eminence of the tuber cinereum located in the anterior part of the infundibulum.

Finally, the upper wall of the third ventricle is a region that is curved and contains the interventricular foramina of Monro, the choroid plexuses, the habenula, the pineal gland, and the posterior white commissure.

Nuclei of the diencephalon

Diencephalon seen from the back of the skull

The diencephalon is made up of four main components that border the third ventricle, which is responsible for dividing the diencephalon into two symmetrical halves. The main nuclei of this brain region are: the thalamus, the hypothalamus, the subthalamus, and the epithalamus.

-Thalamus

The thalamus stands out for being the most voluminous structure of the diencephalon. It is located right in the middle of the brain, above and separated from the hypothalamus through a hypothalamic groove of Monroe.

Its main function is to transmit the sensory stimuli that reach the brain, with the exception of smell; since the olfactory pathways develop in the embryo before the thalamus and smell is the only sense that reaches the cerebral cortex directly.

In order for any meaning to be processed and interpreted by the brain (through the cortex), they must first pass through the thalamus, the region in charge of transmitting each sensitive stimulus to the relevant brain region.

The thalamus is a derivative made up of 80 different neuronal nuclei, which are grouped in different territories. The main nuclei of the thalamus are: the ventrolateral nucleus, the anterior nucleus, the internal nucleus and the thalamic areas.

Ventrolateral nucleus

It is a structure that is divided between a ventral portion and a lateral portion. Fibers from the medial lemniscus and spinothalamic fascicles arrive ventrally, and the lateral geniculate body and medial geniculate body are observed.

The lateral portion, on the other hand, receives abundant fibers from the anterior thalamic peduncle and projection fibers that establish the sensory pathways towards the cortex.

Anterior nucleus

This nucleus is located below the anterior tubercle (anterior territory of the thalamus). It is characterized by receiving mamilothalamic fibers and establishes a connection with the inner nucleus and the nuclei of the midline.

Inner core

This structure receives the afferent projection fibers from the cerebral cortex and other thalamic nuclei (ventrolateral thalamus and hypothalamus). It is responsible for carrying out synapses that regulate visceral activities, as well as connecting the frontal cortical lobe to develop the emotional experience of the human being.

Thalamic zones

These regions allow the thalamus to be divided into different territories. The main ones are: anterior territory (containing the anterior nucleus), ventral territory (containing the anterior ventral nucleus, lateral ventral nucleus and posterior ventral nucleus), posterior territory (containing geniculate nuclei), medial territory (which contains the medianodorsal nucleus and the contromedial nucleus) and the dorsal territory (which contains the dorsal lateral nucleus and the posterior lateral nucleus).

-Hipothalamus

Hypothalamus (yellow)

The hypothalamus is the second great structure of the diencephalon. It is a nuclear region of the brain that is located just below the thalamus.

This structure is the most important brain region for the coordination of essential behaviors, linked to the maintenance of the species. Likewise, it stands out for its close relationship with the hormones of the pituitary, which are regulated by the hypothalamus.

This structure of the diencephalon also plays an important role in the organization of behaviors such as eating, fluid intake, mating, or aggression. As well as the regulation of autonomic and endocrine visceral functions.

Anatomically, the hypothalamus is characterized by containing multiple nuclei of gray matter. Structurally, it borders in front with the terminal lamina, behind with a frontal plane that passes behind the mammillary tubers of the fornix, laterally with the internal capsules and inferiorly with the optic chiasm.

Another important characteristic of the hypothalamus is that it contains two different types of neurons within it: parvocellular neurons and magnocellular neurons.

• Parvocellular neurons are responsible for releasing peptide hormones known as hypophysiotropic factors in the primary plexus of the median eminence. Through this site they travel to the anterior pituitary gland to stimulate the secretion of other hormones such as growth-promoting hormone or prolactin-releasing hormone.
• For their part, magnocellular neurons are the majority cell types in the hypothalamus, they are larger than parvocellular neurons and are responsible for producing neurohypophyseal hormones of a peptide nature, which travel to the neurohypophysis.

Finally, it should be noted that the hypothalamus has a large number of nuclei inside. Each of them contains both parvocellular neurons and magnocellular neurons and they develop specific functions:

• Lateral nuclei: they are hypothalamic structures related to the physiological processes of hunger.
• Preoptic nucleus: it is a small nucleus that is responsible for parasympathetic function.
• Supraoptic nucleus: noted for the production of the antidiuretic hormone ADH.
• Paraventricular nucleus: it is a nucleus that is responsible for generating oxytocin.
• Suprachiasmatic nucleus: it is one of the most important structures of the hypothalamus. It is responsible for regulating the circadian cycle.
• Ventromedial nucleus: it is considered the center of satiety.
• Arcuate nucleus: intervenes in emotional behavior and endocrine activity. It is responsible for releasing the hormone GnRH.
• Mamillary nucleus: it is a hypothalamic region that is involved in memory processes.
• Posterior hypothalamic nucleus: seems to play a fundamental role in the regulation of body temperature.
• Anterior hypothalamic nucleus: is responsible for regulating sweating temperature, as well as inhibiting thyrotropin production.

Subthalamus

The subthalamus is a small structure of the diencephalon that is located below and lateral to the thalamus. Anatomically it represents the continuation of the midbrain within the diencephalon.

It is characterized by containing inside structures such as the black substance or the red nucleus. Likewise, it contains gray matter, the place where the subthalamic nucleus is located.

The function of this brain region is to coordinate motor activities, which is why it is connected to the basal ganglia through the subthalamic tract.

Another important part of the subthalamus is the uncertain zone, a nucleus that is responsible for connecting the diencephalon with the midbrain in order to coordinate vision during motor acts.

-Epithalamus

Epithalamus (red). Source: The original uploader was Mikael Häggström at English Wikipedia. / Public domain

The epithalamus is a small structure just in front of the thalamus. Inside it houses important elements such as the pineal gland, the habenular nuclei and the medullary striae.

The epithalamus also stands out for being a structure belonging to the limbic system, which is why it plays an important role in the development of instinctive behaviors and in the development of sensations of pleasure and / or reward.

The main characteristic of the epithalamus is that it contains one of the most important neuroendocrine glands, the pituitary gland. This is between the superior colliculi, hanging from the posterior part surrounded by pia mater.

Hypophysis (life persona)

The pituitary gland is a structure that contains neurons, glia cells, and specialized secretory cells called pienalocytes. The latter synthesize a highly important hormone such as melatonin.

Melatonin is a hormone that is generated from serotonin and that regulates the sleep-wake cycle. The production of this hormone increases at night and helps the body rest.

As the day progresses and the hours without rest lengthen, the secretion of melatonin decreases. When the amounts of melatonin in the brain are low, the body responds with feelings of tiredness and sleepiness.

Thus, the epithalamus is a main structure in the regulation of sleep processes, since it contains the pineal gland inside.

The other anatomical parts of this region of the diencephalon are: the medullary striae, the habenular nuclei, the habenular striae, the epithelial roof of the third ventricle, and the trigone of the habenula.

This last region is possibly the most important of all. It constitutes a structure that contains two habenular nuclei: one medial and the other lateral.

The habenular nuclei are in charge of receiving afferents from the septal nuclei and projects the interpeduncular nucleus, which is why they are regions involved in the limbic system

Features

The functions of the diencephalon depend mainly on the activities carried out by each of the structures within it and on the relationships that these establish with other areas of the brain.

The activity of the diencephalon can be divided into different elements. The most important are: thalamus, hypothalamic-pituitary connection and epithalamic-epiphysis relationship.

Thalamus

Thalamus, indicated by red arrow

Functionally, the thalamus is characterized by containing gray matter, which is made up of four groups of nuclei: primary, secondary, associative, and reticular.

The primary thalamic nuclei are responsible for receiving the connections of the optic, acoustic and ascending tracts from the spinal cord and brainstem. Subsequently, the neurons of these nuclei send their axons through the internal capsule towards the primary areas of the cerebral cortex.

Functionally, another important area is the ventral posterolateral nucleus. This region receives all the somatic sensitivity of the body (except the head) and the sympathetic visceral information from the spinal cord.

The thalamus is also responsible for receiving all the somatic sensitivity of the body, as well as receiving visual information (through the lateral geniculate nucleus) and acoustic information (through the medial geniculate nucleus).

The associative thalamic nuclei, for their part, are in charge of integrating information from other primary nuclei and the cerebral cortex.

Finally, the reticular nuclei connect with the reticular formation of the brain stem to carry out the bioelectric activity of the diencephalic nuclei themselves and of the cerebral cortex.

Hypothalamic-pituitary connection

Hypothalamus (blue)

The hypothalamus stands out for developing a functioning closely related to its connection with the pineal gland.

In this sense, the diencephalon is also responsible for regulating a wide variety of physiological activities through the connection between the hypothalamus and the pituitary. The most important functions are: emotions, hunger, temperature and sleep.

The hypothalamus is a region responsible for controlling the physiological expression of emotion. This activity is carried out through the regulation of the function of the autonomic nervous system, through its influence on the brain stem.

On the other hand, the hypothalamus is in charge of regulating hunger since it modulates the release of hormones and peptides such as cholecystokinin, the level of glucose or fatty acids in the blood.

Finally, the hypothalamus regulates body temperature, causing an increase or decrease in respiratory rate and sweating.

Epithalamic-epiphysis connection

The epithalamus is a structure of the diencephalon that has connections with the olfactory pathway and is involved in the control of vegetative and emotional functions. Likewise, it seems to have a special importance in regulating people's sexual activity.

The performance of such functions is carried out mainly through the connection of this structure with the pineal gland.

In this sense, the diencephalon intervenes in the regulation of the sleep-wake cycle, since the epithalamus modulates the activity of the pituitary when releasing the hormone melatonin, which is the main responsible for carrying out these functions.

Finally, the diencephalon stands out for its extensive involvement in the limbic system, which is responsible for regulating physiological responses to certain stimuli.

Such activities include the development of involuntary memory, the functioning of attention, the elaboration of emotions and the constitution of elements such as the personality or the behavioral pattern of people.

These actions appear to be developed by the diencephalon, mainly, through the connection between the nucleus of habenula (epithalamus) and the limbic brain.

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