He Diencephalon Is one of the major regions of the brain. It is located just below the Telencephalon (Uppermost region of the brain) and just above the midbrain or midbrain.
The diencephalon is notable for containing in its interior some of the most important structures of the human brain , Such as the thalamus or the Hypothalamus .
In this sense, this brain region has a central location within the brain, is located between the brain hemispheres and the Trunk of the brain And, through it, the majority of fibers that go to the cerebral cortex .
Anatomically, the diencephalon stands out as representing only 2% of the total weight of the central nervous system. However, the connections established 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 the sensory pathways and motorways, so that the diencephalon is a basic structure when connecting the upper structures with the lower structures of the brain, and give rise to such activities.
Also, the diencephalon plays a major role in the limbic system of the brain, and appears to be also involved in the visceral and endocrine systems.
The purpose of this article is to expose the main characteristics of the diencephalon. Explain the structures and regions that comprise it, and review the functions of this region of the brain.
Characteristics of the diencephalon
The diencephalon is a set of gray matter nuclei. That is to say, it constitutes a series of cerebral structures that are characterized by contain nuclei of neurons in its interior.
Thus, when speaking of diencephalon, there is no reference to a single brain structure, but rather to a region of the brain that encompasses a large number of different nuclei and structures.
On the other hand, the diencephalon also contains fascicles of white matter that are in charge to establish multiple connections with different regions of the brain. For this reason, it is a region that is directly related to practically all the cerebral structures. The most important are: the cerebral cortex, the brain nuclei, the brainstem, the spinal cord and the Pituitary gland .
The main functions of the diencephalon are related to the control of the emotional life, due to its high involvement in the limbic system , As well as the transmission and processing of information instinctive (captured by the instincts) and vegetative (generated in the body itself).
This region of the brain is the continuation of midbrain (Middle brain) as it lies just above this. And it establishes the connection between the lower structures of the brain (metencephalon and myelencephalon) with the higher ones (cerebral cortex).
Anatomically, the diencephalon is characterized by six major structures within it. From top to bottom these are: Hypothalamus , Epithelium, thalamus, subthalamus, thalamus and third ventricle.
Unlike the trunk of the brain, which is more isolated and seen externally in almost all its extension, the diencephalon is embedded between the two cerebral hemispheres. Therefore, without practicing cuts to the encephalon, only the posteroinferior face and the apex can be observed. Belongs to the hypothalamus.
Macroscopic anatomy of the diencephalon
The diencephalon is a large region of the brain that contains within it a great variety of structures and regions.
In this sense, when it comes to delimiting its anatomical properties, different organizations and structural divisions can be carried out.
The external macroscopic configuration of the diencephalon (without regard to microscopic structures) is mainly characterized by the presence of optic chiasma And the interpeduncular space of the midbrain.
Specifically, the apex or infundibulum of this brain region is related to the pituitary and optic chiasm. On its lower posteroinferior face, the diencephalon is connected to the interpeduncular space of the midbrain.
In the middle of these two connections, there are two important diencephalic structures: the mamillary bodies and the Tuber cinereum . This last structure is responsible for extending inferiorly with the infundibulum, which in turn continues with the pituitary stem and the pituitary.
In order to visualize the internal macroscopic configuration it is necessary to make stems in the diencephalon.
By a front stem in the structure it is observed that the lateral face is limited by a thick sheet of white substance known as the inner capsule. This capsule extends from the peduncles of the midbrain and, from the thalamus, reaches the cerebral cortex.
The inner capsule of the diencephalon is an important structure as it contains descending and ascending pathways from the brainstem and thalamocortical pathways.
Medially, the diencephalon presents the ependymal cavity, the third ventricle and the continuation of the Aqueducts of Silvio (Referring to the mesencephalon).
In the upper region, the diencephalon is limited by the lateral ventricles of the cerebral hemispheres. In these ventricles the third ventricle opens through the orifices of Monro.
Finally, through a sagittal section, the pathway through which the third ventricle passes and the medial face is covered, which is covered with ependymal epithelium. In the horizontal section in contrast, the upper face is seen in all its extension, with the third ventricle in its middle part.
Third ventricle
The third ventricle is one of the major structures of the diencephalon. It is a cavity with triangular shape that is mainly responsible for dampening any type of trauma on all structures of this region of the brain.
The third ventricle has a very marked lower vertex called infundibular recess. The lateral wall of the ventricle is more extensive and contains the hypothalamic or limiting groove, as well as the interthalamic adhesion, which crosses the ventricle.
As regards its posteroinferior wall, there is an edge that originates the mouth of the Silvio aqueduct, the peduncles of the midbrain, the nipple tubers 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 connecting the two cerebral hemispheres. In its interior also the terminal lamina is observed, which is related to the optical chiasm and the medial eminence of the Tuber cinereum Which is located in the anterior part of the infundibulum.
Finally, the superior wall of the third ventricle is a region that is bent and contains the interventricular foramina of Monro, the choroid plexus, the habenula, the pineal gland and the posterior white commissure.
Diencephalon nuclei
The diencephalon is formed by four main components that border the third ventricle, which is responsible for dividing the diencephalon into two symmetrical halves.
In this sense, the main nuclei of this brain region are: the thalamus, the hypothalamus, the subthalamo and the epithelium.
Thalamus
The thalamus stands out as the most voluminous structure of the diencephalon. It lies right in the middle of the brain, above the hypothalamus and separated from it 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 directly reaches the cerebral cortex).
In this sense, so that any sense can be processed and interpreted by the brain (by the cortex), they must first pass through the thalamus, the region responsible for transmitting each sensitive stimulus to the relevant brain region.
The thalamus is a derivative composed 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 inner nucleus and the thalamic zones.
- Ventrolateral nucleus : Is a structure that is divided between a ventral portion and a lateral portion. In the ventral portion come fibers of the medial lemniscus and spinothalamic fascicles, and the lateral geniculate body and the 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.
- Previous nucleus : This nucleus is located below the anterior tubercle (anterior territory of the thalamus). It is characterized by receiving mamilothalamic fibers and establishes connection with the inner nucleus and the nuclei of the middle line.
- Internal core : This structure receives afferent projection fibers from the cerebral cortex and other thalamic nuclei (ventrolateral thalamus and hypothalamus). It is responsible for conducting synapses that regulate visceral activities, as well as connecting the cortical frontal lobe to develop the emotional experience of the human being.
- Thalamic zones : These regions allow to divide the thalamus in 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), the posterior territory (containing the geniculate nuclei) the medial territory Contains the medianodorsal nucleus and the contromedial nucleus) and the dorsal territory (containing the dorsal lateral nucleus and the posterior lateral nucleus).
Hypothalamus
Hypothalamus
He Hypothalamus Is the second great structure of the diencephalon. It is a nuclear region of the brain that lies 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.
In this sense, this structure of the diencephalon also plays an important role in the organization of behaviors such as feeding, fluid intake, mating or aggression. As well as the regulation of autonomic and endocrine visceral functions.
Anatomically, the hypothalamus is characterized by containing in its interior multiple nuclei of gray matter. Structurally, it borders forward with the terminal sheet, behind with a frontal plane passing behind the mammillary tubers of the Fornix , Laterally with the internal capsules and inferiorly with the optical chiasm.
Another important feature 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 middle eminence. Through this place they travel to the adenohypophysis to stimulate the secretion of other hormones such as growth-stimulating hormone or prolactin-releasing hormone.
- Magnocellular neurons are the major types of cells in the hypothalamus, are larger than the parvocellular and are responsible for producing neurohypophyseal hormones of peptide nature, which travel to the neurohypophysis.
Finally, it should be noted that the hypothalamus presents a large number of nuclei in its interior. Each of them contains both parvocellular neurons and magnocellular neurons and they develop specific functions:
- Lateral nuclei are hypothalamic structures related to the physiological processes of hunger.
- Preoptic nucleus: it is a small nucleus that is in charge of the parasympathetic function.
- Supraoptic nucleus: stands out for the production of antidiuretic hormone ADH.
- Paraventricular nucleus: is a nucleus that is responsible for generating Oxytocin .
- Suprachiasmatic nucleus: it is one of the most important structures of the hypothalamus. It regulates the circadian cycle.
- Ventromedial nucleus: it is considered as the center of satiety.
- Arcuato nucleus: involved in emotional behavior and endocrine activity. It is responsible for releasing the hormone GnRH.
- Mammillary nucleus: is a hypothalamic region that is involved in memory processes.
- Hypothalamic nucleus posterior: 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 the production of thyrotropin.
- Subthalam.
The subthalamo is a small structure of the diencephalon that is located below and laterally to the thalamus. Anatomically represents the continuation of the midbrain within the diencephalon.
It is characterized by containing structures such as the black substance or the red nucleus. It also contains gray matter, where the subthalamic nucleus is located.
The function of this region of the brain is to coordinate motor activities, so it is connected to the basal nuclei through the subthalamic fascicle.
Another important part of the subtalam is the uncertain zone, a nucleus that connects the diencephalon to the midbrain in order to coordinate the vision during the motor acts.
Epithelium
The epithelium is a small structure that lies just in front of the thalamus. Inside, it houses important elements such as Pineal gland , Habenular nuclei and spinal striae.
The epithelium also stands out as a structure belonging to the limbic system, so it plays an important role in the development of instinctive behaviors and in the elaboration of feelings of pleasure and / or reward.
The main features of the epithelium is that it contains one of the most important neuroendocrine glands, the pituitary gland. It is found among the superior colliculi, hanging from the back wrapped by piamadre.
The pineal gland is a structure that contains neurons, glia cells and specialized secretory cells called pienalocitos. The latter synthesize a highly important hormone such as Melatonin .
Melatonin is a hormone that is generated from the Serotonin And to regulate the sleep wake cycle. The production of this hormone increases at night and helps the rest of the body.
As the day progresses and the hours without rest are prolonged, the secretion of melatonin is diminishing. When quantities of melatonin at the cerebral level are scarce, the body responds with feelings of tiredness and sleep.
Thus, the epithelium is a major structure in the regulation of sleep processes, since it contains the pineal gland in its interior.
The other anatomical parts of this region of the diencephalon are: medullary striae, habenular nuclei, habenular striae, the epithelial roof of the third ventricle, and the trine of the habenula.
This latter region is arguably the most important of all. It is a structure that contains two habenular nuclei: one medial and one lateral.
The habenular nuclei are responsible for receiving afferents from the septal nuclei and projecting efferences to the interpeduncular nucleus, so they are regions involved in the limbic system
Functional structure
The diencephalon is a region of the brain that performs a large number of functions.
The functionality of this region depends mainly on the activities performed by each of the structures within it and the relationships they establish with other areas of the brain.
In this sense, the activity of the diencephalon can be divided according to different elements. The most important are: thalamus, hypothalamus-pituitary connection and epithelial-epiphysis relationship.
Thalamus
Functionally, the thalamus is characterized by containing the gray matter, which is formed by 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 fascicles coming from the spinal cord and the brainstem.
Subsequently, the neurons of these nuclei send their axons through the inner capsule to the primary areas of the cerebral cortex.
Functionally, another important area is the posterolateral ventral nucleus. This region receives all the somatic sensibility of the organism (except the head) and the sympathetic visceral information coming from the spinal cord.
In this sense, the thalamus is also responsible for receiving all the somatic sensibility of the body, as well as receiving visual information (through the lateral geniculate nucleus) and acoustic information (through the medial geniculate nucleus).
Associative thalamic nuclei, on the other hand, are responsible for integrating information from other primary nuclei and the cerebral cortex.
Finally, the reticular nuclei connect with the Reticular formation Of the brainstem to perform the bioelectrical activity of the diencephalic nuclei and cerebral cortex.
Hypothalamus-pituitary connection
Hypothalamus
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 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 the control of the physiological expression of emotion . This activity is carried out through the regulation of Function of the autonomic nervous system , By its influence on the trunk of the encephalon.
On the other hand, the hypothalamus is responsible for regulating hunger because it modulates the release of hormones and peptides such as cholecystokinin, glucose level or fatty acids in blood.
Finally, the hypothalamus regulates body temperature causing an increase or decrease in respiratory rate and sweating.
Epithelial-epiphysis connection
The epithelium is a structure of the diencephalon that has connections with the olfactory pathway and intervenes in the control of vegetative and emotional functions. Likewise, it seems to have a special importance in the regulation of the sexual activity of the people.
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 epithelium modulates the activity of the pituitary at the time of releasing the hormone melatonin, which is the main responsible for carrying out such functions.
Finally, the diencephalon is notable for its extensive involvement in the limbic system, which regulates physiological responses to certain stimuli.
Among such activities is 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 seem to be developed by the diencephalon, mainly, through the connection between the nucleus of habénula (epitálamo) and the limbic brain.
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