The diencephalon is one of the two key parts of the forebrain, i.e. prosencephalon in Latin (1). The second key part is the telencephalon or cerebrum. It is interesting to note that its position is not visible by the naked eye from the outside of the brain surface if the skull was removed.
The reason is simple: the diencephalon is hidden and placed under the cerebral hemispheres. In this article, we will elaborate on its importance, anatomical position, embryological development, and, most importantly, functions. Shortly, the diencephalon has numerous roles in our CNS and functioning of our whole organism.
Position, embryological development, and structure
Diencephalon occupies the central part of the brain. Anatomically, we can say it has a central position, as a direct extension of the brain stem (2).
Embryologically, it is linked to the 5th week. Namely, in the 5th embryonic week, the secondary cerebral vesicles form after the prosencephalon is divided into telencephalon (big brain) and diencephalon (midbrain). Precisely, the diencephalon develops from its median region.
In the embryological stage, this region has two lateral plates which are the sensory areas. Moreover, it consists of a roof plate and the 3rd ventricle. Cellular proliferation leads to the development of diencephalon (1).
It is a very important part of our brain as many centers are located in it. Moreover, extremely sensitive nerves are passing through this part of the brain. The diencephalon sits at the top of the brainstem and is closed by the cerebral hemispheres. It consists of four key parts:
- Subthalamus (1).
Except for these main four parts, some other segments and structures build up the “interbrain” or, scientifically, diencephalon.
Those include the pineal gland, the Stria medullaris thalami, the anterior paraventricular nuclei, the posterior paraventricular nuclei, the posterior commissure, the medial habenular nuclei, and the lateral habenular nuclei (1).
Borders of diencephalon
It is interesting to describe the borders of the diencephalon to understand its anatomical position and functions. First of all, it has a roof, the lateral walls, as well as anterior and posterior walls.
Moreover, the structure of these borders is important. The roof consists of tela choroidea. This is a Latin scientific name for two membranes. Those are called pia mater and ependyma. The roof of the diencephalon contains two blood vessels. They enter the third ventricle cavity.
This is the point where the function of the cerebrospinal fluid production takes place too.
The optic chiasm is the key component of the diencephalon floor. Besides, there is the tuber cinereum, tuber infundibulum, posterior perforated segment, the mammillary bodies, and the top segment of the mesencephalic tegmentum (2).
The anterior diencephalon wall contains the fornix column, lamina terminalis, and the anterior commissure. The posterior wall structure contains the Habenular commissures, the stalk of the pineal gland, and the posterior commissure. Medial walls of the thalami build up the diencephalon lateral walls.
Functions of diencephalon
The following centers are located in diencephalon:
- centers of the autonomic nervous system (of the peripheral NS),
- centers for thermoregulation
- hunger and thirst
- centers for the operation of the vegetative nervous system (2).
Moreover, this part of our brain is directly responsible for connecting the parts of the system of inner secretion glands, i.e. the endocrine system with the NS. Moreover, it is interconnected with our limbic system. Namely, it helps manage memories, as well as emotions.
We must note that each part of the diencephalon is responsible for a different function. There are communication pathways between its parts and it makes this brain a diverse and connector body in our organism.
Namely, it connects the limbic system, the basal ganglia, and the visual and audio sensory areas. As a result, it plays an important role in our emotions and motoric activities and coordination.
The upper part of the thalamus transfers information and processes information from the majority of the sensory organs to the cerebrum. The thalamus surrounds the third chamber and represents a relay station for the sensory impulses.
It transfers impulses to the appropriate part of the cortex to be localized and interpreted. The bottom part contains the centers that regulate body temperature, feeling of hunger, satiety, and thirst.
It is interesting to emphasize that each sensory system, except the olfactory one, is linked to the thalamus via a thalamic nucleus. Its role is receiving and sending signals. Namely, thalamic nuclei send the signals to the primary cortical areas.
Moreover, the thalamus is important for sleep and awake state regulation. Similarly, it regulates arousal ad activity. As a result, a state of a coma often results from certain damage to the thalamus.
When it comes to physiological description, size, and shape, we can say that it resembles a walnut. It is around 3 cm long and 2.5 cm wide. Also, the thalamus is around 2 cm high. When the two halves of the thalamus are observed together, we note that the shape resembles a bulb.
They are symmetrically positioned on the third ventricle. The myelinated fibers are the strongest peculiarity of the thalamus. They separate their key subparts. One can distinctly identify the neuron clusters separating these parts.
When it comes to the blood supply of this part of the diencephalon, it comes from four arteries. Those are:
- polar artery,
- paramedian thalamic-subthalamic arteries,
- inferolateral arteries,
- posterior choroidal arteries.
The mammillothalamic tract is the connection between the thalamus and the hippocampus (2).
The hypothalamus is associated with the pituitary gland which controls the operation of all glands with the internal secretion, i.e. the endocrine system glands. Therefore, we can observe this part of the diencephalon as a bridge between the CNS and the endocrine system.
It is located below the thalamus. It is a very important center of the autonomic nervous system. It helps regulate body temperature and controls water balance. It also regulates metabolism.
Moreover, it plays an important role in the limbic system (emotions). The pituitary gland is located below the hypothalamus. As we have already said, it regulates the hormones. Moreover, it secretes neurohormones.
These coordinate the pituitary gland itself. Besides, it reciprocally sends and receives signals between the posterior and anterior pituitary gland.
Other functions of the hypothalamus include:
- Appetite regulation,
- Temperature regulation,
- Sexual dimorphism regulation,
- Processing of fear.
The epithalamus forms the roof of the third brain chamber and contains the pineal gland – the epiphysis (endocrine gland). It includes the choroid plexus – it secrets the cerebrospinal fluid.
Most noteworthy, this part of the diencephalon serves as a connection between the limbic system and other parts of our brain.
Except for the above said, there are several other important functions of the epithalamus. Those are:
- Secretion of melatonin
- Regulation of motoric activities (2).
Subthalamus is another crucial part of our brain we must address when elaborating on the diencephalon. Namely, parts of subthalamus are made from the diencephalon tissue. This part of our brain has strong connections with ganglia. As a result, it participates in motoric activity coordination.
As we have already said, the diencephalon is closely related to the activity of the pituitary gland. The deficiency of the pituitary hormones is a condition well-known in medicine. It is called panhypopituitarism syndrome.
Scientists found that the occurrence of this syndrome is related to diencephalon insults. Also, it can occur due to an impairment of the pituitary gland itself. Hypothalamus insults also lead to this syndrome. Types of insults that lead to it include radiation therapy, necrosis, and a brain injury.
Other conditions related to this syndrome include hypothyroidism if it affects the thyroid gland, failure to thrive, if it affects the growth hormone, etc. In some cases, two or several hormones can be affected at the same time.
Russell’s syndrome or diencephalic syndrome is a rather rare condition. However, it heavily affects the quality of life of a patient. It is a disorder that leads to a euphoric state, hypotension, and hypoglycemia. Moreover, the patient suffers from emaciation and locomotor hyperactivity.
A neoplastic or vascular lesion on diencephalon can lead to disorders or dysfunctions that are related to that specific part of the brain. Scientists found that the neoplastic lesion located on the hypothalamic zone leads to Russell’s syndrome.
Diencephalon is positioned in the forebrain (prosencephalon). It cannot be seen from the outer brain point of view, as it is located under the cerebral hemispheres. This part of the brain has many different, important roles. First of all, it sends and receives information from and to different brain regions.
Secondly, it controls many autonomic functions. It also connects the endocrine system structures. As a result, it affects the hormonal balance in our body. Furthermore, it is related to the limbic system. It affects our emotions and memories.
In addition, it directs sense impulses throughout the body, controls the motoric function, and affects the homeostasis, controls vision, smell, and taste, as well as the perception of touch.
- Chatterjee M, Li JY. Patterning and compartment formation in the diencephalon. Front Neurosci. 2012 May 11;6:66. doi: 10.3389/fnins.2012.00066. PMID: 22593732; PMCID: PMC3349951. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3349951/
- Puelles L. Survey of Midbrain, Diencephalon, and Hypothalamus Neuroanatomic Terms Whose Prosomeric Definition Conflicts With Columnar Tradition. Front Neuroanat. 2019 Feb 27;13:20. doi: 10.3389/fnana.2019.00020. PMID: 30873012; PMCID: PMC6402269. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6402269/