The brain hypothalamus, or subtalamic region, is a small area located below the thalamus in the diencephalon. Despite its small size, the hypothalamus neurons form from 30 to 50 groups of nuclei responsible for various homeostatic indicators of the body, as well as regulating most of the neuroendocrine functions of the brain and the organism as a whole. Hypothalamic neurons have extensive connections with almost all centers and divisions of the central nervous system, and the neuroendocrine connections of the hypothalamus and the pituitary gland deserve special attention. They cause the formation of the so-called functionally unified hypothalamic-pituitary system, responsible for the production of pituitary and hypothalamic hormones and which is the central link between the nervous and endocrine systems. Let's take a closer look at how the hypothalamus works, what it is and what specific body functions are provided by this small area of the brain.
Although the functional activity of the hypothalamus is studied quite well, today there are not enough clear anatomical boundaries defining the hypothalamus. The structure in terms of anatomy and histology is associated with the formation of extensive neuronal connections of the hypothalamic region with other parts of the brain. Thus, the hypothalamus is located in the subtalamic region (below the thalamus, which is why its name occurs) and participates in the formation of the walls and bottom of the third ventricle of the brain. The terminal plate anatomically forms the anterior border of the hypothalamus, and its posterior border is formed by a hypothetical line extending from the posterior commissure of the brain to the caudal region of the mastoid bodies.
Despite its small size, the structural hypothalamic region is divided into several smaller anatomical and functional regions. In the lower part of the hypothalamus, structures such as a gray tubercle, a funnel and a median elevation are distinguished, and the lower one often passes anatomically into the pedicle of the pituitary gland.
Let's look at which nuclei belong to the hypothalamus, what it is, and into which groups they are divided. Thus, the nuclei in the central nervous system imply the accumulation of gray matter (bodies of neurons) in the thickness of the white matter (axon and dendritic terminals - pathways). Functionally, the nuclei provide for the switching of nerve fibers from one nerve cell to another, as well as the analysis, processing and synthesis of information.
Three groups of clusters of bodies of neurons forming the nucleus of the hypothalamus are anatomically distinguished: the anterior, middle, and posterior groups. To date, the exact number of nuclei of the hypothalamus is quite difficult to establish, as different data on their number is given in various domestic and foreign literature sources. The anterior group of nuclei is located in the area of the optic chiasm, the middle group lies in the area of the gray tubercle, and the back group is located in the region of the mastoid, forming the same sections of the hypothalamus.
The anterior group of hypothalamic nuclei includes the supraoptic and paraventricular nuclei, the middle group of nuclei corresponding to the funnel and gray knoll region includes lateral nuclei, as well as dorso-medial, tuberous and ventromedial nuclei, and the posterior group includes the mastoid and posterior nuclei. In turn, the autonomic function of the hypothalamus is provided by the function of nuclear structures, anatomical and functional relationships with the rest of the brain, control of basic behavioral reactions and the release of hormones.
The hypothalamic region secretes highly specific and biologically active substances, which are called “hypothalamic hormones”. The word "hormone" comes from the Greek "excite", i.e., hormones are highly active biological compounds that in nanomolar concentrations can lead to significant physiological changes in the body. Let's consider what hormones the hypothalamus secretes, what it is and what is their regulatory role in the functional activity of the whole organism.
According to their functional activity and the point of application, the hypothalamic hormones are divided into the following groups:
- releasing hormones, or liberins;
- hormones of the posterior lobe of the pituitary gland (vasopressin or antidiuretic hormone and oxytocin).
Functionally, releasing hormones affect the activity and release of hormones by the cells of the anterior pituitary gland, increasing their production. Statin hormones perform the exact opposite function, stopping the production of biologically active substances. The hormones of the posterior lobe of the pituitary gland are actually produced in the supraoptic and paraventricular nuclei of the hypothalamus, and then transported via axon terminals to the posterior pituitary gland. Thus, the hormones of the hypothalamus are a kind of controlling elements that regulate the production of other hormones. Liberins and statins regulate the production of tropic hormones of the pituitary gland, which, in turn, affect the target organs. Let's consider the main functional moments of the hypothalamic region, or what the hypothalamus in the body is responsible for.
Hypothalamus in regulating the function of the cardiovascular system
To date, it has been experimentally shown that electrostimulation of various hypothalamic regions can lead to any of the known neurogenic effects on the cardiovascular system. In particular, by stimulating the hypothalamus centers, it is possible to increase or decrease the level of blood pressure, increase or decrease the heart rate. It has been shown that in various areas of the hypothalamus these functions are organized according to the reciprocal type (that is, there are centers responsible for increasing blood pressure and centers responsible for reducing it): stimulation of the lateral and posterior hypothalamic region leads to an increase in blood pressure and frequency heart contraction, while stimulation of the hypothalamus in the area of the optic chiasm can cause opposite effects. The anatomical basis of regulatory influences of this type are specific centers regulating the activity of the cardiovascular system, located in the reticular regions of the bridge and the medulla oblongata, and extensive neural connections extending from them into the hypothalamus. The regulation functions are provided by the close exchange of information between these areas of the brain.
Participation of the hypothalamic region in maintaining constancy of body temperature
Nuclear formations of the hypothalamic region are directly involved in the regulation and maintenance of body temperature. In the preoptic region is a group of neurons that are responsible for the continuous monitoring of blood temperature.
With an increase in the temperature of the flowing blood, this group of neurons is capable of increasing impulses, transferring information to other structures of the brain, thereby triggering the mechanisms of heat transfer. When the temperature of the blood decreases, the impulses from neurons decrease, which causes the start of heat production processes.
Participation of the hypothalamus in the regulation of the body's water balance
Water-salt balance of the body, vasopressin, hypothalamus - what is it? The answer to these questions is later in this section. Hypothalamic regulation of the water balance of the body is carried out in two main ways. The first of these is to create a sense of thirst and a motivational component, which includes behavioral mechanisms that lead to the satisfaction of the need that has arisen. The second way is to regulate the loss of body fluids from the urine.
Localized center of thirst, causing the formation of the same feelings, in the lateral hypothalamic region. At the same time, sensitive neurons of this area constantly monitor not only the level of electrolytes in the blood plasma, but also osmotic pressure, and with increasing concentration they cause the formation of thirst, which leads to the formation of behavioral reactions aimed at finding water. After the water is found and the feeling of thirst is satisfied, the osmotic pressure of the blood and electrolyte composition are normalized, which returns the impulses of the neurons to normal. Thus, the role of the hypothalamus is reduced to the formation of the vegetative basis of behavioral mechanisms aimed at meeting the emerging nutritional needs.
Regulation of water loss or excretion by the body through the kidneys lies in the so-called supraoptic and paraventricular nuclei of the hypothalamus, which are responsible for the production of a hormone called vasopressin, or antidiuretic hormone. As the name implies, this hormone regulates the amount of water reabsorbed in the collecting nephron tubules. At the same time, vasopressin is synthesized in the aforementioned hypothalamus nuclei, and further along axon terminals it is transported to the back of the pituitary, where it is stored until the required moment. If necessary, the posterior lobe of the pituitary gland secretes this hormone into the blood, which increases the reabsorption of water in the renal tubules and leads to an increase in the concentration of excreted urine and a decrease in the level of electrolytes in the blood.
Participation of the hypothalamus in the regulation of uterine contractile activity
The neurons of the paraventricular nuclei produce a hormone such as oxytocin. This hormone is responsible for the contractility of the muscle fibers of the uterus during childbirth, and in the postpartum period for the contractility of the mammary ducts of the mammary glands. By the end of pregnancy, closer to childbirth, an increase in specific receptors for oxytocin occurs on the surface of the myometrium, which increases the sensitivity of the latter to the hormone. At the time of birth, a high concentration of oxytocin and sensitivity to it of the uterine muscle fibers contribute to the normal course of labor. After childbirth, when the baby takes the nipple, this leads to the stimulation of oxytocin production, which causes a reduction in the milk ducts of the mammary glands and the release of milk.
In addition, in the absence of pregnancy and breastfeeding, as well as in males, this hormone is responsible for developing feelings of love and sympathy, for which he received his second name - “love hormone” or “happiness hormone”.
Participation of the hypothalamus in the formation of hunger and satiety
In the lateral hypothalamic region there are specific centers, organized by reciprocal type, responsible for the formation of a feeling of thirst and satiety. Experimentally, it was shown that electrical stimulation of the centers responsible for developing a feeling of hunger leads to a behavioral response to finding and eating food even in a well-fed animal, and irritation of the saturation center leads to refusal to eat an animal that has been starving for several days.
With the defeat of the lateral hypothalamic region and the centers responsible for the formation of hunger, so-called starvation may occur, which leads to death, and in case of pathology and bilateral lesion of the ventromedial region, insatiable appetite and lack of satiety occur, which leads to the formation of obesity.
The hypothalamus in the field of the mastoid is also involved in the formation of behavioral reactions associated with food. Irritation of this area leads to such reactions as lip licking and swallowing.
Regulation of behavioral activity
Despite its small size, comprising only a few cubic centimeters, the hypothalamus is involved in the regulation of behavioral activity and emotional behavior, being part of the limbic system. At the same time, the hypothalamus has extensive functional connections with the brain stem and the reticular formation of the midbrain, with the anterior thalamic region and the limbic parts of the cerebral cortex, the hypothalamic and hypophysis funnel for the implementation and coordination of the secretory and endocrine functions of the latter.
Pathogenetically, all diseases of the hypothalamus are divided into three large groups, depending on the characteristics of hormone production. Thus, diseases associated with increased hormone production of the hypothalamus, with low hormonal production, as well as with the normal level of hormone production are isolated. In addition, the diseases of the hypothalamus and the pituitary gland are very closely related to each other, due to the common blood supply, anatomical structure and functional activity. Often the pathology of the hypothalamus and pituitary are combined into a common group of diseases of the hypothalamic-pituitary system.
The most common cause leading to the appearance of clinical symptoms is the occurrence of adenoma, a benign tumor from the glandular tissue of the pituitary gland. In this case, as a rule, its occurrence is accompanied by an increase in hormonal production with the corresponding typical manifestation of clinical symptoms. The most common are tumors that produce an excessive amount of corticotropin (corticotropin), somatotropin (somatotropin), thyrotropin (thyrotripinoma), etc.
Among the typical lesions of the hypothalamus should be noted prolactin - a hormonally active tumor that produces prolactin. This pathological condition is accompanied by a clinical diagnosis of hyperprolactinemia and is most characteristic of the female sex. Increased production of this hormone leads to menstrual disorders, the appearance of disorders of the sexual sphere, the cardiovascular system, etc.
Another terrible disease associated with impaired functional activity of the hypothalamic-pituitary system is the hypothalamic syndrome. This condition is characterized not only by hormonal imbalances, but also by the appearance of disorders on the part of the vegetative sphere, disturbances of metabolic and trophic processes. Diagnosis of this condition is sometimes extremely difficult, as individual symptoms are masked under the symptoms of other diseases.
Thus, the hypothalamus, whose functions in life support are difficult to overestimate, is the highest integrative center responsible for controlling the vegetative functions of the body, as well as behavioral and motivational mechanisms. Being in a complex relationship with the rest of the brain, the hypothalamus is involved in the control of almost all vital constants of the body, and its defeat often leads to the emergence of serious diseases and death.