Hypothalamus: What is it? Anatomy, Characteristics, Functions and Pathologies of A.Griguolo

Generality

The hypothalamus is the important structure of the encephalon which takes place just below the thalamus and which directs the activity of the pituitary gland, a major endocrine gland that is fundamental to the life and well-being of the human being.

Of the size of a pea, the hypothalamus is a concentrate of nerve nuclei, which can be divided into three regions (anterior, median and posterior), which are in turn divisible into areas.

Connected to the reticular formation of the brainstem and to the amygdala, the hypothalamus takes part in the maintenance of the body homeostasis, through for example the regulation of the body temperature or the sense of satiety, and to the release of hormones destined to involve the pituitary gland.

The hypothalamus constitutes the anatomical element of connection between the nervous system and the endocrine system.

What is the Hypothalamus?

The hypothalamus is a small but important structure of the encephalon, interposed between the thalamus, superiorly, and the hypophysis, inferiorly.

The hypothalamus is a concentrate of nerve nuclei, which give it a variety of functions, including a role in the so-called limbic system and a major endocrine role.

The hypothalamus, therefore, belongs as much to the central nervous system as to the endocrine system ; to be precise, neurologists describe it as the connecting organ between the nervous system and the endocrine system.

Did you know that ...

The brain of all Vertebrates contains the hypothalamus or an equivalent structure.

Origin of the name

The term "hypothalamus" literally means "under the thalamus" and wants to be a clear reference to the position occupied, in the brain, by the organ to which it refers ("ipo-" is the prefix used to express the concept of "below" or, more generally, of an "inferiority").

Anatomy

Component of the diencephalon, the hypothalamus has the size and shape of a pea, and constitutes less than 1% of the total weight of the brain ; the hypothalamus, therefore, is a small and not very heavy organ.

Linked to the pituitary gland through the so-called infundibulum of the hypothalamus (or pituitary infundibulum ), the hypothalamus can be divided into 3 regions: the anterior region, the median region and the posterior region ; each of these regions can be further subdivided into the so-called hypothalamus areas (or hypothalamic areas ), which are characterized by including specific nerve nuclei within their borders.

Like any other part of the brain, also the hypothalamus is not an isolated structure, but connected to other nerve components, with which it collaborates for a specific purpose.

What is a nerve nucleus?

A nerve nucleus is an aggregate of neuron bodies having an identical role (NB: the neuron is the nerve cell and the body of a neuron is the seat of the cell nucleus); a nerve nucleus, therefore, is a collection of highly specific nerve cells, all of which have the same function.

It is important to point out that the concept of the nervous nucleus is totally different from the concept of the cell nucleus: the latter is the seat of the DNA of a cell and is present in all the cells of the human organism, except red blood cells, platelets and corneal scales of the skin).

Where exactly does the Hypothalamus reside?

Located at the base of the skull, where the ventral portion of the diencephalon extends, the hypothalamus takes its place below and slightly ahead of the thalamus, and above the posterior pituitary, to which it is connected through the already mentioned infundibulum of the pituitary gland.

Together with the proper brain, cerebellum and brainstem, the diencephalon is one of the four main subdivisions of the human brain.

Hypothalamus regions

FRONT REGION

Also known as a supraoptic region, the anterior region of the hypothalamus includes 3 areas:

• The preoptic area, which contains the median preoptic nucleus, the medial preoptic nucleus, the periventricular preoptic nucleus and the ventro-lateral preoptic nucleus;
• The medial area, which includes the supraoptic nucleus, the paraventricular nucleus, the anterior hypothalamic nucleus and the suprachiasmatic nucleus;
• The lateral area, in which the lateral core extends.

As we will see in more detail in the chapter on functions, for most of its extension, the anterior region of the hypothalamus is involved in the production of hormones (therefore it has the function of an endocrine gland), hormones that interact with the pituitary gland or direct it 'activities.

MEDIAN REGION

Also known as the tuberal region, the median region of the hypothalamus includes 2 areas:

• The medial area, in which the dorso-medial nucleus, the ventro-medial nucleus and the arcuate nucleus are enclosed, and
• The lateral area, which includes the lateral nucleus and the lateral tuberal nucleus.

REAR REGION

Also known as the mammillary region, the posterior region of the hypothalamus also includes 2 areas:

• The medial area, in which the mammillary nucleus and the posterior nucleus are located, e
• The lateral area, which houses the lateral nucleus and the tuber-mammillary nucleus.

Hypothalamus connections

As announced, the hypothalamus is in connection with other structures of the brain; in particular, it establishes connections with:

• The so-called reticular formation of the brainstem . Also known as a reticular substance, the reticular formation is a particular component of the brainstem, which brings together neurons responsible for regulating certain activities of the spinal cord, the cerebral cortex and the brainstem itself;

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• The amygdala . The amygdala is an even region of the brain, home to several nerve nuclei, which belongs to the innermost portion of the temporal lobe; similar to an almond in shape and size, the amygdala plays a key role in the formation and memorization of memories associated with emotional events; is responsible for the so-called fear conditioning; participates in the development of emotions such as fear, anger, happiness, etc .; favors the memory of what has caused pain; finally, it is involved in excitement and decision-making processes.

Hypothalamus and Limbic System

Together with the amygdala, the reticular formation of the brainstem and other parts of the brain, the hypothalamus takes part in the constitution of the limbic system .

The limbic system is a collection of encephalic structures, which play a key role in emotional reactions, short-term memory processes, behavior and smell.

The Hypothalamus in Man and Woman

Man has a hypothalamus slightly different from the hypothalamus present in the woman.

The differences between the male and female hypothalamus consist of changes in nerve connections and biochemical sensitivity, and mainly concern the pre-optic area of ​​the anterior region.

The differences between the male and female hypothalamus are the cause of various consequences, such as:

• Man's tendency to be more attracted to the smell and appearance of women. The confirmation of this consequence comes from the lesions of the hypothalamus in man, which entail a decrease in the attraction of the patient towards the female sex;

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• The different modes of secretion of growth hormone in the two sexes. This explains why men and women are different in terms of size.

Blood circulation of the hypothalamus

The influx of oxygenated blood to the hypothalamus is due to the network of arterial vessels known as the circle of Willis (or Willis's polygon ); in fact, the anterior region of the hypothalamus receives oxygen-rich blood from the anterior cerebral arteries and from the anterior communicating artery, the median region of the hypothalamus exploits the supply of oxygenated blood guaranteed by the posterior communicating arteries and, finally, the posterior region receives the oxygenated blood that comes from the basilar artery, from the posterior cerebral arteries and from the posterior communicating arteries .

Function

The hypothalamus has the important task of providing for the maintenance of body homeostasis ; by bodily homeostasis we mean the natural tendency of the human body to reach a condition of stability / balance, both as regards the biochemical and physical sphere and as regards the behavioral sphere.

Furthermore, the hypothalamus is involved in the secretion of neurohormones essential to the functioning of the pituitary gland, that is the major endocrine gland that produces hormones fundamental to the life and well-being of the human being, such as growth hormone, gonadotropin, for example. vasopressin or thyrotropin. This second important function gives the hypothalamus the charge of organ of connection between the central nervous system and the endocrine system.

Finally, it is necessary to point out that the hypothalamus contributes to many other processes and mechanisms of human physiology, such as the stimulation of the gastrointestinal tract, memory, pupillary dilation and learning .

Hypothalamus and Body Homeostasis

The hypothalamus ensures the maintenance of body homeostasis thanks to the ability to:

• Adjust the body temperature through the process of sweating (in hot environments) or the reflection of the shiver (in cold environments);
• Adjust the sense of thirst by controlling the water levels in the body;
• Adjust the appetite and the sense of satiety ;
• Adjust blood pressure and heart rate ;
• Adjust the sleep-wake circadian rhythm .

In particular:

• The regulation of body temperature (or thermoregulation) belongs to the preoptic and medial areas of the anterior region of the hypothalamus, and to the medial area of ​​the posterior region of the hypothalamus;
• The regulation of thirst is due to the medial area of ​​the anterior region of the hypothalamus;
• The regulation of appetite and satiety is the prerogative of the medial area of ​​the median region of the hypothalamus and of the lateral area of ​​the posterior region of the hypothalamus;
• The regulation of blood pressure and heart rate is the task of the medial areas of the median and posterior regions of the hypothalamus;
• The regulation of sleep-wake circadian rhythm lies with the medial area of ​​the anterior region of the hypothalamus and the lateral area of ​​the posterior region of the hypothalamus;

Hypothalamus and Endocrine Activity

Through the anterior region, in particular through the supraoptic nucleus and the paraventricular nucleus of the medial area, the hypothalamus produces:

• Thyrotropin-releasing hormone (or TRH ). It is the hormone responsible for stimulating the pituitary gland, so that the latter produces and releases into circulation the thyroid-stimulating hormone (or TSH ), which, as the name implies, is the hormone that activates the thyroid gland.

  From the hormone TRH-TSH cascade depends, indirectly, the correct functioning of the heart, gastrointestinal tract and muscles;

• Gonadotropin-releasing hormone ( GnRH ). It is the hormone responsible for triggering the production, by the pituitary gland, of the follicle-stimulating hormone ( FSH ) and of the luteinizing hormone ( LH ), ie the hormones stimulating the activity of the gonads (testicles in humans and ovaries in the woman).

  Taken together, the hormones FSH and LH are called gonadotropins;

• Corticotropin-releasing hormone (or CRH). It is the hormone involved in both physical and emotional stress response. Once produced, it interacts with the pituitary gland and stimulates it to secrete the adrenocorticotropic hormone (or ACTH ); the latter has the task of activating the adrenal cortex, so that it produces cortisol, or the so-called stress hormone ;
• The hormone oxytocin . Produced by the hypothalamus but released by the posterior pituitary, oxytocin plays a key role in controlling behavior and emotions, such as sexual arousal, a sense of trust, a sense of motherhood (in women), social recognition or anxiety;
• The vasopressin hormone (or ADH or antidiuretic). Also produced by the hypothalamus but released by the posterior pituitary, vasopressin is the hormone responsible for regulating water levels in the human body; the release of vasopressin, in fact, stimulates the kidneys to absorb water;
• The hormone somatostatin . It is the hypothalamic hormone designed to block the production, by the pituitary gland, of hormones, such as growth hormone or thyroid stimulating hormone.

Moreover, through the median region, specifically through the arcuate nucleus of the medial area, the hypothalamus secretes:

• Growth hormone releasing hormone ( GHRHR ). It is the biological molecule responsible for activating the pituitary gland, so that the latter produces and releases into circulation the growth hormone, that is the hormone on which the growth and development of the human body depends;
• Dopamine designed to inhibit the production of the hormone prolactin by the pituitary gland.

  Prolactin is the pituitary hormone responsible for promoting, in women, breast development, in puberty, and milk production, in the months following the birth.

Did you know that ...

Dopamine is a neurotransmitter, which, however, when it is involved in the aforementioned inhibition process, also dresses as a hormone.

Summary table of the Hypothalamus Hormonal Activity

diseases

Given its involvement in numerous biological processes, the hypothalamus is a brain area that, if not functioning properly, can drastically affect the quality of life of human beings.

When the hypothalamus works inadequately, the doctors talk about hypothalamic dysfunction (or dysfunction of the hypothalamus ).

Causes of Hypothalamus Dysfunction

The causes of hypothalamic dysfunction include:

• Brain traumas involving damage to the hypothalamus;
• Genetic alterations affecting the gene that produces growth hormone-releasing hormone and triggering the condition known as growth hormone deficiency;
• Diseases and genetic defects that are characterized by an anomaly affecting the brain or the hypothalamus. One of these conditions is Prader-Willi syndrome;
• Tumors of the hypothalamus ;
• Eating disorders, such as anorexia nervosa or bulimia ;
• Some autoimmune diseases ;
• Brain surgery is not successful (brain surgery is a very complex procedure).

Possible Symptoms of a Hypothalamus Dysfunction

A dysfunction of the hypothalamus can have various consequences, including: abnormally high or low pressure levels, sudden changes and without plausible reason of body temperature, gain or weight loss for no apparent reason, moments of great appetite or poor appetite, insomnia, infertility, short stature, delayed puberty, frequent urination and dehydration.

Did you know that ...

Hypothalamus dysfunction plays a key role in conditions such as diabetes insipidus and hypopituitarism .

Diabetes insipidus is a syndrome characterized by conspicuous urine production and an insatiable thirst, which may be due to the lack or insufficient secretion of the hormone vasopressin by the hypothalamus and posterior pituitary gland, or the lack of activity of the hormone ADH at the level renal.

Hypopituitarism, on the other hand, is the condition characterized by a decrease in the hormonal activity of the pituitary gland, ie the major endocrine gland that is under the control of the hypothalamus.