Biology

Hypothalamus: what is it, structure, functions and hormones involved

The hypothalamus is located in the brain and is a fundamental organ as we will see below. But before, … have you ever asked yourself the question of why we work the way we do? If we are hungry, thirsty, if we retain fluids, or why are we sleepy sometimes and sometimes not … and even why do we feel that way in love?

The hypothalamus, that tiny structure, is responsible for much of our vital functions. Next, you have all the information.

What is the hypothalamus?

The hypothalamus is considered an endocrine organ and is a region of the brain that, together with the thalamus, forms the diencephalon, which is part of the limbic system. The hypothalamus is essential for life since it is responsible for coordinating vital functions, among other things, as we will see later, controlling the endocrine system and the autonomic nervous system.

It has a very small size, similar to that of a pea although not its shape, and the diversity of neurons it contains is not found in another organ.

Where is the hypothalamus located?

In humans, the diencephalon is located in the center of the brain, below the cerebral cortex and above the brain stem.

Its name refers to the fact that it is located below the thalamus.

Parts or nuclei of the hypothalamus

The hypothalamus is made up of structures called the nucleus, each with a defined and characteristic function:

  • Arcuate nucleus : Participates in the emotional function of the hypothalamus. It is responsible for the release of gonadotropin-releasing hormone (GnRH), also known as luteinizing hormone-releasing hormone (LHRH).
  • Anterior Hypothalamic Nucleus : It is responsible for heat loss through sweating as well as inhibiting the release of thyrotropin in the pituitary.
  • Posterior Hypothalamic Nucleus : Its function is to keep warm when you feel cold.
  • Lateral Nuclei : Regulates the sensation of hunger and thirst.
  • Mamilar nucleus : it is related to memory through its connections with the hippocampus.
  • Paraventricular Nucleus : Regulates the secretion of the pituitary through the synthesis of hormones, such as oxytocin, vasopressin, and adrenocorticotropic hormone-releasing hormone (CRH).
  • Preoptic nucleus : participates in parasympathetic functions that are related to feeding, locomotion and mating.
  • Supraoptic Nucleus : regulates blood pressure and homeostatic balance, through antidiuretic hormone (ADH).
  • Suprachiasmatic nucleus : regulates the circadian cycle.
  • Ventromedial nucleus : it is involved in aggressive and defensive behaviors.
  • Dorsomedial nucleus : regulates the feeling of satiety.

Function of the hypothalamus

More than talking about the function of the hypothalamus, we must talk about functions. The hypothalamus is critical to keeping humans alive as it controls and coordinates many of the vital functions, as well as other important functions.

  • Maintaining body temperature : it is responsible for maintaining constant body temperature by controlling the respiratory rate and sweating through the anterior hypothalamus (parasympathetic), and dissipating heat through the anterior hypothalamus (sympathetic).
  • Regulates appetite and thirst : it regulates it through hormones and peptides such as cholecystokinin, the level of glucose and fatty acids in the blood, and the neuropeptide Y.
  • It regulates sleep and circadian rhythms : it does so through the suprachiasmatic nucleus structure that receives information from the retina, specifically from ganglion cells through the retino-hypothalamic tract. As the retina detects light changes, depending on the presence or not, they send information to the epiphysis (or pineal gland). When there is no light, the epiphysis secretes melatonin to promote sleep and if there is, it reduces its levels to stay awake.

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  • Regulates mating behavior : through the release of oxytocin, which is thought to participate in orgasm and in maternal and paternal behavior. The distention of the uterine cervix and vagina during labor causes the release of oxytocin as well as the stimulation of the nipple by the baby, facilitating labor and breastfeeding.
  • Regulates vital endocrine and visceral functions :
    • Coordinates the female menstrual cycle and spermatogenesis in men, by balancing hormones: gonadotropin-releasing hormone (GnRH, LRHR), luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
    • Regulates blood pressure and kidney function.
    • Regulates hormones and growth factors
  • Participates in the regulation of memory
  • Participate in the available energy level
  • Participate in how we feel love: through a neurotransmitter, phenylethiamine, it produces a pleasant and euphoric sensation, with an increase in adrenaline and norepinephrine that increase the heart rate and blood pressure causing the sensations of falling in love.

Hypothalamic pituitary axis

The pituitary gland or pituitary gland and the hypothalamus are connected through the infundibulum. Their relationship is very important and close since both are needed to exist. The function of the pituitary is to spread the effects of hormones and hypothalamic factors to the rest of the glands and target organs in the rest of the body.

The pituitary gland lies under the hypothalamus, and is protected by the bony structure called the sella turcica.

Hypothalamic Hormones and Hypothalamic Factors

The hypothalamus is capable of secreting hormones but its main function is to release inhibitory or stimulating factors into the blood so that they act on other organs and hormones.

Hormones secreted by the hypothalamus

The hypothalamus secretes a few hormones, which are classified as neurohormones, some of them mentioned above:

Antidiuretic hormone (ADH)

This hormone is one of the most important. It is produced by the hypothalamus in the supraoptic and paraventricular nuclei and accumulates in the neurohypophysis that secretes it. This hormone is released when there is a decrease in plasma volume or an increase in osmolarity (plasma concentration).

This hormone is also known as vasopressin and is in charge of regulating the water balance, which is why it acts on the kidneys and is important for the regulation of blood pressure.

Its main functions are:

  • Increase water reabsorption and prevent water loss due to gastric problems by acting on the final portion of the distal tubule and the renal collecting tubes. Water reabsorption will cause:
  • Retention of water from the kidneys to excrete only just enough to eliminate waste products
  • Increased blood volume
  • Increased venous return
  • Increase in the volume of the beat and therefore, increase in cardiac output.
  •  It generates vasoconstriction by acting on smooth muscle, and increases peripheral vascular resistance.
  • It acts as a neurotransmitter by inhibiting discharges from the supraoptic and paraventricular nucleus that also produce oxytocin. ADH or vasopressin also acts on the brain amygdala as a fear hormone.
  • It alters blood pressure, and acts as an antipyretic and analgesic when administered intracerebrally.
  • It seems that it participates in the formation of memory, but its implication is not yet clear.
  • It is inhibited by alcohol consumption, so water reabsorption does not occur. This reason leads to frequent use of the bathroom when consuming alcohol.

Oxytocin

Its name in Greek means rapid birth (oxys “rapid” and tokos “birth”). Like ADH, this hormone is produced by the supraoptic and paraventricular nuclei of the hypothalamus and is stored and released by the neurohypophysis. It also acts as a neurotransmitter in the brain.

As mentioned in the previous point, it is related to maternal and paternal behavior, sexual stimulation, increases confidence and reduces social fear and may be related to autism.

Hypothalamic factors

Gonadotropin-releasing hormone (GnRH)

It acts on the pituitary gland, stimulating the production and release of luteinizing hormone (LH) and follicle- stimulating hormone (FSH). As I said before, it coordinates the female menstrual cycle and male spermatogenesis. It is a decapeptide.

Thyrotropin-releasing hormone (TRH)

It is a tripeptide and its function is to stimulate the secretion of prolactin and thyrotropin (TSH) that regulates the production of thyroid hormones by the thyroid gland.

Adrenocorticotropin Hormone Releasing Hormone (CRH or CRF)

Stimulates ACTH secretion together with ADH and angiotensin II that enhance ACTH secretion. Oxytocin inhibits CRH-mediated ACTH secretion.

CRH is secreted anteriorly from the paraventricular nuclei in their lateral position alongside TRH-secreting neurons.

CRH increases with pregnancy and during delivery. In addition, it is related to stress and energy balance due to its role in ACTH secretion. Its maximum peak occurs in the mornings.

Somatocrinin or somatrotropin-releasing hormoan (STH) or growth hormone releasing factor

It is produced in the arcuate nucleus of the hypothalamus and stimulates the release of pituitary growth hormone.

Somatostatin or somatropin release inhibitory hormone (GIH)

It inhibits the secretion of somatotropin, insulin, glucagon, pancreatic polypeptide, and TSH. It is secreted by the periventricular region of the hypothalamus.

Prolactin release inhibitory factor (PIF)

It inhibits pituitary prolactin secretion and is secreted by the hypothalamic arcuate nucleus.

Angiotensin II (AII)

Stimulates the action of corticotropin-releasing hormone. Angiotensin II increases blood pressure since it stimulates the Gq protein in vascular smooth muscle cells, acts on the Na / H exchanger in the proximal tubules of the kidney, stimulating Na reabsorption and hydrogen secretion, which implies an increase blood volume, pressure and pH.

Diseases caused by dysfunctions of the hypothalamus

  • Hypothalamic amenorrhea : it is produced by the suppression of the thalamic-pituitary-ovarian axis. It is associated with stress, weight loss and physical exercise. Its importance is that it prevents the accumulation of bone mass in young women with its consequent impact over time with a higher incidence of osteoporosis. It is produced by suppressing the release of gonadotropin-releasing hormone (GnRH). Of the three factors, the most important is the caloric deficit since it appears that leptin plays an important role in the regulation of hypothalamic dysfunction.
  • Diabetes insipidus : caused by dysfunction of the hypothalamus in the production of ADH. The secretion of this hormone is very low and people who suffer from this diabetes go to the bathroom very frequently, being able to become dehydrated for this reason.
  • Korsakoff syndrome : or anterograde amnesia. Those who suffer from it are unable to generate new long-term memories, filling in the gaps in their memory through inventions to compensate for forgetfulness. It is produced by alcoholism but also by the alteration of the hippocampus.

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