The hypothalamic and pituitary neuroendocrine system and its functioning

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Anthony Golden
The hypothalamic and pituitary neuroendocrine system and its functioning

The endocrine system is made up of neurons and glands that produce and secrete hormones, which are chemicals produced in the body that regulate the activity of cells and organs. These hormones regulate the body's growth, metabolism (the body's physical and chemical processes), and sexual development and function. Hormones are released into the bloodstream and can affect one or more organs throughout the body..

Contents

  • Function of hormones
  • Hypothalamus and pituitary hormones
  • Pituitary gland
  • Neurohypophysis system
    • Functions of oxytocin
    • Functions of vasopressin
  • Adenohypophysis system: hypothalamic pituitary portal system
    • How are hormones released?
    • What are the hormones secreted by the anterior pituitary?
  • Axes of the hypothalamic pituitary portal system
    • Hypothalamic-pituitary corticoadrenal axis
    • Adrenocortical hormones
    • What happens when there is a deficiency of adrenocortical hormones?
    • What happens when there is an excess of adrenocortical hormones?
    • Hypothalamic pituitary thyroid axis
    • Thyroid hormones
    • What happens when there is a deficiency of thyroid hormones (hypothyroidism)?
    • What happens when there is an excess of thyroid hormones (hyperthyroidism)?
    • What are the main functions of sex hormones?
    • Prolactin axis
    • Growth hormone axis
  • Hormones not released by pituitary action
  • Hormones of the adrenal medulla
  • Hormones of the pancreas
  • Other hormones
    • References

Function of hormones

Hormones are chemical messengers created by the body. They transfer information from one set of cells to another to coordinate the functions of different parts of the body.

The main glands of the endocrine system are the hypothalamus, the pituitary gland, the thyroid, the parathyroids, the adrenal glands, the pineal body, and the reproductive organs (ovaries and testes). The pancreas is also part of this system; has a role in the production of hormones and also in digestion.

Hormones control a large number of physiological functions (metabolism, alert reactions, homeostasis, growth, reproduction, pain, etc.), but they are also heavily involved in behavior.

The glands can be the following two types:

  • Exocrine glands, which secrete their products through ducts (digestive, sweat, and mammary glands).
  • Endocrine glands, which secrete hormones into the blood and act on target organs or tissues.

Hypothalamus and pituitary hormones

The almond-sized hypothalamus lies below the thalamus and sits just above the brain stem. All vertebrate brains have a hypothalamus. Its main function is to maintain homeostasis (stability of the internal environment) in the body.

One of the main functions of the hypothalamus is to control the endocrine system, and it does so through neurosecretory cells, which are specialized neurons that, instead of secreting a neurotransmitter, release a hormone into the bloodstream..

The hypothalamus links the nervous and endocrine systems through the pituitary gland. Its function is to secrete releasing hormones and to inhibit hormones that stimulate or inhibit (as the name implies) the production of hormones in the anterior pituitary. Specialized neuronal clusters called neurosecretory cells in the hypothalamus produce the hormones Antidiuretic Hormone (ADH) and Oxytocin (OXT) and transport them to the pituitary, where they are stored for later release..

The pituitary is located at the base of the brain, attached to the hypothalamus by a stalk (the median eminence), and consists of two very different parts, which function independently and have different embryological origins:

  • Posterior pituitary or neurohypophysis, considered as an extension of the hypothalamus. Stores and releases two hormones synthesized by the hypothalamus.
  • Anterior pituitary or adenohypophysis, has no nerve connection and acts like a real gland. Secretes hormones that go to endocrine glands or tissues.

The control that the hypothalamus exerts over the pituitary is carried out in the following two ways:

  • Directly, synthesizing hormones (oxytocin and vasopressin) that travel through axons from hypothalamic cells to the neurohypophysis. From there they are released into general circulation.
  • Indirectly, by synthesizing hormones (release factors) that are secreted in the portal vessel of the median eminence and carried to the adenohypophysis. These hormones stimulate or inhibit the secretory activity of the cells of the anterior pituitary..

Thus, the hypothalamus communicates neurally with the posterior pituitary, and via the bloodstream with the anterior pituitary.

Pituitary gland

The pituitary gland is a small gland in the brain. It is known as the master gland because the hormones it produces affect the production of many other hormones and functions in the body..

Attached to the hypothalamus, it is a reddish-gray body the size of a pea that stores hormones from the hypothalamus and releases them into the bloodstream. The pituitary is divided into an anterior lobe and a posterior lobe, each of which has different functions..

Hormones produced by the anterior pituitary gland affect adrenocortical function, sexual development, growth, skin pigmentation, and thyroid function. If the anterior part of the pituitary gland does not function properly, there is retarded growth and decreased function of all other glands controlled by this part of the pituitary gland except the parathyroid glands. When abnormal pituitary function occurs, there is overgrowth or acromegaly.

The posterior pituitary gland is the back of the pituitary. It secretes antidiuretic hormone (ADH) that affects water retention in the body and oxytoxin, facilitating uterine union and contractions. Lack of ADH causes diabetes insipidus, leading to excess urination and possibly dehydration.

Neurohypophysis system

The secretion of the posterior pituitary consists of the release of the following two hormones:

  • Oxytocin
  • Vasopressin or antidiuretic hormone (ADH)

These hormones are produced in two nuclei of the hypothalamus that contain large neurons, the magnocellular ones. The hypothalamic nuclei are as follows:

  • Supraoptic
  • Paraventricular

The axons of the cells of these nuclei through the median eminence to the neurohypophysis, where they come into contact with the blood capillaries of the general circulation and release the aforementioned hormones..

Vasopressin and oxytocin are peptides that are synthesized as prohormones in the bodies of magnocellular neurons and are transported in vesicles along the axons to the neurohypophysis. It is in this pathway that the hormones oxytocin and vasopressin themselves are formed..

Functions of oxytocin

They are functions related to reproduction. These functions are as follows:

  • Stimulate the secretion of milk by the breasts during lactation.
  • Promote uterine contractions at the time of fertilization and delivery.

Functions of vasopressin

  • Cause the reabsorption of water in the kidneys, therefore, decrease the production of urine.
  • Contribute to homeostasis: regulate blood volume, electrolyte balance and blood pressure (increases it).

Adenohypophysis system: hypothalamic pituitary portal system

The anterior pituitary gland functions as a true endocrine gland, as it is made up of neurosecretory cells. But, in addition, it is also under strict hormonal control by the hypothalamus.

Hypothalamic hormones are generally small peptides and are called releasing factors or releasing hormones, and inhibitory factors or inhibitory hormones, depending on whether they act by stimulating or inhibiting hormone secretion from the anterior pituitary gland..

How are hormones released?

There are hypothalamic nuclei, in the periventricular zone (for example, the arcuate, the periventricular, the medial preoptic area) that synthesize and send the releasing or inhibiting factors in the portal circulation (the capillaries of the median eminence). From there they are transported to the adenohypophysis, where they stimulate or inhibit cells that secrete pituitary hormones..

Adenohypophyseal hormones act on other glands in the body, stimulating the release of hormones into the blood. Some of these glands are the adrenal glands, the thyroid, the gonads, the mammary glands.

What are the hormones secreted by the anterior pituitary?

Of the hormones secreted by the anterior pituitary gland, four are tropic hormones, that is, they target another gland on which they act to regulate their hormonal production. These are the following:

  • Adrenocorticotropic hormone or corticotropin (ACTH). The acronym with which hormones are usually known corresponds to their name in English (ACTH, adrenocorticotropic hormone).
  • Thyroid-stimulating hormone (TSH) or thyrotropin. They include follicle stimulating hormone (FSH) and luteinizing hormone (LH)

Apart from these tropic hormones, the anterior pituitary also secretes:

  • Prolactin
  • Growth hormone (GH) or somatotropin

Taking into account the target organ of the pituitary hormones, we can distinguish different hormonal axes:

  • Hypothalamic-pituitary corticoadrenal axis
  • Hypothalamic-pituitary thyroid axis
  • Gonadal hypothalamic-pituitary axis
  • Prolactin axis
  • Growth hormone axis

Axes of the hypothalamic pituitary portal system

Hypothalamic-pituitary corticoadrenal axis

The main control of this axis is exerted by the hormone ACTH of the anterior pituitary; when ACTH reaches the adrenal gland, hormones are released. ACTH secretion is controlled by the hypothalamic hormone CRH and also by the level of adrenocortical (or adrenocortical) hormones in the blood. If the level of adrenocortical hormones decreases, the secretion of CRH and ACTH occurs.

Adrenocortical hormones

Glucocorticoids:

  • They increase the level of glucose in the blood, accelerate the breakdown of proteins.
  • In high concentrations, they have anti-inflammatory effects.

Mineralocorticoids:

  • They cause retention of sodium ions and elimination of potassium ions in the urine.

What happens when there is a deficiency of adrenocortical hormones?

Addison's disease, which consists of an under-functioning of the adrenal glands. It has the following consequences: tiredness, apathy, cognitive deficits, depression, etc..

What happens when there is an excess of adrenocortical hormones?

In situations of chronic stress, a large amount of glucocorticoids are released and that causes a depression in the immune system, an increase in blood pressure, damage to the nervous tissue (for example, in the hippocampus). and muscle, growth inhibition, infertility, etc..

Hypothalamic pituitary thyroid axis

The main control of this axis is exerted by the hormone TSH of the anterior pituitary; When TSH reaches the thyroid gland, the release of thyroid hormones occurs. TSH secretion is controlled by the hypothalamic hormone TRH and also by the level of thyroid hormones in the blood. If the level of thyroid hormones decreases, the secretion of TRH and TSH occurs.

Thyroid hormones

  • The main role is to regulate metabolic processes and especially the use of carbohydrates.
  • It also influences growth and development, both body and nervous system.

What happens when there is a deficiency of thyroid hormones (hypothyroidism)?

If it is during development, there is an arrest of body growth, facial malformations and reduction in the size and cellular structure of the brain. This leads to mental retardation and is called cretinism..

If it occurs later, behavioral disorders such as apathy, depression, delayed speech, etc. are observed..

What happens when there is an excess of thyroid hormones (hyperthyroidism)?

Generally, physiological and behavioral changes: insomnia, irritability, nervousness, increased heart rate and blood pressure, changes in temperature, decreased weight, etc..

What are the main functions of sex hormones?

Androgens:

  • They promote the development, growth and maintenance of the male reproductive organs.
  • They promote the development of male secondary sexual characteristics (body shape, tone of voice, beard, etc.).
  • They stimulate protein metabolism.

Estrogens:

  • They promote the development, growth and maintenance of the female reproductive organs.
  • They promote the development of female secondary sexual characteristics (body shape, breasts, hair pattern, etc.).

Progestins:

  • Prepare the walls of the uterus for implantation of the fertilized egg.
  • Prepare the breasts to secrete milk.

Prolactin axis

Prolactin stimulates the production of milk by the mammary glands. During lactation, the hypothalamus reduces dopamine secretion so that a sufficient level of prolactin is produced and milk production does not stop.

Growth hormone axis

Growth hormone or somatotropin stimulates the body's growth by producing substances that regulate bone growth. It is controlled by GHRH, which stimulates its production, and somatostatin, which inhibits it..

GH shortage produces dwarfism, while excess produces gigantism. However, if the excess is in adulthood it no longer produces gigantism because the bones cannot grow in length, but acromegaly does occur, characterized by an increase in some tissues such as the jaw and the joints of the hands and feet.

Hormones not released by pituitary action

So far we have dealt with all those hormones, the secretion of which is under the control of the tropic hormones of the pituitary. Next we will explain those hormones that escape from this hypothalamic-pituitary control.

Hormones of the adrenal medulla

The inner region of the adrenal glands constitutes the adrenal medulla and releases the following hormones:

  • Adrenalin
  • Noradrenaline

The main function of these hormones is to prepare the body for situations of great effort or tension (it provides a greater blood supply to the heart, skeletal muscles and the brain) and they trigger different metabolic processes that provide the energy necessary for these organs work properly (increases blood glucose and oxygen).

Hormones of the pancreas

The pancreas is a gland that secretes several hormones, including the following:

  • Insulin
  • Glucagon

Insulin is released as a result of increased blood sugar levels and its function is to stimulate the uptake of glucose by the tissues and to transform excess glucose into glycogen (it is stored in the liver and muscles) and in triglycerides (in adipose tissue).

Glucagon is released after a while without eating, as blood glucose levels drop. Glucagon causes a rise in glucose by causing liver glycogen to break down and convert to glucose.

Other hormones

There are many other hormones, such as the ones listed below:

  • Gastrointestinal (cholecystokinin, gastrin, etc.)
  • Calcium regulators (parathyroid, calcitonin)
  • Melatonin

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References

Bradford, H.F. (1988). Fundamentals of neurochemistry. Barcelona: Labor.

Carlson, N.R. (1999). Physiology of behavior. Barcelona: Ariel Psychology.

Carpenter, M.B. (1994). Neuroanatomy. Fundamentals. Buenos Aires: Editorial Panamericana.

Delgado, J.M .; Ferrús, A .; Mora, F .; Rubia, F.J. (eds) (1998). Neuroscience Manual. Madrid: Synthesis.

Diamond, M.C .; Scheibel, A.B. and Elson, L.M. (nineteen ninety six). The human brain. Work book. Barcelona: Ariel.

Guyton, A.C. (1994) Anatomy and physiology of the nervous system. Basic neuroscience. Madrid: Editorial Médica Panamericana.

Kandel, E.R .; Shwartz, J.H. and Jessell, T.M. (eds) (1997) Neuroscience and Behavior. Madrid: Prentice Hall.

Martin, J.H. (1998) Neuroanatomy. Madrid: Prentice Hall.

Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.


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