Articles

The Neuroendocrine System: hypothalamus, pituitary gland and its functioning

The Neuroendocrine System: hypothalamus, pituitary gland and its functioning

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

Content

  • 1 Function of hormones
  • 2 Hypothalamus and pituitary hormones
  • 3 Pituitary gland
  • 4 Neurohypophysis system
  • 5 Adenohypophysis system: pituitary hypothalamus portal system
  • 6 Axes of the pituitary hypothalamus portal system
  • 7 Hormones not released by pituitary action
  • 8 Hormones of the adrenal medulla
  • 9 Hormones of the pancreas
  • 10 Other hormones

Hormone function

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, pituitary, thyroid, parathyroid, adrenal glands, pineal body and reproductive organs (ovaries and testicles). The pancreas is also part of this system; It has a role in the production of hormones and also in digestion.

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

The glands can be the following two types:

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

Hypothalamus and pituitary hormones

The hypothalamus of the size of an almond is located under the thalamus and is located 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 by means of neurosecretory cells, which are specialized neurons that instead of secreting a neurotransmitter release a hormone in the bloodstream.

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

The pituitary gland is located at the base of the brain, attached to the hypothalamus by a stem (the middle 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. It stores and releases two hormones synthesized by the hypothalamus.
  • Anterior pituitary or adenohypophysisIt 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 on the pituitary gland is carried out in the following two ways:

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

Thus, the hypothalamus communicates neurally with the posterior pituitary, and by blood with the anterior pituitary.

Pituitary gland

The pituitary gland is a small gland in the brain. Is known like 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.

The hormones produced by the pituitary gland Anterior affect adrenocortical function, sexual development, growth, skin pigmentation and thyroid function. If the anterior part of the pituitary gland does not work properly, delayed growth and decreased function of all other glands controlled by this part of the pituitary gland, except for the parathyroid glands, occur. When abnormal pituitary function occurs, there is excess growth or acromegaly.

The posterior pituitary gland It is the back of the pituitary gland.Secretes antidiuretic hormone (ADH) that affects water retention in the body and oxytoxin, which facilitates the union and uterine contractions. Lack of ADH causes diabetes insipidus, which leads to excess urine and possibly dehydration.

Neurohypophysis system

The secretion of the posterior pituitary consists in 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. The hypothalamic nuclei are the following:

  • Supraoptic
  • Paraventricular

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

Vasopressin and oxytocin are peptides that are synthesized as prohormones in the soma of magnocellular neurons and transported in vesicles along the axons to the neurohypophysis. This is when the hormones oxytocin and vasopressin themselves are formed.

Oxytocin Functions

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

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

Vasopressin functions

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

Adenohypophysis system: pituitary hypothalamus portal system

The adenohypophysis works like a true endocrine gland, since it is formed by neurosecretory cells. But, in addition, it is also under strict hormonal control by the hypothalamus.

Hormones hypothalamus They 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 the hormonal secretion of the anterior pituitary gland.

How are hormones released?

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

Adenohypophyseal hormones act on other glands in the body, and stimulate the release of hormones in 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 gland?

Of the hormones secreted by the adenohypophysisFour are tropic hormones, that is, they have 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 commonly 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 adenohypophysis also secretes:

  • Prolactin
  • Growth hormone (GH) or somatotropin

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

  • Corticoadrenal hypothalamus-pituitary axis
  • Hypothalamic-pituitary thyroid axis
  • Hypothalamus-pituitary gonadal axis
  • Prolactin shaft
  • Growth hormone axis

Axes of the pituitary hypothalamus portal system

Corticoadrenal hypothalamus-pituitary axis

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

Adrenocortical hormones

Glucocorticoids:

  • Increase blood glucose level, accelerate protein degradation.
  • In high concentrations, they have anti-inflammatory effects.

The mineralocorticoids:

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

What happens when there is an adrenocortical hormone deficit?

Addison's disease, which consists of a hypofunctioning of the adrenal glands. It has the following consequences: tiredness, apathy, cognitive deficits, depression, etc.

What happens when there is an excess of adrenal corticosteroid hormones?

In situations of chronic stress, a large amount of glucocorticoids is released and that means that in the medium-long term there is 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.

Thyroid pituitary hypothalamus axis

The main control of this axis is exercised by the TSH hormone of the anterior pituitary gland; 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, 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 deficit of thyroid hormones (hyperthyroidism)?

If it is during development, there is a arrest of body growth, facial malformations and reduction of brain cell size and structure. This entails mental retardation and is called cretinism.

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

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, temperature changes, weight loss, etc.

What are the main functions of sex hormones?

Androgens:

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

The progestogens:

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

Prolactin shaft

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

Growth hormone axis

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

GH shortage produces dwarfismwhile the excess produces giantism. However, if the excess is in adulthood it no longer produces gigantism because the bones cannot grow in length, but it does occur acromegaly, 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 treated all those hormones, the secretion of those under the control of the tropic hormones of the pituitary gland. Next we will explain those hormones that escape this hypothalamic-pituitary control.

Adrenal medulla hormones

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

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

Pancreas hormones

The pancreas is a gland that secretes several hormones, among which are the following:

  • Insulin
  • Glucagon

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

Glucagon is released after a while without eating, as blood glucose levels decrease. Glucagon causes an increase in glucose because it causes liver glycogen to degrade and become glucose.

Other hormones

There are many other hormones, such as those listed below:

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

Visit here our Visual and interactive brain atlas

References

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

Carlson, N.R. (1999). Behavioral physiology. Barcelona: Ariel Psychology.

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

Delgado, J.M .; Ferrús, A .; Mora, F .; Blonde, 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: Pan American Medical Editorial.

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.

Related tests
  • Depression test
  • Goldberg depression test
  • Self-knowledge test
  • how do others see you?
  • Sensitivity test (PAS)
  • Character test