Neurohypophysis: structure, functions and associated diseases

Our body and the organs that compose it work in harmony, just like a clockwork mechanism, to maintain our physical health and that all the functions and activities of the organism can develop efficiently.

One of the pieces of this machinery is the neurohypophysis, a small organ of the endocrine system which has an essential role in the regulation and release of some of the most important hormones for proper human functioning, both physical and psychological.

• Related article: "Parts of the human brain (and functions)"

What is the neurohypophysis?

Within the endocrine system, made up of a large number of organs and hormone-producing structures, we find the neurohypophysis. This organ forms the posterior part of the pituitary gland.

One of the main differences that exists between the neurohypophysis and the rest of the pituitary to which it belongs is that, due to its different embryological origin, its structure is not glandular as the pituitary gland is. former. Also, this has a growth directed toward the hypothalamus, so its functions also differ from those of the rest of the structure.

In contrast, the neurohypophysis is largely a collection of axonal projections from the hypothalamus which open into the posterior area of ​​the anterior pituitary. The main parts into which the pituitary gland is divided are the median eminence, the infundibulum, and the pars nervosa, which we will discuss in the next point.

Regarding the elements or pieces that make up the mass of the neurohypophysis, this It is made up of a series of cells called pituitary cells., which can be considered as supporting glial cells.

Finally, despite the fact that at first glance the neurohypophysis may appear to be a gland that secretes more hormones, it is actually a kind of warehouse for the substances secreted in the hypothalamus

While it is true, the neuronal cells of the supraoptic and paraventricular hypothalamic nuclei secrete vasopressin and oxytocin which is stored in the vesicles of the axons from the neurohypophysis, which releases these hormones in response to electrical impulses coming from the hypothalamus.

• You may be interested in: "Pituitary gland (hypophysis): the link between neurons and hormones"

Structure

As we mentioned earlier, the posterior area of ​​the pituitary, or neurohypophysis, consists mainly of neuronal projections of magnocellular neurosecretory cells extending from the supraoptic and paraventricular nuclei of the hypothalamus

The neuropituitary hormones known as oxytocin and vasopressin are stored and released in the axons of these neurosecretory cells. They are released into the neurohypophysial capillaries.. From there part of them enter the circulation of the bloodstream, while others return to the pituitary system.

Although the differentiation of the various parts of the pituitary may vary by classification, most sources include the following three structures:

1. median eminence

The area of ​​the neurohypophysis known as the median eminence is that which is attached to the infundibulum. This takes the form of a small swelling and is one of the seven areas of the brain that do not have blood brain barrier, which means that It is an organ with leaky capillaries..

The main function of the median eminence is to act as a gateway for the release of hypothalamic hormones. However, it also shares continuous perivascular spaces with the adjacent hypothalamic arcuate nucleus, suggesting a possible sensory role.

2. infundibulum

The infundibulum is the connection between the hypothalamus and the posterior pituitary. This carries axons from the magnocellular neurosecretory cells of the hypothalamus to the posterior pituitary, where they release their neurohypophyseal hormones (oxytocin and vasopressin) into the blood.

3. pars nervosa

Also known as neural lobe or posterior lobe, this region constitutes most of the neurohypophysis and is the storage site for oxytocin and vasopressin. On many occasions this is considered synonymous with the neurohypophysis, however it is only a part of it.

Finally, some classifications also include the middle pituitary as part of the neurohypophysis, but this is not usual.

functions

Despite the fact that, as mentioned at the beginning of the article, the neurohypophysis is often mistakenly considered a hormone-producing gland, Its main function is not to synthesize these substances, but to store and release the two hormones classically related to this organ: oxytocin and vasopressin.

Initially, these hormones are synthesized in the hypothalamus, transported, and released in the posterior pituitary. After their production, they are stored in the regrouped neurosecretory vesicles, before being secreted into the neurohypophysis through the bloodstream.

1. oxytocin

Oxytocin is a neuropeptide hormone which is characterized by playing an essential role in social ties, sexual reproduction in both sexes and in being of vital importance both during and after childbirth.

2. vasopressin

Also known as antidiuretic hormone (ADH), arginine vasopressin (AVP), or argipressin. The main functions of this peptide hormone include increasing the amount of free solute water reabsorbed into the circulation and contraction of arterioles, which increases peripheral vascular resistance and increases blood pressure.

In addition, it is also given a possible third function related to the release of vasopressin in certain areas of the brain. This release could play an important role in social behavior, sexual motivation, bonding between people, and the mother's response to stress.

What happens if it fails? Associated diseases

An injury, degeneration or alteration in the functioning of the neurohypophysis can have as consequently a deregulation of the secretion of the two hormones described in the section former.

Insufficient secretion of vasopressin can lead to the development of diabetes insipidus, a condition in which the body loses the ability to store and concentrate urine and which causes the person to excrete up to 20 liters of diluted urine per day.

On the other hand, an increase in the amount of vasopressin released into the blood is the main cause of Syndrome of Inappropriate Secretion of Antidiuretic Hormone (SIADH), a disease of the neurohypophysis caused mostly by drugs and that causes all kinds of gastrointestinal, neuromuscular, respiratory and neurological symptoms.