Our body and the organs that compose it work in harmony, just as a watchmaker would, to maintain our physical health so that all the functions and activities of the body can be carried out effectively.

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

What is the neurohypophysis?

Within the endocrine system, formed by a large number of organs and structures producing hormones, we find the neurohypophysis. This organ forms the back of the pituitary gland.

One of the main differences between the neuro-pituitary 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 anterior pituitary is. Furthermore, the anterior pituitary has a growth directed towards 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 that lead to the back of the anterior pituitary. The main parts into which the pituitary gland is divided are the medial eminence, infundibulum, and pars nervosa, which we will discuss in the next section.

As for the elements or pieces that make up the mass of the neurohypophysis, this is composed of a series of cells called pythuites , which can be considered as glial support cells.

Finally, although at first glance the neuro-pituitary may appear to be just another hormone-secreting gland, it is actually a kind of storehouse for substances secreted in the hypothalamus.

While it is true, the neuronal cells of the hypothalamic supra-optic and paraventricular nuclei secrete vasopressin and oxytocin that is stored in the axon vesicles of the neuro-pituitary gland, which release these hormones in response to electrical impulses coming from the hypothalamus.

Structure

As mentioned above, the posterior area of the pituitary, or neurohypophysis, consists primarily of neuronal projections of magnocellular neurosecretory cells extending from the supra-optic and paraventricular nuclei of the hypothalamus.

In the axons of these neurosecretory cells, the neurophyseal hormones known as oxytocin and vasopressin are stored and released. These are released into the neurophyseal capillaries . From there some 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 according to classifications, most sources include the following three structures:

1. Middle eminence

The area of the neuro-pituitary known as the medial eminence is that which is attached to the infundibulum. It takes the form of a small swelling and is one of the seven areas of the brain that have no blood-brain barrier, which means that is an organ with permeable capillaries .

The main function of the Middle Eminence is to act as a gateway for the release of hypothalamic hormones. However, it also shares continuous perivascular spaces with the adjacent hypothalamic arched nucleus, indicating a possible sensory role.

2. Infundibulum

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

3. Pars nervosa

Also known as the neural lobe or posterior lobe , this region constitutes the major part of the neuro-pituitary gland and is the storage site for oxytocin and vasopressin. It is often considered to be synonymous with the neuro-pituitary, but it is only one part of the pituitary.

Finally, some classifications also include the middle pituitary gland as part of the neuro-pituitary, but this is not common.

Functions

Despite the fact that, as mentioned at the beginning of the article, on many occasions the neurohypophysis is mistakenly considered to be 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 gland. After their production, they are stored in the regrouped neurosecretory vesicles, before being secreted into the neuro-pituitary gland via the bloodstream.

1. Oxytocin

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

2. Vasopressin

Also known as antidiuretic hormone (ADH), vasopressin arginine (AVP), or argipressin. The main functions of this hormone peptide include increasing the amount of water without solute reabsorbed into the circulation and the contraction of the arterioles, which increases peripheral vascular resistance and raises 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 may play an important role in social behavior, sexual motivation, bonding, and the mother’s response to stress.

What happens if this fails? Associated diseases

A lesion, degeneration or alteration in the functioning of the neurohypophysis may result in a deregulation of the secretion of the two hormones described in the previous section.

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 causes you to excrete up to 20 liters of dilute 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 Inadequate Antidiuretic Hormone Secretion (SIADH), a disease of the neuro-pituitary caused mostly by drugs and which causes all kinds of gastrointestinal, neuromuscular, respiratory and neurological symptoms.