Neurohypophysis: structure, functions and associated diseases
Our body and the organs that compose it work in harmony, just as a watchmaking machinery would, in order to maintain our physical health and that all the functions and activities of the organism can be developed effectively.
One of the pieces of this machinery is the neurohypophysis, a small organ of the endocrine system It has an essential role in the regulation and release of some of the most important hormones for proper human functioning, both physical and psychological.
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What is the neurohypophysis?
Within the endocrine system, formed by a large number of organs and structures that produce hormones, we find the neurohypophysis. This organ forms the back of the pituitary gland.
One of the main differences between the neurohypophysis and the rest of the pituitary gland to which it belongs is that, due to its different embryological origin, its structure is not glandular as the anterior pituitary gland is. In addition, this it has a growth directed towards the hypothalamus , so their functions also differ from those of the rest of the structure.
On the contrary, the neurohypophysis is, to a large extent, a collection of axonal projections of the hypothalamus that flow into the posterior area of the anterior pituitary. The main parts in which the pituitary gland is divided are the middle eminence, the infundibulum and the pars nervosa, which we will discuss in the next point.
As for the elements or pieces that make up the mass of the neurohypophysis, this is composed of a series of cells called pituicitos , which can be considered as glial cells of support.
Finally, although at first glance the neurohypophysis may seem like a 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 supraoptic and paraventricular hypothalamic nuclei secrete vasopressin and oxytocin that is stored in the vesicles of axons of the neurohypophysis, which releases these hormones in response to electrical impulses coming from the hypothalamus.
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As mentioned above, the posterior zone of the pituitary, or neurohypophysis, consists mainly of projections neurons of magnocellular neurosecretory cells that extend from the supraoptic and paraventricular nuclei of the hypothalamus.
In the axons of these neurosecretory cells are stored and released neurohipofisarias hormones known as oxytocin and vasopressin. These are released to the neurohypophyseal 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 gland may vary according to the classifications, most sources include the following three structures:
1. Average Eminence
The area of the neurohypophysis known as the middle 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 a blood-brain barrier, which means that It is an organ with permeable capillaries .
The main function of the middle eminence is to act as the gateway for the release of hypothalamic hormones. However, it also shares continuous perivascular spaces with the adjacent hypothalamic arcuate nucleus, indicating a possible sensory role.
The infundibulum is the connection between the hypothalamus and the posterior pituitary. It carries the axons from the magnocellular neurosecretory cells of the hypothalamus to the posterior pituitary, where they release their neurohypophyseal hormones (oxytocin and vasopressin) in the blood.
3. Pars nervosa
Also known as neural lobule or posterior lobe , this region constitutes most of the neurohypophysis and is the place of storage of oxytocin and vasopressin. In many occasions this is considered as synonymous with the neurohypophysis, however it is only a part of it.
Finally, some classifications also include the middle pituitary gland as part of the neurohypophysis, but this is not usual.
Although, as mentioned at the beginning of the article, in many cases the neurohypophysis is mistakenly considered as 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 in the neurohypophysis through the bloodstream.
Oxytocin is a neuropeptide hormone which is characterized by its an essential role in social bonds, sexual reproduction in both sexes and in being of vital importance both during and after delivery.
Also known as antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin. The main functions of this peptide hormone include the increase in the amount of water without solute reabsorbed in the circulation and the contraction of the arterioles, which increases peripheral vascular resistance and increases blood pressure .
In addition, it is also granted 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, the bond between people and the mother's response to stress.
What happens if it 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 may lead to the appearance of diabetes insipidus , a condition in which the body loses the ability to store and concentrate urine and 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 in the blood is the main cause of the 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.