Like the adenohypophysis, the neurohypophysis is a part of the pituitary gland (hypophysis). However, it is not a gland itself but a component of the brain. Its role is to store and provide two important hormones.
What is the neurohypophysis?
The neurohypophysis (posterior pituitary) is the smaller component of the pituitary gland, along with the adenohypophysis (anterior pituitary). However, unlike the adenohypophysis, the neurohypophysis is not a gland. It cannot produce hormones. But it performs the task of storing two important hormones, ADH and oxytocin. Developmentally, it is a part of the brain. During pregnancy, it develops from an outpouching of the diencephalon. The adenohypophysis, on the other hand, arises from an outpouching of the oral bay and becomes an endocrine gland. Despite their different origins and functions, however, the two organs merge into a single functional unit in the form of the pituitary gland. As the posterior pituitary lobe, the neurohypophysis in humans represents the posterior part of the pituitary gland. However, this only applies to humans. In other mammals, such as predators or horses, it is completely enclosed by the adenohypophysis. Therefore, the term posterior pituitary cannot be generalized in the context of the neurohypophysis.
Anatomy and structure
The neurohypophysis is composed of the infundibulum (pituitary stalk) and the lobus nervosus. The pituitary stalk is connected to the eminentia mediana. The eminentia mediana is a neurohemal organ that releases neuropeptides formed by neurons into the blood. The neuropeptides thus reach the adenohypophysis via the pituitary-portal vein system and function here as releasing or inhibiting hormones. Thus, the eminentia mediana represents an important junction between the nervous and endocrine systems. The lobus nervosus is the posterior part of the neurohypophysis. The effector hormones oxytocin and ADH produced in the hypothalamus are stored there. The storage of these hormones is ensured by their binding to certain carrier proteins (neurophysins). Certain specialized cells of the neurohypophysis, the pituicytes, can induce proteolytic cleavage of the hormones from the carrier proteins and their release into the bloodstream when needed.
Function and Tasks
The neurohypophysis is responsible for storing the hormones vasopressin (ADH) and oxytocin and releasing them when needed. The two hormones are first bound to so-called neurophysins and travel through axons (nerve cell processes) from the hypothalamus to the posterior lobe of the pituitary gland. As the interface between the nervous and endocrine systems, the neurohypophysis determines the body’s need for these hormones and initiates their release on this basis. Vasopressin, also called antidiuretic hormone, regulates the body’s water balance. It prevents too much water from being excreted from the body through the urine. Thus, when the body is undersupplied with water or when there is too much fluid loss, the blood thickens. As a result, the amount of blood decreases and blood pressure drops. In the hypothalamus, these changes are registered by certain nerve cells, with the result that the production of vasopressin increases. This increased production of vasopressin is in turn the signal for the neurohypophysis to release the hormone from its store. In response, the vasopressin curbs further excretion of fluid. The other hormone, oxytocin, performs several functions in the body. It is responsible for inducing labor and providing milk in the breast during breastfeeding. It also imparts a stimulating effect during sexual intercourse. The release of oxytocin by the neurohypophysis is triggered by various stimuli, such as childbirth, the sucking reflex during breastfeeding, or sexual intercourse.
Diseases
Diseases of the neurohypophysis mainly involve the dysregulation of vasopressin production and release. In the context of oxytocin, pathological processes are very rare. The posterior pituitary may be affected by various benign or malignant disorders that affect hormone production or its storage. A deficiency of vasopressin triggers the so-called diabetes insipidus. Here, the excretion of very high amounts of urine occurs. The body loses a lot of water and there is a severe feeling of thirst as a result of its dehydration (exsiccosis).In extreme cases, fluid loss occurs up to 20 liters per day. The severe feeling of thirst causes people to drink very large quantities of fluid. However, this does not prevent dehydration because the drunk fluid is immediately excreted. Other symptoms of diabetes insipidus include fatigue, dry skin and very low blood pressure. The constant loss of water also increases the sodium concentration in the blood. This leads to states of confusion, seizures or coma. Vasopressin deficiency can have a variety of causes. For example, the production and storage of vasopressin may be disturbed by tumors or cysts in the hypothalamus or neurohypophysis. The tissue of the hypothalamus or posterior pituitary may also be damaged by inflammatory processes in meningitis or tuberculosis. Surgery, radiation, infarction or hemorrhage can also affect both organs. The same applies to an accident with a craniocerebral trauma. Very often, the hypothalamus or neurohypophysis is also damaged by so-called autoimmune reactions. In this case, the immune system attacks the tissue of these organs. In rare cases, too much vasopressin (ADH) is also produced. This particularly affects premature babies who are ventilated. It also occurs in bronchial carcinomas. Here, a lot of water is retained in the body and the sodium concentration drops. Recent studies also suggest that, in addition to antidiuretic properties, vasopressin may also have an influence on the psyche. Thus, there is a likelihood that neurohypopituitarism disorders also have psychological effects.
