The pars nervosa, a component of the pituitary gland located at the base of the brain, plays a significant role in the body’s endocrine system. It serves as a specialized storage and release site for specific hormones. Once released, these hormones travel through the bloodstream to influence distant organs and maintain various physiological processes. Understanding its function provides insight into how the body regulates fluid balance, reproductive processes, and certain social behaviors.
Location and Structure
The pars nervosa constitutes the posterior lobe of the pituitary gland, residing within a bony depression called the sella turcica. Unlike the anterior pituitary, which produces its own hormones, the pars nervosa is composed of neural tissue, making it part of the neurohypophysis. This neural connection is maintained through the hypothalamic-hypophyseal tract, a bundle of nerve fibers originating in the hypothalamus.
These nerve fibers extend directly into the pars nervosa, transporting hormones produced in the hypothalamic nuclei. The pars nervosa contains specialized glial cells known as pituicytes. These cells do not produce hormones but act as supporting structures, enveloping the nerve terminals and assisting in the release of stored molecules into the bloodstream. Therefore, the pars nervosa functions primarily as a neurohemal organ, facilitating the transfer of neurohormones into the circulatory system.
Hormones It Releases and Their Functions
The pars nervosa is responsible for releasing two primary hormones into the bloodstream: Vasopressin and Oxytocin. These hormones are not synthesized within the pars nervosa but are produced by neurons in the hypothalamus. Once synthesized, they are transported to the nerve terminals within the pars nervosa, where they await release.
Vasopressin, also known as Antidiuretic Hormone (ADH), plays a central role in regulating the body’s water balance and blood pressure. When the body detects dehydration or increased blood osmolarity, ADH is released. It acts on the kidneys to increase water reabsorption back into the bloodstream, concentrating urine and conserving body fluid. At higher concentrations, ADH can also cause vasoconstriction, leading to an increase in blood pressure.
Oxytocin is another hormone released from the pars nervosa, known for its roles in reproductive physiology. During childbirth, oxytocin stimulates strong contractions of the uterine smooth muscle, facilitating labor. After birth, it triggers the milk ejection reflex, causing milk release in lactating mothers in response to suckling. Beyond these reproductive functions, oxytocin also influences social behaviors, promoting bonding and trust.
Common Conditions Linked to Pars Nervosa Dysfunction
Dysfunction of the pars nervosa often involves imbalances in Vasopressin (ADH) regulation. One notable condition is Diabetes Insipidus (DI), characterized by the body’s inability to regulate water balance. This occurs either due to insufficient production or release of ADH from the pars nervosa, known as central DI, or when the kidneys do not respond adequately to ADH, termed nephrogenic DI. Individuals with DI experience excessive thirst and produce large volumes of dilute urine, leading to dehydration if fluid intake is not maintained.
Conversely, the Syndrome of Inappropriate Antidiuretic Hormone (SIADH) results from excessive ADH release. In SIADH, the pars nervosa releases too much ADH, leading to excessive water reabsorption by the kidneys. This results in fluid retention and diluted blood sodium levels (hyponatremia). Symptoms can range from mild to severe neurological complications like confusion, seizures, and coma due to a significant drop in sodium levels.