Posterior Pituitary (Neurohypophysis)

1. Overview

The posterior pituitary, also known as the neurohypophysis, is the posterior lobe of the pituitary gland and functions primarily as a site for the storage and release of neurohormones. Unlike the anterior pituitary, it does not synthesize its own hormones. Instead, it stores and secretes two key hormones—antidiuretic hormone (ADH) and oxytocin—which are synthesized in the hypothalamus. These hormones are critical for water balance, blood pressure regulation, and reproductive functions.

2. Location

The posterior pituitary is located at the base of the brain, within the sella turcica of the sphenoid bone, directly beneath the hypothalamus. It is connected to the hypothalamus by the infundibulum (pituitary stalk). Through this connection, the hypothalamic neurons extend axons into the posterior pituitary, enabling the transport and release of hormones.

3. Structure

The posterior pituitary is composed primarily of:

• Axon terminals of hypothalamic neurons: These originate in the supraoptic and paraventricular nuclei of the hypothalamus.

• Pituicytes: Specialized glial cells that support and regulate the axons and hormone release.

The posterior pituitary is not a gland in the traditional sense; it does not produce hormones but acts as a neurohemal organ, releasing hormones into systemic circulation when triggered by nerve impulses from the hypothalamus.

4. Function

The posterior pituitary releases two major hormones:

• Antidiuretic Hormone (ADH) / Vasopressin:

  • Promotes water reabsorption in the kidneys by acting on the collecting ducts.

  • Constricts blood vessels (vasoconstriction), helping to raise blood pressure.

• Oxytocin:

  • Stimulates uterine contractions during labor.

  • Triggers milk ejection (let-down reflex) in lactating mothers.

  • Plays roles in social bonding, sexual behavior, and maternal-infant bonding.

5. Physiological role(s)

The hormones secreted by the posterior pituitary have essential systemic effects:

• Water and electrolyte balance: ADH maintains blood osmolality and volume by reducing water loss via the kidneys.

• Blood pressure regulation: Through vasoconstriction, ADH contributes to systemic vascular resistance and arterial pressure.

• Reproductive physiology: Oxytocin facilitates childbirth and lactation and may influence emotional bonding and trust.

• Stress adaptation: Both hormones are involved in acute physiological responses to stress (e.g., water retention, childbirth facilitation).

6. Clinical Significance

Disorders of the posterior pituitary primarily relate to deficiencies or excesses of ADH and, less commonly, oxytocin:

• Diabetes insipidus:

  • Central diabetes insipidus: Caused by insufficient ADH production or release, leading to excessive urination (polyuria) and thirst (polydipsia).

  • Treated with desmopressin, a synthetic ADH analog.

• Syndrome of Inappropriate ADH Secretion (SIADH):

  • Excessive ADH secretion causes water retention, hyponatremia, and low serum osmolality.

  • Can result from CNS disorders, tumors, or certain medications.

• Oxytocin disorders:

  • Clinical deficiencies are rare but may impair labor progression or lactation.

  • Synthetic oxytocin (e.g., Pitocin) is used to induce or augment labor and control postpartum hemorrhage.

• Pituitary stalk damage:

  • Injury or tumors affecting the infundibulum may disrupt ADH and oxytocin release, leading to endocrine dysfunctions including diabetes insipidus.