The neurohypophysis is the posterior (back) portion of the pituitary gland, a pea-sized structure at the base of the brain. Unlike the front portion of the pituitary, which produces its own hormones, the neurohypophysis is essentially an extension of the brain itself. It stores and releases two hormones, vasopressin and oxytocin, that are actually made by nerve cells in the hypothalamus and transported down long nerve fibers into the gland. From there, these hormones enter the bloodstream to regulate water balance, blood pressure, labor contractions, and breastfeeding.
How It Connects to the Brain
The neurohypophysis has two main parts: the infundibular stalk (sometimes called the pituitary stalk) and the pars nervosa, which is the bulbous lower portion where hormones are actually released. The infundibular stalk acts as a bridge, carrying bundles of nerve fibers from the hypothalamus down into the pars nervosa. These fibers come from clusters of large neurons in two specific areas of the hypothalamus: the supraoptic nucleus, which sits just above the optic nerves where they cross, and the paraventricular nucleus nearby. Together, this highway of nerve fibers is called the hypothalamic-neurohypophysial tract.
Because the neurohypophysis doesn’t make its own hormones but instead receives them from the hypothalamus, many anatomists consider it a direct extension of the central nervous system rather than a true gland. This sets it apart from the anterior pituitary, which is a proper hormone-producing gland with its own secretory cells.
Different Origins, Same Gland
The two halves of the pituitary gland develop from completely different tissues during embryonic life. The neurohypophysis forms from brain tissue. It starts as a small outpouching from the floor of the developing brain (the ventral diencephalon) and grows downward. In human embryos, this process occurs between weeks five and seven of gestation. The anterior pituitary, by contrast, develops from the roof of the mouth. Cells from the oral cavity push upward to meet the descending brain tissue, and the two fuse together into a single gland. This dual origin explains why the front and back halves of the pituitary look so different under a microscope and function in such different ways.
What’s Inside the Neurohypophysis
If you were to look at a slice of neurohypophysis under a microscope, you wouldn’t see the typical clusters of hormone-producing cells found in other glands. Instead, you’d find a dense network of unmyelinated nerve fiber endings and specialized support cells called pituicytes. The nerve endings are packed with tiny granules of vasopressin and oxytocin, waiting to be released.
Pituicytes play a surprisingly active role in controlling hormone release. Under resting conditions, pituicytes wrap themselves around the nerve endings and physically wedge between the nerve terminals and nearby blood vessels. This creates a barrier that limits how much hormone can reach the bloodstream. When the body needs a surge of vasopressin or oxytocin, pituicytes retract their projections, exposing the nerve terminals directly to the capillary walls and allowing a flood of hormone to pass through. It’s a built-in gating mechanism that fine-tunes hormone delivery.
Threading through the entire neurohypophysis is a dense capillary network supplied by branches of the superior and inferior hypophyseal arteries. These capillaries have small pores (fenestrations) that allow hormones to pass easily from the nerve terminals into the blood. Venous blood drains through hypophyseal veins into the surrounding dural venous sinuses of the brain.
Vasopressin and Water Balance
Vasopressin, also known as antidiuretic hormone (ADH), is the neurohypophysis’s primary tool for regulating how much water your kidneys retain. When your blood becomes too concentrated (a sign of dehydration), or when blood volume drops, specialized sensors trigger the hypothalamus to send vasopressin down into the neurohypophysis for release. Once in the bloodstream, vasopressin reaches the kidneys and acts on the collecting ducts, the final stretch of the tiny tubes that filter urine. It causes these ducts to insert water channels into their walls, allowing water to flow back into the body rather than being lost in urine. The result is more concentrated urine and better-preserved hydration.
Vasopressin also constricts blood vessels, which helps maintain blood pressure during dehydration or blood loss. This dual action, water retention plus blood vessel tightening, makes it critical for cardiovascular stability.
Oxytocin in Labor and Breastfeeding
Oxytocin drives two powerful reflexes. During childbirth, it stimulates rhythmic contractions of the uterine muscle. As labor progresses, stretching of the cervix sends signals back to the hypothalamus, triggering more oxytocin release in a positive feedback loop that intensifies contractions until delivery.
After birth, oxytocin controls the milk let-down reflex. When a baby suckles, nerve signals travel from the nipple to the hypothalamus, which triggers bursts of oxytocin from the neurohypophysis. The hormone reaches the breast and causes tiny muscle cells surrounding the milk-producing glands to contract, squeezing milk into the ducts and out through the nipple. This release happens in intermittent pulses rather than a continuous stream, which is why breastfeeding mothers often feel the let-down as a distinct tingling sensation that comes in waves.
What Happens When It Malfunctions
Problems with neurohypophysial function typically involve too much or too little vasopressin, and both extremes create serious electrolyte imbalances.
Central Diabetes Insipidus
When the neurohypophysis can’t release enough vasopressin, the kidneys lose their ability to concentrate urine. The hallmark is producing more than three liters of very dilute urine per day, with urine concentration falling below 300 milliosmoles per kilogram (far more watered-down than normal). People with this condition experience extreme thirst and need to drink constantly to keep up with fluid losses. Common causes include head trauma, brain surgery near the pituitary, tumors, or inflammatory conditions that damage the hypothalamic neurons or the neurohypophysis itself. Doctors can distinguish this from kidney-based forms of the same condition by giving a synthetic version of vasopressin: if the kidneys respond by concentrating urine more than 50%, the problem lies in the neurohypophysis rather than the kidneys.
Syndrome of Inappropriate ADH (SIADH)
The opposite problem occurs when too much vasopressin is released regardless of the body’s actual need. The kidneys retain excessive water, diluting the blood’s sodium concentration. This condition, called SIADH, is one of the most common causes of low sodium in hospitalized patients, affecting roughly 30% of them to some degree. Symptoms of low sodium range from mild nausea and headache to confusion and seizures in severe cases. SIADH can be triggered by lung diseases, certain medications, brain injuries, and some cancers that produce vasopressin-like substances on their own.