Located at the base of a brain structure called the hypothalamus, the median eminence normally secretes substances that control the pituitary gland, which releases many of the body’s hormones. The median eminence is typically located behind the location where the optic nerves cross and is also near the neural connection between the hypothalamus and pituitary gland. Nerve extensions called axons extend into the interior of the structure from other parts of the brain. The outer part generally features a network of nerve endings and small portal blood vessels that transport substances to the pituitary.
Neurons that release substances such as dopamine, Growth-Hormone Releasing Hormone (GHRH), somatostatin, and Corticotrophin Releasing Hormone (CRH) usually end in the median eminence. Compounds released from these cells can enter the blood whenever necessary. The internal portal system typically comprises three cellular layers which control secretion based on the signals sent and received. Nerve terminals and neurological components called glial cells are located in the region, although synapses, or communicating ends of nerve cells, are generally not found here.
The median eminence typically acts as a link between neurological hormone control and the endocrine system. Reproduction, growth, stress, and metabolism can be regulated by activity in this part of the brain. The blood barrier in the structure is generally porous, so it can respond to chemical signals in the circulatory system like other parts of the brain called circumventricular organs. It is also part of the hypophyseal portal system, which is where nerve signals are typically sent to the body’s endocrine glands.
Under a microscope, the median eminence usually has four different zones, which are differentiated by the types of cells located in each. The areas include the ventricular, myelinated axon, neural profile, and capillary zones. Terminals of nerve cells in the capillary zone are normally connected to openings in small blood vessels where secretions can pass into the blood.
The overall function of the median eminence can change with age. Also able to secrete Gonadotropin-Releasing Hormone (GnRH), the region can slow down the release of this substance; it typically does so when a mammal is not reproducing; the pattern in which this occurs often depends on age and species. Damage to the median eminence, which can disrupt the communication between the different cells, sometimes affects the function of the pituitary gland. Abnormal hormone production can then cause various symptoms, including extreme thirst or frequent urination.
