The Neurohypophysial System

There are two distinct nuclei located in the hypothalamus known as the supraoptic nuclei (SON) and the parvaventricular nuclei (PVN). The majority of these neurones travel down from the hypothalamus, through the infundibulum, to the posterior pituitary (neurohypophysis). The area of the capillary plexus in the posterior pituitary is known as the pars nervosa. These large neurosecretory nerves are called magnocellular neurones.

Some of the axons originating from the parvaventricular nuclei terminate at different areas other than the pars nervosa in the posterior pituitary. Some of the axons terminate at the median eminence (primary capillary plexus) and, as they are smaller than the magnocellular neurones, they are called parvocellular neurones. Some of the other axons terminate in other areas of the brain including the wall of the third ventricle, the hippocampus, the dorsal nucleus of the vagus and the nucleus tractus solitarius.

The neurohypophysis secretes two main hormones:

• oxytocin
• vasopressin (also known as anti-diuretic hormone, ADH).