Friday, November 6, 2009

Median Eminence

The median eminence is the region of the brain which lies below the third cerebral ventricle and behind the optic chiasm. It is bounded laterally by the arcuate nucleus, rostral to the neural stalk that connects the posterior pituitary gland to the hypothalamus.

The median eminence consists of two zones, an internal zone and an external zone. The internal zone is made up of axons of the magnocellular neurosecretory neurons of the supraoptic nucleus and paraventricular nucleus which project to the posterior lobe of the pituitary gland. The external zone is composed of the convoluted blood vessels of the hypothalamo-pituitary portal system, and the nerve endings of many neuroendocrine neurons, such as the dopamine neurons of the arcuate nucleus and somatostatin neurons from the periventricular neucleus. These portal vessels transport releasing factors released by neuroendocrine neurones of the hypothalamus to the anterior pituitary gland.

The median eminence is of great physiological importance, as it is integral to the hypophyseal portal system, which connects the hypothalamus with the anterior lobe of the pituitary gland. It is in this structure that the secretions of the hypothalamus (releasing and inhibiting regulatory hormones) collect before entering the portal system.