Sunday, November 20, 2011

Neuroendocrine System

The neuroendocrine system is the system which overlaps the nervous and endocrines systems, comprising the hypothalamus, and the anterior and posterior areas of the hypophysis (pituitary gland). In the neuroendocrine system, the hypophysis is under the control of the hyopothalamic centers. Unmyelinated fiber bundles from the hypothalamus run in the hypophysial stalk and in the hypophysial posterior lobe. Severing the hypophysial stalk leads to retrograde cellular changes, namely, to extensive loss of neurons in the nuclei of the tuber cinereum when transection is carried out at a high level.

Neurons in the hypothalamus produce substances that migrate within the axons into the hypophysis and enter there into the bloodstream. This endocrine function of neurons is called neurosecretion. The substances are produced in the perikarya and appear there as small secretory droplets. The cells, which are genuine neurons with dendrites and axons, represent a transitional stage between neurons and secretory cells. Both these cell types are of ectodermal origin and are closely related with respect to physiology and metabolism. Both of them produce a specific substance that they secrete in response to a nervous or humoral stimulus: the neurons release transmitter substances (neurotransmitters) and the secretory cells release their secretion. Transitional forms between the two cell types are the neurosecretory cells and the endocrine cells, both of which release their secretion into the bloodstream.

Corresponding to the structure of the hypophysis with an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis), there are two different fiber systems extending from the hypothalamus to the hypophysis, namely, the tuberoinfundibular tract and the hypothalamohypophysial tract. In both, the coupling of the neural system with the endocrine system is achieved by the sequential arrangement of nerve fibers and capillaries (neurovascular chain).