|MadSci Network: Anatomy|
Dear Michelle, The adrenal medulla releases its hormones, norepinephrine, epinephrine, and enkephalin, into the blood in response to a number of stimuli. One potent stimulus is a fall in blood glucose. Blood glucose levels seem to be monitored by a part of the brain called the ventromedial hypothalamus (see my paper "The brain response to 2-deoxy glucose is blocked by a glial drug" in the journal Pharmacology, Biochemistry, & Behavior, vol. 67, pp. 233-237, 2000, and also "Local ventromedial hypothalamus glucose perfusion blocks counterregulation during systemic hypoglycemia in awake rats" by MA Borg, et al in Journal of Clinical Investigation, vol. 99, pp. 361-365, 1997). When low blood sugar levels are detected by the hypothalamus, a series of interconnected neurons send a signal to a portion of the spinal cord that controls both the sympathetic nervous system and also sends nerves to the adrenal medulla. It is this signalling from higher portions of the brain that activates the adrenal medulla. Other stimuli that activate this entire system can be painful or frightening events that also activate the hypothalamus, particularly a cluster of neurons called the paraventricular nucleus of the hypothalamus. The adrenal medulla helps us cope temporarily with painful stimuli by secreting enkephalin, a small peptide (protein) molecule that binds to opioid receptors in the nervous system. Enkephalin diminishes the sensation of pain.
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