Re: How does the adrenal medulla regulate its secretion?

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.