Chromaffin cells are stimulated by the splanchnic nerve to secrete adrenaline (epinephrine), noradrenaline, and enkephalin endogenous ligands, which are small opiate-like peptides, which are responsible for the euphoria that runners sometimes feel.
Chromaffin cells originate in the embryonic neural crest. In the fifth week of (human) fetal development, neuroblast cells migrate from the neural crest to form the sympathetic chain and preaortic ganglia. The cells migrate a second time to the adrenal medulla. Chromaffin cells also settle near the sympathetic ganglia, vagus nerve, paraganglia, and carotid arteries. In lower concentrations, extra-adrenal chromaffin cells also reside in the bladder wall, prostate, and behind the liver.