THE ADRENAL MEDULLA
The cells of the adrenal medulla, called chromaffin cells, are homologous with sympathetic postganglionic neu-rons. The adrenal medulla may in fact be considered a modified sympathetic ganglion. The adrenal medulla se-cretes two hormones. One is norepinephrine, which is also the primary neurotransmitter of sympathetic post-ganglionic neurons. The other medullary hormone is epinephrine.
General activation of the sympathetic system during stress, fear, or anxiety is accompanied by increased se-cretion of adrenal medullary hormones, which consist primarily of epinephrine in the human. The secretory activity of the adrenal medulla is regulated by the CNS.
Some blood-borne substances of endogenous origin, such as histamine, angiotensin, and bradykinin, can di-rectly stimulate the chromaffin cells to secrete epineph-rine and norepinephrine. A variety of exogenously ad-ministered drugs, such as cholinomimetic agents and caffeine, can directly stimulate the secretion of adrenal medullary hormones. The neuronally induced secretion of medullary hormones is antagonized by ganglionic blocking agents.