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.