The adrenal medulla
is part of the adrenal gland
. It is located at the center of the gland, being surrounded by the adrenal cortex
Composed mainly of hormone-producing chromaffin cells
, the adrenal medulla is the principal site of the conversion of the amino acid tyrosine
into the catecholamines adrenaline
(norepinephrine), and dopamine
In response to stressors such as exercise or imminent danger, medullary cells release catecholamines into the blood in an 17:3 ratio of adrenaline to noradrenaline.
Notable effects of adrenaline and noradrenaline include increased heart rate and blood pressure, blood vessel constriction, bronchiole dilation, and increased metabolism, all of which are characteristic of the fight-or-flight response. Release of catecholamines is stimulated by nerve impulses, and receptors for catecholamines are widely distributed throughout the body.
Medullary cells are derived from the embryonic neural crest
and, as such, are simply modified neurons
In particular, they are modified postganglionic cells of the sympathetic nervous system that have lost their axons and dendrites, receiving innervation from corresponding preganglionic fibers.
Moreover, as the synapses between pre- and postganglionic fibers are called ganglia, the adrenal medulla is actually a ganglion of the sympathetic nervous system.