Adrenal medulla

The adrenal medulla is part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of cells that secrete epinephrine (adrenaline), norepinephrine (noradrenaline), and a small amount of dopamine in response to stimulation by sympathetic preganglionic neurons.

Basic
The adrenal medulla consists of irregularly shaped cells grouped around blood vessels. These cells are intimately connected with the sympathetic division of the autonomic nervous system (ANS). In fact, these adrenal medullary cells are modified postganglionic neurons, and preganglionic autonomic nerve fibers lead to them directly from the central nervous system.

Function
Rather than releasing a neurotransmitter, the cells of the adrenal medulla secrete hormones.

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 epinephrine, norepinephrine, and dopamine.

Because the ANS exerts direct control over the chromaffin cells the hormone release can occur rather quickly. In response to stressors such as exercise or imminent danger, medullary cells release catecholamines into the blood in a 17:3 ratio of adrenaline to noradrenaline.

Notable effects of adrenaline and noradrenaline include increased heart rate and blood pressure, blood vessel constriction in the skin and gastrointestinal tract, bronchiole dilation, and decreased 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.

Origin
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 Autonomic nervous system that have lost their axons and dendrites, receiving innervation from corresponding preganglionic fibers. The cells form clusters around large blood vessels.

Moreover, as the synapses between pre- and postganglionic fibers are called ganglia, the adrenal medulla is actually a ganglion of the sympathetic nervous system.

Pathology
Neoplasms including:
 * Pheochromocytoma (most common), a catecholamine-secreting tumor of the adrenal medulla
 * Neuroblastoma, a neuroendocrine tumor of any neural crest tissue of the sympathetic nervous system
 * Ganglioneuroma, a tumor in the nerve cells of the peripheral nervous system