The adrenal medulla (Latin: medulla glandulae suprarenalis) 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 chromaffin cells that secrete catecholamines, including epinephrine (adrenaline), norepinephrine (noradrenaline), and a small amount of dopamine, in response to stimulation by sympathetic preganglionic neurons.
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). These adrenal medullary cells are modified postganglionic neurons, and preganglionic autonomic nerve fibers lead to them directly from the central nervous system. The adrenal medulla affects energy availability, heart rate, and basal metabolic rate. Recent research indicates that the adrenal medulla may receive input from higher-order cognitive centers in the prefrontal cortex as well as the sensory and motor cortices, providing credence to the idea that there are psychosomatic illnesses.
Chromaffin cells are derived from the embryonic neural crest, and are modified postganglionic sympathetic neurons. They are modified postganglionic sympathetic neurons of the autonomic nervous system that have lost their axons and dendrites, receiving innervation from corresponding preganglionic fibers. The cells form clusters around fenestrated capillaries where they release norepinephrine and epinephrine into the blood.
Because the ANS, specifically the sympathetic division, 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 the catecholamines adrenaline and noradrenaline into the blood. Adrenaline composes about 85% of the released catecholamines, and noradrenaline the other 15%.
Notable effects of adrenaline and noradrenaline include increased heart rate and blood pressure, blood vessel constriction in the skin and gastrointestinal tract, smooth muscle (bronchiole and capillary) 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.
- 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
The adrenal medulla may be poorly formed or absent in cases of absent adrenal gland. The deficiency in circulating catecholamines is mildly symptomatic due to compensation by the autonomous nervous system, except in episodes of hypoglycemia where glycogenolysis cannot be stimulated by circulating epinephrine .
In dopamine beta hydroxylase deficiency, the entire body cannot efficiently produce epinephrine and norepinephrine from dopamine, this results in severe dysautonomia but most crucially due to autonomous nervous system failure which requires epinephrine and norepinephrine as neurotransmitters, dopamine being used in this pathology as an inadequate substitute.
- Carmichael, Stephen W. (1997-01-01), Bittar, E. Edward; Bittar, Neville (eds.), "Chapter 8 - The Adrenal Medulla", Principles of Medical Biology, Molecular and Cellular Endocrinology, Elsevier, 10: 207–225, doi:10.1016/s1569-2582(97)80035-9, retrieved 2020-12-17
- Hinson, Joy; Raven, Peter; Chew, Shern (2010-01-01), Hinson, Joy; Raven, Peter; Chew, Shern (eds.), "The Adrenal Glands Part I", The Endocrine System (Second Edition), Churchill Livingstone, pp. 53–60, doi:10.1016/b978-0-7020-3372-8.00005-7, ISBN 978-0-7020-3372-8, retrieved 2020-12-17
- Dum, Richard (2016). "Motor, cognitive, and affective areas of the cerebral cortex influence the adrenal medulla". Proceedings of the National Academy of Sciences of the United States of America. 113 (35): 9922–9927. doi:10.1073/pnas.1605044113. PMC 5024624. PMID 27528671.
- Introduction to Autonomics, Part 2 - Page 5 of 12 anatomy module at med.umich.edu
- Fung, M. M.; Viveros, O. H.; O’Connor, D. T. (16 November 2007). "Diseases of the adrenal medulla". Acta Physiologica. 192 (2): 325–335. doi:10.1111/j.1748-1716.2007.01809.x. PMC 2576282. PMID 18021328.
- Robertson, D; Haile, V; Perry, SE; Robertson, RM; Phillips JA, 3rd; Biaggioni, I (July 1991). "Dopamine beta-hydroxylase deficiency. A genetic disorder of cardiovascular regulation". Hypertension. 18 (1): 1–8. doi:10.1161/01.hyp.18.1.1. PMID 1677640.
- Anatomy photo:40:04-0202 at the SUNY Downstate Medical Center - "Posterior Abdominal Wall: Blood Supply to the Suprarenal Glands"
- Anatomy Atlases - Microscopic Anatomy, plate 15.292 - "Adrenal Gland"