Adrenal Medulla (AM)
The adrenal medulla is the inner part of the adrenal gland, located at the center of each gland atop the kidneys. It is composed primarily of chromaffin cells, which are modified postganglionic sympathetic neurons derived from neural crest cells. These cells secrete catecholamine hormones—mainly epinephrine (adrenaline) and norepinephrine (noradrenaline)—directly into the bloodstream in response to stimulation by the sympathetic nervous system124.
Structure and Innervation
- The adrenal medulla forms about 10% of the adrenal gland’s mass and is surrounded by the adrenal cortex.
- Chromaffin cells cluster around fenestrated capillaries and receive direct innervation from preganglionic sympathetic neurons originating in the spinal cord (T10–L1 segments).
- Unlike typical sympathetic ganglia, the adrenal medulla releases hormones into the circulation rather than neurotransmitters into synapses234.
Function
- The adrenal medulla is the principal site for converting the amino acid tyrosine into catecholamines: epinephrine (~85%), norepinephrine (~15%), and small amounts of dopamine.
- Upon stress stimuli such as exercise, low blood sugar, or imminent danger, preganglionic neurons release acetylcholine, which activates chromaffin cells to release catecholamines rapidly.
- These hormones mediate the fight-or-flight response, which includes increasing heart rate and blood pressure, dilating airways, redistributing blood flow to muscles and vital organs, and elevating blood glucose to prepare the body for rapid action1235.
Clinical Significance
- Disorders of the adrenal medulla are rare but can be severe.
- Pheochromocytoma is a tumor of chromaffin cells causing excessive catecholamine secretion, leading to symptoms like episodic hypertension, headaches, sweating, and palpitations.
- Such tumors may be discovered incidentally or present with hypertensive crises triggered by mild stress or certain activities15.
- Other rare conditions include neuroblastoma (a malignant tumor of neural crest origin in children) and dopamine beta-hydroxylase deficiency affecting catecholamine synthesis12.
Summary Table
| Feature | Description |
|---|---|
| Location | Inner core of adrenal gland, surrounded by adrenal cortex |
| Cell Type | Chromaffin cells (modified postganglionic sympathetic neurons) |
| Hormones Produced | Epinephrine (~85%), norepinephrine (~15%), dopamine (small amounts) |
| Innervation | Preganglionic sympathetic fibers from spinal cord (T10–L1) |
| Function | Rapid release of catecholamines mediating fight-or-flight response |
| Clinical Disorders | Pheochromocytoma, neuroblastoma, catecholamine synthesis disorders |
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