Nerve Supply of the Adrenal Glands

The nerve supply of the adrenal glands is anatomically and functionally significant, particularly for the adrenal medulla. While the adrenal cortex is largely under hormonal control (via ACTH), the adrenal medulla receives direct neuronal innervation from the sympathetic nervous system, allowing for rapid systemic catecholamine release during stress. The adrenal medulla functions essentially as a specialized sympathetic ganglion that secretes neurotransmitters into the bloodstream.

Sympathetic Innervation

Both adrenal glands receive rich sympathetic innervation via the thoracic splanchnic nerves. These fibers primarily influence the adrenal medulla and facilitate catecholamine release.

Preganglionic Sympathetic Fibers

• Originate in the intermediolateral cell columns of the spinal cord at T5–T9 levels
• Travel via the greater splanchnic nerves (and to a lesser extent, lesser splanchnic nerves)
• Pass through the sympathetic chain ganglia without synapsing
• Terminate directly on chromaffin cells in the adrenal medulla

These fibers release acetylcholine, which binds to nicotinic receptors on chromaffin cells, triggering the exocytosis of catecholamine-containing granules (epinephrine and norepinephrine) into the bloodstream.

Medullary Response

• Activation results in the systemic release of epinephrine (~80%) and norepinephrine (~20%)
• Response is part of the acute “fight or flight” reaction, increasing heart rate, blood glucose, and vascular tone

Sensory and Visceral Afferents

• Visceral afferent fibers from the adrenal region travel along sympathetic pathways (especially T10–T12)
• These fibers carry pain signals, which may be stimulated in cases of hemorrhage, tumor, or capsular distension
• Referred pain: May present in the flank, epigastrium, or back due to dermatomal overlap

Parasympathetic Innervation

There is no significant parasympathetic innervation of the adrenal glands. The cortex is controlled hormonally, and the medulla operates under sympathetic regulation only.

Innervation of the Adrenal Cortex

Unlike the medulla, the adrenal cortex lacks direct functional neural control. However, sympathetic fibers may innervate blood vessels within the cortex to modulate blood flow and possibly influence cortical hormone output indirectly.

• Vasomotor fibers: Regulate cortical perfusion, which may affect hormone delivery and release
• No direct synapses occur on cortical steroidogenic cells

Clinical Significance

Stress Response

• Neuronal stimulation of the adrenal medulla results in rapid catecholamine release during stress
• This complements the slower hormonal release of cortisol from the adrenal cortex

Pheochromocytoma

• Neural input can trigger dangerous surges in catecholamine output from adrenal tumors
• Perioperative sympathetic stimulation must be managed carefully to avoid hypertensive crises

Spinal Cord Injuries

• High-level thoracic cord injuries may impair sympathetic input to the adrenal medulla
• This may blunt the adrenal stress response in affected individuals

Pharmacological Considerations

• Drugs that block nicotinic receptors (e.g., ganglionic blockers) can inhibit adrenal medullary activity
• Sympathomimetic agents (e.g., epinephrine) mimic adrenal medullary output for therapeutic use

Neural Pathway Overview