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Thyroid and Adrenal Glands

2. Adrenal Glands

2.6. Adrenal Medulla

1. Introduction
  • The adrenal medulla is the inner region of the adrenal gland, surrounded by the adrenal cortex. 
  • It functions as a neuroendocrine organ, meaning it acts as both a nerve tissue and an endocrine gland.
  • It plays a critical role in the body’s acute stress response (the “fight or flight” reaction), working in close coordination with the sympathetic division of the autonomic nervous system.
2. Structure and Composition

  • The adrenal medulla is composed mainly of chromaffin cells, which are modified postganglionic sympathetic neurons.

  • These cells lack axons and dendrites; instead, they release their secretions—catecholamines—directly into the bloodstream.

  • The medulla receives direct innervation from preganglionic sympathetic fibers that originate from the thoracic spinal cord (T5–T9 segments).

3. Hormones of the Adrenal Medulla

The chromaffin cells secrete two main catecholamines:

  1. Epinephrine (adrenaline) – about 80% of total secretion

  2. Norepinephrine (noradrenaline) – about 20%

These hormones are synthesized from the amino acid tyrosine through a series of enzymatic steps.

Synthesis Pathway:

Tyrosine → DOPA → Dopamine → Norepinephrine → Epinephrine

(The final conversion of norepinephrine to epinephrine requires the enzyme phenylethanolamine N-methyltransferase (PNMT), which is stimulated by cortisol from the adrenal cortex—showing cortical–medullary interaction.)

4. Regulation of Secretion
  • The adrenal medulla is neurogenically controlled, not hormonally.
  • Direct sympathetic stimulation (via acetylcholine release from preganglionic neurons) triggers chromaffin cells to secrete catecholamines.
  • Secretion is rapid and short-lived, matching the body’s immediate needs during stress.
  • Common stimuli include:
    • Physical or emotional stress
    • Hypoglycemia
    • Exercise
    • Trauma
    • Exposure to cold or heat

5. Physiological Effects of Catecholamines

Catecholamines act on adrenergic receptors (α and β types) distributed throughout the body, producing widespread effects that prepare the body for action:

System Effect Receptor Type (main)
Cardiovascular ↑ Heart rate, ↑ contractility, ↑ cardiac output, vasoconstriction in skin and viscera, vasodilation in skeletal muscles β₁, α₁, β₂
Respiratory Bronchodilation, ↑ respiratory rate β₂
Metabolic ↑ Glycogenolysis, ↑ gluconeogenesis, ↑ lipolysis → ↑ blood glucose and free fatty acids β₂
Musculoskeletal ↑ Blood flow to muscles, ↑ performance β₂
Pupillary Pupil dilation (mydriasis) α₁
Digestive/Urinary ↓ GI motility, ↓ bladder tone α₂, β₂

These effects collectively enhance alertness, energy availability, and physical readiness.
6. Clinical Correlation
Pheochromocytoma:
  • A rare catecholamine-secreting tumor of chromaffin cells causing episodic hypertension, palpitations, sweating, and anxiety.
  • Diagnosis: Elevated urinary VMA and metanephrines.
  • Treatment: Surgical removal after stabilization with α- and β-blockers.
Adrenal Medullary Hypofunction:
  • Rare, but may occur in generalized adrenal insufficiency; reduces stress tolerance and glucose regulation.
Summary
  • The adrenal medulla acts as part of the sympathoadrenal system, releasing catecholamines in response to stress.
  • ·Epinephrine and norepinephrine prepare the body for “fight or flight.”
  • Control is neural, rapid, and transient, unlike the slower hormonal regulation of the adrenal cortex.
  • Clinical relevance: Overactivity (pheochromocytoma) or underactivity affects cardiovascular and metabolic stability.