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Human Body Study

Sympathoadrenal Axis

The sympathoadrenal axis CNS stress centres signal directly down preganglionic sympathetic fibres to the adrenal medulla, whose chromaffin cells release a catecholamine surge — about 80% epinephrine and 20% norepinephrine — into the bloodstream, driving the acute fight-or-flight response. There is no hypothalamic-releasing hormone, no pituitary relay, and no classic feedback loop; the response terminates when the stressor resolves and the catecholamines clear within minutes. Trigger: acute stressor (threat, exercise, hypoglycaemia, hypovolaemia) CNS · stress centres hypothalamus → brainstem → thoracic spinal cord preganglionic sympathetic (splanchnic nerve, ACh) Adrenal medulla chromaffin cells (modified post-ganglionic neurons) epinephrine (~80%) norepinephrine (~20%) Fight-or-flight (effects in seconds) ↑ heart rate · ↑ contractility · ↑ blood pressure bronchodilation · pupil dilation ↑ glycogenolysis · ↑ lipolysis · ↑ blood glucose blood diverted to muscle · ↓ digestion (skin pallor, sweating, tremor) No classic feedback loop. Response terminates when central autonomic drive ends — catecholamines clear within minutes. Compare with the slower, sustained HPA / cortisol response.

Signaling chain

Section titled “Signaling chain”

The fast counterpart to the HPA axis. Neural rather than hormonal at its origin: stress activates the sympathetic nervous system, and preganglionic sympathetic fibers directly stimulate the adrenal medulla (chromaffin cells), which releases epinephrine (~80%) and norepinephrine (~20%) into the bloodstream. No hypothalamic-releasing or pituitary trophic hormone is involved.

Function

Section titled “Function”

Mediates the acute “fight-or-flight” response — effects occur within seconds, far faster than the cortisol response. Catecholamines increase heart rate and contractility, raise blood pressure, dilate bronchioles and pupils, mobilize glucose (glycogenolysis, lipolysis), redirect blood flow to muscle, and suppress non-essential functions like digestion.

Feedback

Section titled “Feedback”

Not a classic endocrine feedback loop — it is switched on and off by central autonomic control. The response terminates when the stressor resolves; catecholamines are rapidly cleared (half-life of minutes).

Clinical relevance

Section titled “Clinical relevance”
  • Pheochromocytoma — catecholamine-secreting adrenal medullary tumor; episodic hypertension, palpitations, headache, sweating.
  • Chronic sympathetic overactivity contributes to hypertension and cardiovascular disease.
  • Pharmacologic targets: β-blockers, α-blockers; epinephrine itself is used in anaphylaxis and cardiac arrest.

Key labs

Section titled “Key labs”

Plasma free metanephrines, 24-hr urinary metanephrines and catecholamines (workup for pheochromocytoma).