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Solve questions no 2 of physiology
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Physiology Question 2 - Samjhana Shrestha (Pheochromocytoma)
Clinical Scenario Recap: A 51-year-old woman presents with pounding heart, throbbing headache, visual disturbances, feeling hot with cold hands/feet, and nausea/vomiting. Her 24-hour urinary VMA (vanillylmandelic acid) test is positive - pointing to a pheochromocytoma (tumor of the adrenal medulla chromaffin cells).
Part a. Relationship between the Adrenal Medulla and the Autonomic Nervous System (ANS) [1 mark]
The adrenal medulla is a specialized ganglion of the sympathetic division of the ANS. Here is the key anatomical and functional relationship:
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Preganglionic neurons - Their cell bodies lie in the thoracic spinal cord. Their axons travel via the greater splanchnic nerve directly to the adrenal medulla (bypassing any autonomic ganglion).
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Synaptic transmission - The preganglionic fibers synapse on chromaffin cells (the secretory cells of the adrenal medulla) and release acetylcholine (ACh), which activates nicotinic receptors on chromaffin cells.
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Secretory output - When activated, chromaffin cells release catecholamines directly into the bloodstream:
- ~80% Epinephrine (adrenaline)
- ~20% Norepinephrine (noradrenaline)
- (This differs from sympathetic postganglionic neurons, which release only norepinephrine)
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Why mainly epinephrine? - The enzyme phenylethanolamine-N-methyltransferase (PNMT) converts norepinephrine to epinephrine. This enzyme is present in the adrenal medulla (activated by cortisol from the adjacent adrenal cortex) but not in peripheral sympathetic neurons.
In pheochromocytoma, the tumor is too distant from the adrenal cortex to receive cortisol, so PNMT is not activated - hence pheochromocytomas secrete mainly norepinephrine (explaining the profound vasoconstriction and hypertension in this patient).
- Costanzo Physiology 7th Edition, p. 58-59
Part b. Effects of Sympathetic and Parasympathetic Nervous System on Cardiovascular, Gastrointestinal, Urinary, and Respiratory Systems [4 marks]
1. Cardiovascular System
| Effect | Sympathetic (Adrenergic) | Parasympathetic (Cholinergic) |
|---|---|---|
| Heart rate | Increases (β1 receptors) | Decreases (M2 receptors, vagal) |
| Force of contraction | Increases (β1) | Slight decrease (mainly atria) |
| Blood pressure | Increases (vasoconstriction via α1) | Decreases (vasodilation in some beds) |
| Blood vessels (skin, viscera) | Constriction (α1) | Little direct effect |
| Coronary vessels | Dilation (β2) | Slight constriction |
In this patient: Excess catecholamines from the pheochromocytoma activated β1 receptors (→ pounding heart, tachycardia) and α1 receptors (→ vasoconstriction → cold hands/feet, severe hypertension). Headache was secondary to extreme hypertension.
2. Gastrointestinal System
| Effect | Sympathetic | Parasympathetic |
|---|---|---|
| Motility (peristalsis) | Decreases/Inhibits | Increases |
| Secretions (salivary, gastric) | Decreases | Increases |
| Sphincters | Contracts (closed) | Relaxes |
| Defecation reflex | Inhibits | Promotes (sacral S2-S4) |
In this patient: Excess sympathetic drive suppressed GI motility/secretion, contributing to her nausea and vomiting.
3. Urinary System
| Effect | Sympathetic | Parasympathetic |
|---|---|---|
| Detrusor muscle (bladder wall) | Relaxes (β2) - allows filling | Contracts - promotes voiding |
| Internal urethral sphincter | Contracts (α1) - urinary retention | Relaxes |
| Micturition reflex | Inhibits | Promotes (sacral S2-S4) |
- Sympathetic = "store urine" (relaxes bladder, closes sphincter)
- Parasympathetic = "void urine" (contracts bladder, opens sphincter)
4. Respiratory System
| Effect | Sympathetic | Parasympathetic |
|---|---|---|
| Bronchi / airways | Bronchodilation (β2 receptors) | Bronchoconstriction (M3 receptors) |
| Respiratory rate | Increases | Decreases |
| Mucus secretion | Decreases | Increases |
- The sympathetic "fight-or-flight" response dilates airways (β2) to maximize oxygen intake during stress/exertion.
- The parasympathetic ("rest and digest") response narrows airways and increases secretions.
Summary: Sympathetic vs. Parasympathetic - The Big Picture
| Sympathetic | Parasympathetic | |
|---|---|---|
| Role | Fight or flight ("alarm reaction") | Rest and digest (restorative) |
| Neurotransmitter | Norepinephrine (+ epinephrine from adrenal medulla) | Acetylcholine |
| Receptors | Adrenergic (α, β) | Muscarinic (M), Nicotinic (N) |
| Overall state | Catabolic, aroused | Anabolic, conserving |
- Costanzo Physiology 7th Edition, pp. 58-59; Guyton and Hall Textbook of Medical Physiology, pp. 762-763
Connection to the clinical case: Samjhana's pheochromocytoma was secreting excess catecholamines (mainly norepinephrine), causing massive sympathetic stimulation - hence her tachycardia, hypertension, headache, visual disturbances, cold extremities (vasoconstriction), feeling hot (impaired heat dissipation from skin), and nausea/vomiting (GI effects). The positive urinary VMA test confirmed excess catecholamine metabolism.
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