from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark answer on comparison of sympathetic and parasympathetic nervous system
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Comparison of Sympathetic and Parasympathetic Nervous System
(Guyton and Hall, Textbook of Medical Physiology)
The autonomic nervous system has two divisions - sympathetic and parasympathetic - that generally act reciprocally on visceral organs, though most organs are dominantly controlled by one or the other.
1. Origin
- Sympathetic: Thoracolumbar outflow - arises from the intermediolateral horn cells of the spinal cord segments T1 to L2.
- Parasympathetic: Craniosacral outflow - arises from certain cranial nerves (III, VII, IX, X) and sacral spinal nerves S2-S4.
2. Ganglia
- Sympathetic: Ganglia lie close to the spinal cord (paravertebral sympathetic chain and prevertebral ganglia), so the preganglionic fiber is short and the postganglionic fiber is long.
- Parasympathetic: Ganglia lie in or very near the wall of the effector organ, so the preganglionic fiber is long and the postganglionic fiber is very short.
3. Neurotransmitters
- Sympathetic: Preganglionic fibers release acetylcholine (cholinergic); most postganglionic fibers release norepinephrine (adrenergic), except sweat glands and some blood vessels in skeletal muscle, which are cholinergic.
- Parasympathetic: Both preganglionic and postganglionic fibers release acetylcholine (cholinergic throughout).
4. Receptors
- Sympathetic: Acts through alpha (α1, α2) and beta (β1, β2, β3) adrenergic receptors on effector organs.
- Parasympathetic: Acts through muscarinic cholinergic receptors on effector organs (nicotinic receptors at the ganglionic synapse in both systems).
5. Mode of action and effects on organs
- Sympathetic stimulation prepares the body for "fight or flight": it dilates the pupils, increases heart rate and force of contraction, dilates bronchioles, decreases gut motility and secretion, contracts sphincters, causes vasoconstriction in skin and viscera with vasodilation in skeletal muscle, and increases blood glucose (glycogenolysis) and sweating.
- Parasympathetic stimulation promotes "rest and digest" functions: it constricts the pupils, decreases heart rate, constricts bronchioles, increases gut motility and glandular secretion, relaxes sphincters, and stimulates bladder emptying and salivation.
6. Discharge pattern
- Sympathetic: Can act diffusely as a "mass discharge" affecting many organs simultaneously (e.g., in stress or emergency reactions), and its effects are prolonged because norepinephrine and co-released epinephrine (from the adrenal medulla) persist in the circulation.
- Parasympathetic: Acts more discretely on specific organs (e.g., only the eye or only the gut) with localized, short-lived, and more precisely controlled reflex responses (e.g., pupillary light reflex, defecation reflex).
(Reference: Sympathetic and parasympathetic effects on organs are summarized in Table 61.2, "Autonomic Effects on Various Organs of the Body" - Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition, Chapter 61.)
Summary Table
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | Thoracolumbar (T1-L2) | Craniosacral (III, VII, IX, X; S2-S4) |
| Ganglia | Near spinal cord (short preganglionic, long postganglionic) | Near/in effector organ (long preganglionic, short postganglionic) |
| Neurotransmitter (postganglionic) | Norepinephrine (adrenergic) | Acetylcholine (cholinergic) |
| Receptors | Alpha and beta adrenergic | Muscarinic cholinergic |
| Action pattern | Diffuse, mass discharge, prolonged | Discrete, localized, brief |
| Overall effect | Fight or flight (catabolic) | Rest and digest (anabolic) |
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