Autonomic Nervous System in Action
Core wiring, transmitters, receptors and organ effects of the sympathetic and parasympathetic divisions.
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Questions Covered in This Set
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What is the fundamental organisational rule of the autonomic efferent pathway?
It is a two-neuron system: a preganglionic neuron with its cell body in the CNS synapses in a peripheral ganglion onto a postganglionic neuron that reaches the target — unlike the single alpha motor neuron of somatic control.
Where do sympathetic vs parasympathetic preganglionic cell bodies live?
Sympathetic: thoracolumbar cord T1–L2/L3 lateral horn. Parasympathetic: craniosacral outflow — brainstem nuclei of CN III, VII, IX, X plus sacral S2–S4.
Why is sympathetic output 'mass-acting' while parasympathetic output is discrete?
Sympathetic preganglionic fibres are short and diverge onto 20+ postganglionic cells (long axons), producing widespread responses; parasympathetic fibres are long with ganglia in the target organ and divergence of only ~1:3, giving organ-specific effects.
Which transmitter and receptor are used at ALL autonomic ganglia?
Acetylcholine acting on nicotinic (Nn) receptors — which is why ganglion blockers abolish both divisions.
Name the three exceptions to 'postganglionic sympathetic = noradrenaline'.
(1) Sweat glands — ACh on muscarinic receptors; (2) adrenal medulla — a modified ganglion releasing ~80% adrenaline into blood; (3) some renal vasculature using dopamine.
List the signature β1 receptor effects.
Via ↑cAMP: ↑heart rate (SA node), ↑AV conduction, ↑contractility, and renin release from juxtaglomerular cells.
What does β2 stimulation do?
Bronchodilation, skeletal-muscle and coronary vasodilation, uterine relaxation, hepatic glycogenolysis, and K⁺ shift into cells.
Contrast M2 and M3 muscarinic receptor effects.
M2 (↓cAMP, ↑K⁺ via GIRK): slows heart rate and AV conduction. M3 (IP₃/Ca²⁺): glandular secretion, GI and bladder contraction, pupil constriction, bronchoconstriction.
Why is 'parasympathetic vasodilation' largely a myth?
Most blood vessels have no parasympathetic innervation; tone is set by tonic sympathetic discharge, so vasodilation occurs mainly by withdrawing sympathetic tone (plus local metabolites, NO, and genital/salivary exceptions).
Why is resting heart rate ~70 when the intrinsic SA node rate is ~100/min?
Resting vagal (M2) tone dominates and slows the SA node; atropine blocks muscarinic receptors and unmasks the intrinsic pacemaker rate.
