Vagal short postganglionic fibres
Postganglionic · M2 nodes M3 smooth muscle & glands NO / VIP
Millimetre-long cholinergic neurones in the heart, airways and gut wall. They release acetylcholine onto M2 receptors at the SA and AV nodes and M3 receptors on bronchial and gut smooth muscle and glands, while non-adrenergic non-cholinergic (NO, VIP) neurones mediate receptive relaxation and sphincter opening.
Traced from the start
- Parasympathetic (craniosacral) outflow
- Dorsal motor nucleus of vagus and nucleus ambiguus (CN X)
- Vagal terminal (intramural) ganglia
- Vagal short postganglionic fibres
Detail
- Heart (M2)
- Gi-coupled: opens K+ channels and lowers cAMP in nodal cells → slower SA discharge, slower AV conduction and longer AV refractory period; negligible effect on ventricular contractility
- Airways (M3)
- Bronchoconstriction and mucus secretion — the resting bronchomotor tone that antimuscarinics remove
- Gut (M3 and NANC)
- Increased motility and secretion (parietal cell acid via M3 and gastrin); NO/VIP neurones relax the lower oesophageal sphincter and fundus
When it goes wrong
Symptomatic sinus bradycardia or AV block from excess vagal tone (inferior MI, vasovagal)
Atropine 0.5–1 mg IV blocks M2 and restores rate; ineffective in a transplanted heart or in infranodal (Mobitz II, complete) block
Digoxin for rate control in atrial fibrillation increases vagal tone at the AV node
Slower ventricular response at rest but poor control on exertion when sympathetic drive dominates; toxicity produces AV block
Practise this structure
1 question in the bank tagged Acetylcholine Release.