Heart and bronchial smooth muscle
Target & receptor · β1 SA/AV/ventricle M2 SA/AV β2 bronchi M3 bronchi
The heart receives balanced sympathetic (β1) and vagal (M2) input, with vagal tone dominant at rest. Bronchial smooth muscle receives vagal M3 constrictor tone but almost no sympathetic nerves; its β2 relaxation depends on circulating adrenaline or inhaled agonists.
Traced from the start
- Sympathetic (thoracolumbar) outflow
- Intermediolateral cell column T1–L2 via white rami communicantes
- Superior cervical ganglionParavertebral sympathetic chain ganglia (including stellate)
- Parasympathetic (craniosacral) outflow
- Dorsal motor nucleus of vagus and nucleus ambiguus (CN X)
- Vagal terminal (intramural) ganglia
- Adrenal medulla
- Cardiac and pulmonary sympathetic nervesVagal short postganglionic fibresCirculating adrenaline and noradrenaline
- Heart and bronchial smooth muscle
Detail
- Sympathetic on heart (β1)
- Faster SA rate, faster AV conduction, greater contractility and faster relaxation (lusitropy); coronary β2 dilatation
- Vagus on heart (M2)
- Slower SA rate and AV conduction; resting vagal tone keeps intrinsic rate of ~100 down to ~70
- Airways
- β2 relaxation (circulating adrenaline, salbutamol); M3 constriction and mucus (blocked by ipratropium, tiotropium)
When it goes wrong
Non-selective β-blocker (propranolol, timolol eye drops) given to an asthmatic
Bronchospasm from loss of β2 tone — use cardioselective agents cautiously or avoid; β-blockers also blunt the tachycardia of hypovolaemia and hypoglycaemia
Carotid sinus massage or Valsalva manoeuvre in supraventricular tachycardia
Reflex vagal surge slows AV conduction and can terminate an AV-nodal re-entrant rhythm; adenosine mimics this at A1 receptors
Practise this structure
1 question in the bank tagged Denervated Heart.