Autonomic Nervous System
Overview
The Autonomic Nervous System (ANS) controls involuntary visceral function. It is divided into the Sympathetic (thoracolumbar, T1-L2) and Parasympathetic (craniosacral, CN III/VII/IX/X + S2-S4) branches. Understanding ANS pharmacology is essential for interpreting cardiovascular and imaging drug effects.
Adrenergic Receptors (Sympathetic)
- Alpha-1 (postsynaptic): Vascular smooth muscle contraction (vasoconstriction), pupillary dilation, internal urethral sphincter contraction.
- Alpha-2 (presynaptic): Inhibitory autoreceptor — reduces further noradrenaline release.
- Beta-1 (cardiac-predominant): Increases heart rate, conduction velocity, and contractility. Stimulates renin release.
- Beta-2 (non-cardiac): Bronchodilation, vasodilation in skeletal muscle, uterine relaxation, glycogenolysis.
Cholinergic Receptors (Parasympathetic)
- Nicotinic (Nm): At ALL ganglionic synapses (both ANS divisions) and the neuromuscular junction.
- Muscarinic M2 (cardiac): Bradycardia, reduced AV conduction.
- Muscarinic M3 (visceral): Glandular secretion, GI smooth muscle contraction, bronchoconstriction, detrusor contraction.
WarningNon-Selective Beta-Blockers and Asthma
Non-selective beta-blockers like propranolol block Beta-2 receptors in addition to Beta-1, precipitating fatal bronchospasm in asthmatic patients. ALWAYS use cardioselective Beta-1-blockers (atenolol, bisoprolol, metoprolol) when beta-blockade is needed in patients with reactive airways disease.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
ALL preganglionic neurons (sympathetic AND parasympathetic) release acetylcholine at nicotinic receptors. The adrenal medulla is a modified sympathetic ganglion, directly releasing adrenaline (80%) and noradrenaline (20%) into the bloodstream. Sweat glands are innervated by CHOLINERGIC sympathetic fibres (an important exception!).