Autonomic nervous system

Also known as: ANS, visceral motor system, involuntary nervous system

The visceral motor system controlling smooth muscle, cardiac muscle and glands through two-neuron chains, organised into sympathetic, parasympathetic and enteric divisions and directed by the hypothalamus and brainstem.

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Overview

Neurons from CNS to effector2 (pre- and postganglionic)

Preganglionic fibresThinly myelinated B fibres (3–15 m/s)

Postganglionic fibresUnmyelinated C fibres (0.5–2 m/s)

The ANS maintains internal homeostasis, adjusting heart rate, vascular tone, airway calibre, gut motility and secretion, pupil size, sweating, bladder function and sexual responses, mostly without conscious control. Every peripheral autonomic motor pathway is a two-neuron chain. A preganglionic neuron in the brainstem or spinal cord sends a thinly myelinated B fibre to an autonomic ganglion, where it synapses on a postganglionic neuron whose unmyelinated C fibre reaches the effector. Preganglionic neurons always release acetylcholine onto nicotinic (N_N) receptors.

The sympathetic division (thoracolumbar, T1–L2) prepares the body for exercise and emergencies. Its ganglia lie near the cord, and most postganglionic fibres release noradrenaline onto α and β adrenoceptors. The parasympathetic division (craniosacral: CN III, VII, IX, X and S2–S4) promotes digestion, rest and repair. Its ganglia lie in or near the target, and postganglionic fibres release ACh onto muscarinic receptors. The enteric nervous system is an intrinsic network in the gut wall that the other two divisions modulate.

Visceral afferents run alongside the efferents. Most parasympathetic-route afferents (vagus, glossopharyngeal) carry reflex information such as blood pressure, blood gases and stretch to the nucleus of the solitary tract, whereas sympathetic-route afferents carry visceral pain, which is referred to the dermatomes of the same spinal segments. Central control comes from the hypothalamus (the "head ganglion" of the ANS), the NTS and ventrolateral medulla, the periaqueductal grey, the amygdala, the insula and the prefrontal cortex.

Parts & subdivisions

Sympathetic division
Thoracolumbar outflow (T1–L2); short preganglionic, long postganglionic fibres; paravertebral and prevertebral ganglia; adrenal medulla.
Parasympathetic division
Craniosacral outflow (III, VII, IX, X; S2–S4); long preganglionic, short postganglionic fibres; terminal and intramural ganglia.
Enteric division
Myenteric and submucosal plexuses of the gut wall; can function autonomously.
Cholinergic receptors
Nicotinic N_N (ganglia, adrenal medulla): ligand-gated cation channels. Muscarinic (G-protein-coupled): M1 (CNS, enteric neurons, gastric secretion; Gq), M2 (heart; Gi → slows SA node and AV conduction), M3 (smooth muscle, glands, endothelium → NO; Gq).
Adrenergic receptors
α1 (Gq): vasoconstriction, pupil dilation, sphincter contraction. α2 (Gi): presynaptic inhibition of noradrenaline release, reduced insulin release, central sympatholysis (clonidine). β1 (Gs): ↑ heart rate, contractility and conduction; renin release. β2 (Gs): bronchodilation, vasodilation in skeletal muscle, uterine relaxation, glycogenolysis. β3 (Gs): lipolysis, detrusor relaxation.
Organs with sympathetic innervation only
Most blood vessels, eccrine sweat glands, arrector pili muscles, adrenal medulla and juxtaglomerular cells. Their tone is graded by raising or lowering sympathetic firing.
Central autonomic network
Hypothalamus (paraventricular nucleus and others), nucleus of the solitary tract, rostral ventrolateral medulla, dorsal motor nucleus of vagus and nucleus ambiguus, periaqueductal grey, amygdala, insula and anterior cingulate cortex.

Function

Regulates cardiac output and blood pressure (baroreceptor reflex)
Controls airway calibre and secretions
Regulates gastrointestinal motility, secretion and sphincters
Controls pupil size and accommodation (near response)
Thermoregulation: sweating, skin blood flow, piloerection
Controls micturition, defecation and sexual function
Mobilises energy (glycogenolysis, lipolysis) during stress

Physiology: Most organs receive dual innervation, usually antagonistic (heart rate, pupil, gut) but sometimes complementary (salivary secretion, male sexual function). Both divisions fire tonically, so an organ's state can be shifted in either direction. Sympathetic preganglionic fibres diverge widely (one fibre reaches ≈10–20 or more postganglionic neurons), allowing the mass "fight-or-flight" discharge. Parasympathetic responses are more discrete and organ-specific.

Clinical significance

Autonomic toxidromes

• Anticholinergic poisoning (atropine, antihistamines, tricyclic antidepressants, Datura): "hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter, full as a flask", i.e. hyperthermia, dry skin, flushing, mydriasis with cycloplegia, delirium and urinary retention, plus tachycardia.
• Cholinergic poisoning (organophosphate insecticides, nerve agents that inhibit acetylcholinesterase) gives the muscarinic DUMBELS syndrome: Diarrhoea, Urination, Miosis, Bronchorrhoea/Bradycardia, Emesis, Lacrimation, Salivation. Nicotinic effects follow: fasciculations, weakness, paralysis. Treatment is atropine plus pralidoxime.

Autonomic failure

Diabetic autonomic neuropathy, Parkinson disease, multiple system atrophy and pure autonomic failure cause orthostatic hypotension (a fall of ≥20 mmHg systolic within 3 min of standing without a compensatory rise in heart rate). They also cause gastroparesis, constipation or diarrhoea, bladder dysfunction, erectile dysfunction, anhidrosis and "silent" (painless) myocardial ischaemia.

Vasovagal syncope

The commonest cause of fainting. A trigger (prolonged standing, pain, emotion) produces sudden sympathetic withdrawal and vagal activation, so vasodilation and bradycardia lower cerebral perfusion. The patient has prodromal pallor, sweating and nausea, and recovers rapidly when lying flat.

Mnemonics

Anticholinergic toxidrome

"Hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter, full as a flask."

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.