Sympathetic nervous system

Also known as: thoracolumbar division, fight-or-flight system

The thoracolumbar (T1–L2) division of the ANS that mobilises the body for exertion, stress and thermoregulation through noradrenergic postganglionic fibres and adrenaline from the adrenal medulla.

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Overview

Outflow segmentsT1–L2 (L3)

Adrenal medulla output≈80% adrenaline, ≈20% noradrenaline

White ramiOnly at T1–L2

Grey ramiTo all 31 spinal nerves

Preganglionic sympathetic neurons lie in the intermediolateral cell column (lateral horn) of spinal segments T1–L2 (L3). Their axons leave through the ventral roots and reach the sympathetic trunk through white rami communicantes, which exist only at T1–L2. Inside the trunk a preganglionic fibre can follow one of four routes:
1. synapse in the paravertebral ganglion at the same level;
2. ascend or descend in the trunk and synapse at another level, which is how cervical, lower lumbar and sacral ganglia are supplied;
3. pass through the trunk without synapsing as a splanchnic nerve to a prevertebral ganglion (coeliac, aorticorenal, superior or inferior mesenteric) supplying the abdominopelvic viscera;
4. run directly to the adrenal medulla, whose chromaffin cells are modified postganglionic neurons that secrete adrenaline (≈80%) and noradrenaline (≈20%) into the blood.

Postganglionic fibres to the body wall and limbs rejoin every spinal nerve through grey rami communicantes to supply sweat glands, arrector pili and blood vessels. Fibres to the head travel on the carotid arteries from the superior cervical ganglion. Fibres to the thoracic viscera form the cardiac and pulmonary plexuses. Because preganglionic fibres are short and diverge widely, and because adrenaline circulates, activation tends to be widespread. The "fight or flight" response combines tachycardia, increased contractility, redistribution of blood to muscle, bronchodilation, pupil dilation, hyperglycaemia, lipolysis, sweating and inhibition of digestion and voiding.

Location: Preganglionic neurons in spinal segments T1–L2; paravertebral ganglia along the vertebral column from the skull base to the coccyx; prevertebral ganglia around the abdominal aorta.

Parts & subdivisions

Eye
Pupil dilation (mydriasis) via α1 receptors on dilator pupillae; elevation of the upper lid via the superior tarsal (Müller) muscle; slight relaxation of the ciliary muscle (β2) for far vision.
Glands of the head
Scant viscous saliva (α1/β); vasoconstriction of the nasal mucosa. Lacrimal glands are minimally affected.
Heart
β1: ↑ heart rate (chronotropy), ↑ contractility (inotropy), ↑ AV conduction (dromotropy). Coronary vessels dilate (metabolic, β2).
Lungs
Bronchodilation (β2, mainly via circulating adrenaline); reduced bronchial secretion.
Blood vessels
α1 vasoconstriction in skin, gut and kidney; β2 vasodilation in skeletal muscle (adrenaline). Tonic sympathetic tone sets peripheral resistance and venous capacitance.
Gastrointestinal tract
↓ motility and secretion (α2, β2); contraction of sphincters (α1); reduced splanchnic blood flow.
Liver, fat and pancreas
Glycogenolysis and gluconeogenesis (β2, α1); lipolysis (β1, β3); ↓ insulin (α2) and ↑ glucagon release.
Kidney
Renin release from juxtaglomerular cells (β1); afferent arteriolar constriction; ↑ Na⁺ reabsorption.
Bladder
Detrusor relaxation (β3) and internal urethral sphincter contraction (α1), favouring storage.
Skin
Thermoregulatory sweating via cholinergic sympathetic fibres on muscarinic (M3) receptors; adrenergic palmar "stress" sweating; piloerection (α1, arrector pili); cutaneous vasoconstriction.
Reproductive organs
Emission (contraction of ductus deferens, seminal vesicles and prostate, α1) and closure of the internal urethral sphincter during ejaculation; uterine effects vary with hormonal state (β2 relaxation).
Adrenal medulla
Preganglionic ACh → nicotinic receptors on chromaffin cells → release of adrenaline and noradrenaline into the blood.

Anatomy

Development: Preganglionic neurons arise from the basal plate intermediate zone. Ganglion neurons and adrenal chromaffin cells derive from the neural crest; neuroblastoma, the commonest extracranial solid tumour of childhood, arises from these sympathoadrenal precursors.

Function

Fight-or-flight mobilisation in exercise, stress and emergencies
Maintains vasomotor tone and blood pressure (baroreceptor reflex efferent limb)
Thermoregulation: sweating, skin blood flow, piloerection
Mobilises glucose and fatty acids
Promotes urinary continence and seminal emission

Physiology: Transmitters: preganglionic ACh (nicotinic N_N) → postganglionic noradrenaline (α and β). The exceptions are ACh at eccrine sweat glands (M3), dopamine at renal vascular smooth muscle (D1), and adrenal chromaffin cells, which receive preganglionic ACh directly. Noradrenaline action ends mainly by reuptake (NET, uptake-1), with breakdown by MAO and COMT. Cotransmitters are ATP and neuropeptide Y.

Clinical significance

Horner syndrome

Interruption of the sympathetic supply to the head causes ipsilateral ptosis, miosis and anhidrosis. A classic cause is an apical lung (Pancoast) tumour invading the cervicothoracic chain. See the sympathetic trunk for the full pathway.

Phaeochromocytoma

A catecholamine-secreting tumour of adrenal chromaffin cells causes paroxysmal headache, sweating and tachycardia with hypertension. Plasma or urine metanephrines are diagnostic. An α-blocker (phenoxybenzamine) is started before any β-blocker, to avoid unopposed α-vasoconstriction.

Sympathectomy and hyperhidrosis

Endoscopic thoracic sympathectomy (division of the chain at T2–T3) treats severe palmar hyperhidrosis. It may cause compensatory truncal sweating.

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