The autonomic nervous system: organization !

Concept
Exam priority: high

The autonomic system runs the organs without conscious control: the sympathetic side prepares for action, the parasympathetic side for rest.

Updated 2026-11-01

Also called: Sympathetic and parasympathetic divisions, Visceral motor system

See it in 3D: Hypothalamus · Medulla oblongata · Sympathetic trunk · Vagus nerve · Celiac plexus · Splanchnic nerves · Right adrenal gland · Left adrenal gland

Key points

Sympathetic: thoracolumbar (T1–L2), short preganglionic and long postganglionic fibres, ganglia in the paravertebral chain or prevertebral (celiac, mesenteric).
Parasympathetic: craniosacral (III, VII, IX, X; S2–S4), long preganglionic and short postganglionic fibres; the vagus carries about 75% of all parasympathetic fibres.
All preganglionic neurons release ACh onto nicotinic receptors; parasympathetic postganglionic neurons release ACh onto muscarinic receptors; most sympathetic postganglionic neurons release norepinephrine.
Exceptions: sympathetic fibres to sweat glands release ACh; the adrenal medulla is a modified ganglion that secretes mostly epinephrine (≈80%) into blood.
Most organs have dual, opposing innervation; most blood vessels, sweat glands, piloerector muscles and the adrenal medulla are sympathetic only.
Thoracic splanchnic nerves are sympathetic (to celiac and mesenteric ganglia); pelvic splanchnic nerves (S2–S4) are parasympathetic.

What it is

The autonomic nervous system uses a two-neuron chain: a preganglionic neuron in the CNS and a postganglionic neuron in a ganglion. Sympathetic preganglionic neurons lie in the lateral horn from T1 to L2 and synapse close to the cord. Parasympathetic ones lie in brainstem nuclei (cranial nerves III, VII, IX, X) and in S2–S4 and synapse near or inside the target organ. The hypothalamus and brainstem centres set a continuous background output (tone) that reflexes adjust moment to moment.

The two divisions compared

Feature	Sympathetic	Parasympathetic
Origin	Lateral horn T1–L2	Brainstem nuclei (III, VII, IX, X) and S2–S4
Ganglia	Paravertebral chain and prevertebral	Near or inside the target organ
Preganglionic fibre	Short, myelinated (white ramus)	Long
Postganglionic transmitter	Norepinephrine (ACh at sweat glands)	Acetylcholine
Spread	Wide divergence: mass discharge	Local, organ by organ
Typical setting	Exercise, fear, cold, blood loss	Digestion, rest, voiding

Clinical notes

Horner syndrome: interruption of the sympathetic path to the face (apical lung tumour, carotid dissection) gives ptosis, miosis and anhidrosis on the same side.
Autonomic dysreflexia in cord injury above about T6: a stimulus below the lesion, such as a full bladder, triggers massive sympathetic discharge, severe hypertension and reflex bradycardia.
Diabetic autonomic neuropathy: orthostatic hypotension, gastroparesis, resting tachycardia, bladder dysfunction.
Vasovagal syncope: a sudden vagal surge with sympathetic withdrawal causes bradycardia and vasodilation.

Test yourself

Think of the answer, then tap the question to see it.

Why does a lesion of the cervical sympathetic chain make the pupil small?

The dilator muscle of the pupil has only sympathetic innervation. Without it the parasympathetic sphincter acts unopposed, causing miosis, as in Horner syndrome.

Why is the adrenal medulla called a modified sympathetic ganglion?

It receives preganglionic sympathetic fibres that release ACh onto nicotinic receptors. Its chromaffin cells act as postganglionic neurons without axons and secrete epinephrine and norepinephrine into blood.

A patient with a T4 cord injury gets a pounding headache, severe hypertension and bradycardia when the bladder fills. Why?

The bladder stimulus triggers spinal sympathetic reflexes below the lesion with massive vasoconstriction. Baroreceptors slow the heart through the vagus, but brain signals to stop the vasoconstriction cannot pass T4.

Why does sympathetic activation affect the whole body at once while parasympathetic effects stay local?

Each sympathetic preganglionic fibre diverges to many postganglionic neurons and the adrenal medulla adds hormone to blood. Parasympathetic fibres reach ganglia inside each organ and act one organ at a time.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 15.1 Divisions of the Autonomic Nervous System (OpenStax (Rice University), 2022); 15.3 Central Control (OpenStax (Rice University), 2022); Neuroanatomy, Cranial Nerve 10 (Vagus Nerve) (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 61). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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