Autonomic Nervous System — Anatomy & Ganglia

The autonomic nervous system (ANS) is the involuntary, visceromotor division of the nervous system that governs smooth muscle, cardiac muscle, and glands. For NEET PG, the anatomical wiring — outflow levels, ganglion locations, rami communicantes, and named cranial ganglia — is far more heavily tested than the pharmacology, so this note is built around localisation, relays, and classic clinical correlates such as Horner's syndrome.

Definition & overall plan

The ANS is a two-neuron motor pathway: a preganglionic neuron whose cell body sits in the central nervous system (brainstem or spinal cord), and a postganglionic neuron whose cell body sits in a peripheral ganglion. This is the single most important structural difference from the somatic motor system, which uses a single neuron from the anterior horn to the muscle.

The ANS has two divisions:

• Sympathetic — thoracolumbar outflow (T1–L2/L3), "fight or flight."
• Parasympathetic — craniosacral outflow (CN III, VII, IX, X and S2–S4), "rest and digest."

High-yield: Preganglionic fibres of BOTH divisions are myelinated (white) and release acetylcholine. Sympathetic postganglionic fibres are mostly adrenergic (noradrenaline) — the major exceptions being sweat glands and the arrector pili/some vessels in skeletal muscle, which are cholinergic sympathetic. The adrenal medulla is a modified sympathetic ganglion innervated directly by preganglionic fibres.

Feature	Sympathetic	Parasympathetic
Outflow	Thoracolumbar (T1–L2)	Craniosacral (III, VII, IX, X; S2–S4)
Preganglionic fibre	Short	Long
Postganglionic fibre	Long	Short
Ganglion location	Near CNS (paravertebral/prevertebral)	Near/in target organ
Preganglionic transmitter	ACh (nicotinic)	ACh (nicotinic)
Postganglionic transmitter	Noradrenaline (mostly)	ACh (muscarinic)
Divergence	High (1:many) → diffuse response	Low → discrete response

Sympathetic system — anatomy

Cell bodies of origin

Preganglionic sympathetic neurons arise from the lateral horn (intermediolateral cell column, IML) of spinal segments T1 to L2 (sometimes L3). This is why the sympathetic outflow is described as thoracolumbar.

High-yield: The lateral horn exists ONLY between T1 and L2. There is no lateral horn in the cervical, lower lumbar, or sacral cord — yet the effects of sympathetic activity reach the whole body (head, limbs, pelvis) because fibres travel up and down the sympathetic chain before relaying.

The route: white and grey rami communicantes

This is one of the most repeated NEET PG concepts.

Flow of a typical sympathetic fibre: Lateral horn (T1–L2) → ventral root → spinal nerve → white ramus communicans → sympathetic chain ganglion → (relay here OR travel up/down the chain OR pass straight through to a prevertebral ganglion) → postganglionic fibre exits via grey ramus communicans → spinal nerve → target.

	White ramus communicans	Grey ramus communicans
Fibre type	Preganglionic (myelinated)	Postganglionic (unmyelinated)
Colour reason	Myelin = white	No/little myelin = grey
Levels present	Only T1–L2	All 31 spinal nerves
Direction	Spinal nerve → ganglion	Ganglion → spinal nerve

High-yield: White rami are restricted to T1–L2; grey rami connect to every spinal nerve. That is the anatomical basis for sympathetic supply reaching the head and limbs despite the limited outflow.

Sympathetic chain (paravertebral) ganglia

A pair of sympathetic chains (sympathetic trunks) runs alongside the vertebral column from the base of the skull to the coccyx, where they fuse at the midline ganglion impar.

• Typical counts: 3 cervical (superior, middle, inferior), 11–12 thoracic, 4–5 lumbar, 4–5 sacral ganglia.
• The inferior cervical ganglion often fuses with the first thoracic ganglion to form the stellate (cervicothoracic) ganglion.
• Superior cervical ganglion (SCG): largest; lies at C2–C3 level (opposite transverse processes); supplies postganglionic sympathetics to the head and neck, including the eye (dilator pupillae, superior tarsal/Müller's muscle), sweat glands of the face, and salivary glands (vasomotor). Fibres travel along the internal and external carotid plexuses.

Prevertebral (collateral) ganglia

These lie anterior to the aorta, around the origins of its major branches, and serve the abdominal and pelvic viscera. Preganglionic fibres reaching them form the splanchnic nerves and pass through the chain without synapsing.

Splanchnic nerve	Spinal origin	Relay ganglion	Target
Greater splanchnic	T5–T9	Coeliac ganglion	Foregut, adrenal medulla (preganglionic direct)
Lesser splanchnic	T10–T11	Aorticorenal/superior mesenteric	Midgut, kidney
Least splanchnic	T12	Renal/aorticorenal	Kidney
Lumbar splanchnics	L1–L2	Inferior mesenteric ganglion	Hindgut, pelvic viscera

High-yield: The adrenal medulla receives preganglionic sympathetic fibres directly (via the greater splanchnic nerve) — there is no postganglionic relay. Chromaffin cells act as modified postganglionic neurons, secreting adrenaline (~80%) and noradrenaline into the blood.

Parasympathetic system — anatomy

The parasympathetic (craniosacral) outflow has two parts.

Cranial outflow — CN III, VII, IX, X

Preganglionic cell bodies lie in brainstem nuclei; CN X (vagus) supplies thoracic and abdominal viscera up to the left colic (splenic) flexure — the junction of midgut and hindgut. Below this, sacral parasympathetics take over.

Sacral outflow — S2, S3, S4

Preganglionic neurons lie in the intermediolateral/sacral parasympathetic nuclei of S2–S4. They emerge as pelvic splanchnic nerves (nervi erigentes) and relay in ganglia in the walls of the pelvic viscera.

High-yield mnemonic: "S2, 3, 4 keep the genitals (and pee) off the floor." Pelvic splanchnic nerves (S2–S4) are parasympathetic and mediate erection (point) — sympathetic mediates ejaculation (shoot): "Point and Shoot — Parasympathetic Points, Sympathetic Shoots."

The four cranial parasympathetic ganglia

This table is among the most frequently asked single-fact clusters in NEET PG.

Ganglion	Preganglionic (CN)	Postganglionic target	Function
Ciliary	CN III (Edinger–Westphal nucleus)	Sphincter pupillae, ciliary muscle	Pupillary constriction, accommodation
Pterygopalatine (sphenopalatine)	CN VII (superior salivatory nucleus, via greater petrosal n.)	Lacrimal gland, nasal & palatine glands	Lacrimation, nasal secretion
Submandibular	CN VII (via chorda tympani → lingual n.)	Submandibular & sublingual glands	Salivation
Otic	CN IX (inferior salivatory nucleus, via lesser petrosal n.)	Parotid gland (via auriculotemporal n.)	Salivation

High-yield: Two ganglia belong to CN VII (pterygopalatine + submandibular) and they are distinguished by route — greater petrosal → pterygopalatine → lacrimal; chorda tympani → submandibular → salivary. The otic ganglion (CN IX) relays to the parotid via the auriculotemporal nerve (a branch of V3) — a classic "hitchhiking fibre" MCQ.

Sympathetic fibres pass through these ganglia without relaying (they have already relayed in the SCG); only parasympathetic fibres synapse there.

Horner's syndrome — the oculosympathetic pathway

Horner's syndrome results from interruption of the three-neuron sympathetic supply to the eye and face. Localising the lesion along this pathway is a guaranteed exam favourite.

Three-neuron flow:

1. First-order (central) neuron — hypothalamus → descends through brainstem and cervical cord → synapses in the ciliospinal centre of Budge (C8–T2 / IML at T1).
2. Second-order (preganglionic) neuron — exits T1 → over the lung apex / subclavian artery → ascends the sympathetic chain → synapses in the superior cervical ganglion.
3. Third-order (postganglionic) neuron — travels along the internal carotid artery plexus → through the cavernous sinus → via the ophthalmic nerve / long ciliary nerves to the dilator pupillae and superior tarsal (Müller's) muscle.

Clinical triad (and tetrad)

• Ptosis (partial — loss of Müller's muscle, not levator)
• Miosis (loss of dilator pupillae)
• Anhidrosis (loss of facial sweating — extent depends on lesion level)
• Apparent enophthalmos + ipsilateral flushing (loss of vasoconstriction)

High-yield: Ptosis in Horner's is partial because the levator palpebrae superioris (CN III, somatic) is intact — only the sympathetic superior tarsal muscle fails. Contrast with CN III palsy, which gives complete ptosis + mydriasis (the opposite pupil change).

Lesion level	Classic cause	Anhidrosis pattern
Central (1st order)	Lateral medullary (Wallenberg) syndrome, hypothalamic lesion	Whole half of body
Preganglionic (2nd order)	Pancoast (apical lung) tumour, cervical rib	Face only
Postganglionic (3rd order)	Carotid dissection, cluster headache, cavernous sinus lesion	Minimal/none

High-yield: A Horner's syndrome with arm pain/wasting points to a Pancoast tumour invading the lower trunk of the brachial plexus (T1) and the sympathetic chain. Cocaine drops fail to dilate a Horner's pupil (blocks NA reuptake but there is no NA being released); apraclonidine causes pupil reversal (dilates the affected, denervation-supersensitive pupil).

Autonomic innervation of pelvic organs

Coordinated sympathetic and parasympathetic supply governs micturition, defecation, and sexual function.

Organ/function	Sympathetic	Parasympathetic
Detrusor (bladder wall)	Relaxes (β3) — storage	Contracts (M3) — voiding
Internal urethral sphincter	Contracts (α1) — continence	Relaxes
Erection	—	Pelvic splanchnic (S2–4)
Emission/Ejaculation	Hypogastric (L1–2)	—
Gut distal to splenic flexure	Lumbar splanchnics → IMG	Pelvic splanchnics (S2–4)

High-yield: Continence (storage) is dominantly sympathetic (hypogastric nerve, T11–L2 → inferior mesenteric/hypogastric plexus); voiding is parasympathetic (pelvic splanchnic S2–4). The external urethral sphincter is somatic (pudendal nerve, S2–4 — Onuf's nucleus).

The inferior hypogastric (pelvic) plexus is the great mixing station of pelvic autonomics, receiving hypogastric nerves (sympathetic) and pelvic splanchnic nerves (parasympathetic).

Investigation / localisation pearls

Although primarily anatomy, the "investigation of choice" framing appears for autonomic lesions:

• Horner's syndrome: confirm with apraclonidine (or cocaine) eye drops; localise post- vs preganglionic with hydroxyamphetamine. Imaging: CT chest/apex (Pancoast), CT/MR angiography of carotid (dissection, especially with neck pain).
• Pupillary light-near dissociation (Argyll Robertson — neurosyphilis; Adie's tonic pupil — postganglionic parasympathetic/ciliary ganglion damage). Adie's pupil is the classic ciliary ganglion lesion: large, poorly reactive to light, slow tonic near response, denervation supersensitivity to dilute pilocarpine (0.1%) which constricts it.

Complications / clinical correlates

• Pancoast tumour: Horner's + brachial plexus (T1) involvement → hand wasting.
• Frey's syndrome (gustatory sweating): after parotid surgery, regenerating parasympathetic secretomotor fibres (originally to parotid, via otic ganglion) misroute to sweat glands of the skin → sweating while eating.
• Carotid artery dissection: painful postganglionic Horner's.
• Spinal cord injury above T6: autonomic dysreflexia — unopposed sympathetic surge below the lesion.

Key differentials

• Horner's ptosis vs CN III palsy ptosis: partial + miosis (Horner) vs complete + mydriasis + "down-and-out" eye (CN III).
• Adie's tonic pupil vs Argyll Robertson pupil: both show light-near dissociation; Adie's is a large unilateral pupil (ciliary ganglion), Argyll Robertson is small, bilateral, irregular (neurosyphilis, accommodates but does not react).
• Sympathetic vs parasympathetic ganglion location: paravertebral/prevertebral (near CNS) vs intramural (in organ wall).

Recently asked / exam angle

• "White ramus communicans is present at which spinal levels?" → T1–L2 only (grey at all levels).
• "Which cranial parasympathetic ganglion relays for the parotid gland?" → Otic ganglion (CN IX, lesser petrosal, postganglionic via auriculotemporal nerve).
• "Lacrimal gland secretomotor relay?" → Pterygopalatine ganglion (CN VII, greater petrosal).
• "Second-order Horner's lesion is most commonly caused by?" → Pancoast (apical lung) tumour.
• "Site of synapse for greater splanchnic nerve?" → Coeliac ganglion (prevertebral).
• "Nerve mediating penile erection?" → Pelvic splanchnic nerves (S2–S4), parasympathetic.
• "Adrenal medulla is innervated by?" → Preganglionic sympathetic fibres (greater splanchnic), no relay.
• "Ciliospinal centre of Budge lies at?" → C8–T2 (T1).
• "Müller's muscle (superior tarsal) is supplied by?" → Sympathetic fibres; its loss causes partial ptosis in Horner's.
• Integrated image-based MCQs on the sympathetic chain and rami are increasingly common.

Rapid revision

1. Sympathetic = thoracolumbar (T1–L2 lateral horn); parasympathetic = craniosacral (III, VII, IX, X; S2–4).
2. White rami (preganglionic, myelinated) only T1–L2; grey rami (postganglionic) at all 31 spinal nerves.
3. Sympathetic: short pre-, long postganglionic; parasympathetic: long pre-, short postganglionic.
4. All preganglionic fibres + parasympathetic postganglionic = cholinergic; most sympathetic postganglionic = adrenergic (sweat glands are cholinergic exception).
5. Four cranial parasympathetic ganglia: Ciliary (III), Pterygopalatine (VII), Submandibular (VII), Otic (IX).
6. Otic ganglion → parotid via auriculotemporal nerve (V3 hitchhiking).
7. Adrenal medulla = modified sympathetic ganglion, direct preganglionic supply.
8. Greater splanchnic (T5–9) → coeliac ganglion; lumbar splanchnics → inferior mesenteric ganglion.
9. Horner's = partial ptosis + miosis + anhidrosis; lesion in 3-neuron oculosympathetic pathway; Pancoast tumour is the classic preganglionic cause.
10. Cocaine drops fail to dilate Horner's pupil; apraclonidine reverses it.
11. Erection = parasympathetic (S2–4, point); ejaculation = sympathetic (L1–2, shoot).
12. Bladder: parasympathetic (S2–4, M3) voids; sympathetic stores; external sphincter = somatic pudendal (Onuf's nucleus).