Unit 17 — Autonomic Nervous System · Question Bank

Origin — Lower Centre

Preganglionic sympathetic cell bodies lie in the lateral grey horn of spinal cord segments T1 to L2 (thoracolumbar outflow). Preganglionic fibres exit via the anterior root, join the spinal nerve, and reach the sympathetic chain via white rami communicantes (myelinated, T1–L2 only).

Paravertebral Chain (Sympathetic Trunk)

• Two chains of ganglia lying on either side of the vertebral column, from base of skull to coccyx
• Ganglia connected by interganglionic branches forming the trunk
• Numbers: 3 cervical + 10–12 thoracic + 4–5 lumbar + 2–3 sacral + 1 coccygeal (ganglion impar)
• Inferior cervical ganglion + T1 often fuse = stellate (cervicothoracic) ganglion

Four Patterns of Distribution from the Chain

• Pattern 1 — Synapse in chain, return via grey rami communicantes: Postganglionic fibres re-enter spinal nerve via grey ramus (unmyelinated, all levels) → distributed to blood vessels, sweat glands, arrector pili of the body wall and limbs
• Pattern 2 — Synapse in chain, leave via named nerve branches: Postganglionic fibres pass directly from the chain as named branches (e.g. cervical cardiac branches, pulmonary branches) to reach heart, lungs, oesophagus, trachea
• Pattern 3 — Pass through chain without synapsing → synapse in prevertebral ganglion: Preganglionic fibres form splanchnic nerves (greater T5–9, lesser T10–11, least T12) → synapse in coeliac, SMA, or renal ganglia → postganglionic fibres travel along blood vessels to supply abdominal viscera
• Pattern 4 — Adrenal medulla: Preganglionic fibres pass directly (via greater splanchnic) to the adrenal medulla (modified postganglionic) → release adrenaline + noradrenaline into bloodstream

Main Effects of Sympathetic Stimulation (“Fight or Flight”)

• Heart: increased rate + force (tachycardia); coronary dilation
• Lungs: bronchodilation (β2 receptors)
• Blood vessels: skin + splanchnic vasoconstriction (α); skeletal muscle vasodilation (β2)
• Eyes: pupil dilation (dilator pupillae, α1); upper eyelid retraction (superior tarsal)
• GI: decreased motility + sphincter contraction
• Skin: sweating (cholinergic exception); piloerection
• Adrenal medulla: adrenaline + noradrenaline secretion
• Liver: glycogenolysis → raised blood glucose

Overview — Craniosacral Outflow

The parasympathetic division is the craniosacral outflow. Preganglionic neurons are in cranial nerve nuclei (brainstem) and sacral cord S2–S4. Preganglionic fibres are long and myelinated; postganglionic fibres are short and unmyelinated. Both release acetylcholine. Ganglia are located close to or within the wall of the target organ.

Cranial Component — Four Cranial Ganglia

• Ciliary ganglion (CN III): Preganglionic fibres from Edinger-Westphal nucleus via CN III → ciliary ganglion in orbit → short ciliary nerves → sphincter pupillae (miosis) + ciliary muscle (accommodation)
• Pterygopalatine ganglion (CN VII): Preganglionic via greater petrosal nerve (branch of CN VII) → pterygopalatine ganglion in pterygopalatine fossa → via CN V2 branches → lacrimal gland + nasal mucosa + palatine glands (secretomotor)
• Submandibular ganglion (CN VII): Preganglionic via chorda tympani → lingual nerve (CN V3) → submandibular ganglion on hyoglossus → submandibular gland + sublingual gland (secretomotor)
• Otic ganglion (CN IX): Preganglionic via tympanic nerve → tympanic plexus → lesser petrosal nerve → otic ganglion (below foramen ovale) → auriculotemporal nerve (CN V3) → parotid gland (secretomotor)

Vagus Nerve (CN X) — No Named Peripheral Ganglion

CN X is the dominant cranial parasympathetic nerve. It supplies the thoracic viscera (heart, lungs) and abdominal viscera to the left colic (splenic) flexure. Ganglion cells are located in the walls of the target organs (intramural ganglia). Effects: bradycardia, bronchoconstriction, increased GI motility.

Sacral Component — Pelvic Splanchnic Nerves (S2–S4)

• Preganglionic fibres arise from sacral parasympathetic nucleus in lateral grey horn of S2–S4
• Exit as pelvic splanchnic nerves (nervi erigentes)
• Targets: descending + sigmoid colon, rectum; urinary bladder (detrusor muscle → micturition); erectile tissue (penis/clitoris → erection via NO-mediated vasodilation); uterus
• Ganglion cells are in the pelvic plexus and in organ walls

Pupillary Light Reflex (Parasympathetic, Constriction)

• Afferent limb (CN II): Light detected by retinal photoreceptors → ganglion cells → optic nerve → optic chiasm → optic tract → pretectal nucleus of midbrain (NOT lateral geniculate nucleus — this bypasses the visual cortex)
• Efferent limb (CN III — bilateral): Pretectal nucleus → bilateral Edinger-Westphal nuclei (CN III parasympathetic nucleus) → preganglionic fibres travel in CN III → ciliary ganglion (in orbit) → short ciliary nerves → sphincter pupillae → miosis
• Bilateral projection explains the consensual reflex: light in one eye constricts both pupils

Accommodation Reflex (Near Vision)

• Visual cortex perceives blurred near image → signals to pretectal area → Edinger-Westphal nucleus
• CN III → ciliary ganglion → ciliary muscle contracts → zonule (suspensory ligament) relaxes → lens rounds up (increases convexity) → focus for near objects
• Simultaneously: medial rectus muscles contract (convergence) + sphincter pupillae contract (miosis) = near reflex triad

Sympathetic Pathway to the Eye

• First neuron: Hypothalamus → ipsilateral ciliospinal centre (C8–T2) in lateral grey horn
• Second neuron: Preganglionic → white ramus → T1 sympathetic chain → ascends to superior cervical ganglion (at C2–C3) → synapses
• Third neuron: Postganglionic fibres travel along internal carotid artery → through cavernous sinus → via nasociliary nerve branch (CN V1) → long ciliary nerves → dilator pupillae (mydriasis) + superior tarsal (Müller's) muscle (lid retraction) + inferior tarsal muscle

Horner's Syndrome

Interruption anywhere along the three-neuron sympathetic arc to the eye produces:

• Ptosis (partial) — loss of superior tarsal muscle
• Miosis — dilator pupillae paralysed; sphincter unopposed
• Anhydrosis (ipsilateral face) — loss of sympathetic (cholinergic) sweat gland supply
• Enophthalmos (apparent) — loss of inferior tarsal muscle tone
• Causes by level: Central = stroke, MS, syringomyelia; Pre-ganglionic = Pancoast tumour (T1 root), cervical rib, carotid dissection, thyroid carcinoma; Post-ganglionic = carotid/cavernous sinus pathology

Greater Splanchnic Nerve

• Origin: Preganglionic sympathetic fibres from T5–T9 (pass through chain without synapsing)
• Course: Descends obliquely over vertebral bodies → passes through the crus of the diaphragm
• Relay: Coeliac ganglion (largest prevertebral ganglion, at level T12/L1 around coeliac artery)
• Targets: Foregut (stomach, liver, gallbladder, spleen, pancreas, upper duodenum) via coeliac plexus branches
• Adrenal medulla: Some preganglionic fibres from the greater splanchnic nerve bypass the coeliac ganglion and terminate directly on chromaffin cells of the adrenal medulla (modified postganglionic neurons → adrenaline + noradrenaline release)

Lesser Splanchnic Nerve

• Origin: T10–T11
• Course: Passes through diaphragm (or crus)
• Relay: Superior mesenteric ganglion (around SMA origin at L1)
• Targets: Midgut (lower duodenum, jejunum, ileum, ascending + transverse colon to splenic flexure)

Least (Lowest) Splanchnic Nerve

• Origin: T12 (when present; variable)
• Relay: Renal plexus / aorticorenal ganglion
• Targets: Kidney, proximal ureter

Adrenal Medulla — Special Case

The adrenal medulla is embryologically derived from neural crest cells and is functionally a modified sympathetic ganglion. It is innervated directly by preganglionic sympathetic fibres (mainly from the greater splanchnic nerve), bypassing any postganglionic relay. The chromaffin cells = modified postganglionic neurons. On stimulation they secrete adrenaline (80%) + noradrenaline (20%) into the bloodstream, providing a widespread, amplified sympathetic response during “fight-or-flight.”

Mechanism of Referred Pain

Referred pain arises from the convergence of visceral and somatic afferent fibres on the same second-order neurons in the dorsal horn of the spinal cord. The brain cannot distinguish which source activated that neuron and defaults to the more familiar somatic interpretation → pain is “felt” in the dermatome supplied by those spinal segments rather than at the diseased organ. Visceral pain is dull, diffuse, poorly localised, and often midline; once parietal peritoneum or adjacent somatic structures are irritated, pain becomes sharp and well-localised.

Cardiac Pain (Angina / MI)

• Cardiac afferents travel with sympathetic fibres via the cardiac plexus → enter spinal cord at T1–T5
• These levels also receive somatic afferents from the medial left arm (T1–T2) + precordium (T2–T5) + jaw (upper cardiac via C3/C4 referral)
• Pain is referred to: left arm + shoulder + jaw + precordium
• Clinical: silent MI occurs in diabetic autonomic neuropathy (afferent fibres damaged); right coronary territory can refer to right arm

Appendicitis — Two-Phase Pain Migration

• Phase 1 (visceral, early): Appendiceal inflammation activates visceral afferents → travel via mesenteric plexus → enter spinal cord at T10 → referred to periumbilical region (T10 dermatome = level of umbilicus). Pain is dull, poorly localised, central/periumbilical. Nausea is common (same spinal level).
• Phase 2 (somatic, late): Inflammation spreads to parietal peritoneum overlying the appendix → somatic afferents (ilioinguinal + iliohypogastric, L1) activated → pain is sharp, well-localised to McBurney's point (RIF). Guarding, rebound tenderness, and Rovsing's sign appear.

Two Further Clinical Examples

• Diaphragmatic irritation (e.g. sub-phrenic abscess, ruptured spleen → haemoperitoneum): phrenic nerve afferents (C3–C5) → referred pain to ipsilateral shoulder tip (Kehr's sign). Supraclavicular fossa dermatome = C3–C4 → same segments as phrenic.
• Renal/ureteric colic: ureteric smooth muscle spasm activates T10–L1 afferents → referred pain from loin to groin (T10–L1 dermatomes), often radiating to the ipsilateral testis/labium majus. Haematuria distinguishes from appendicitis; pain is typically colicky.