Autonomic nervous system anatomy

Two-neurone outflow

• All preganglionic autonomic fibres release acetylcholine. Postganglionic sympathetic fibres synthesise noradrenaline (and co-transmitters such as neuropeptide Y and ATP); the cholinergic sympathetic fibres to sweat glands are the exception.
• Only T1–L2 nerves carry white rami; grey rami join every spinal nerve so that sweat glands, arrector pili and vessels of all dermatomes are supplied.
• The adrenal medulla is a modified sympathetic ganglion: it receives preganglionic fibres directly (from the thoracic splanchnic nerves, about T10–L1) and releases adrenaline into the blood.
• The hypothalamus is the highest centre directly controlling autonomic reflexes.

Ganglia: autonomic versus sensory

• Autonomic ganglia contain synapses and multipolar neurones.
• Dorsal root and cranial sensory ganglia contain pseudounipolar neurones with no synapses; the single process splits into peripheral and central branches.

Splanchnic nerves and gut supply

• The vagus carries parasympathetic fibres to the gut as far as the distal transverse colon (about two-thirds along, near the splenic flexure). After a truncal vagotomy at the hiatus, the descending and sigmoid colon keep their supply through the pelvic splanchnic nerves.
• Highly selective vagotomy cuts only the branches to the acid-secreting body and fundus and preserves the terminal "crow's foot" branches of the nerves of Latarjet to the antrum and pylorus, so gastric emptying is kept without a drainage procedure.

Parasympathetic ganglia of the head

• Postganglionic fibres to the lacrimal gland leave the pterygopalatine ganglion with the maxillary nerve (zygomatic branch) and reach the gland through a communication with the lacrimal nerve. The facial nerve is the source; branches of V are only the delivery route.
• The sensory root of the ciliary ganglion comes from the nasociliary nerve (V1); its sympathetic root from the internal carotid plexus.

Sympathetic pathway to the head and Horner syndrome

• Features: mild ptosis (Müller's muscle), miosis, anhidrosis (none in many third-order lesions, because facial sweat fibres follow the external carotid), apparent enophthalmos, iris heterochromia if congenital.
• Stellate ganglion block (fused inferior cervical and first thoracic ganglia) produces an ipsilateral Horner syndrome and a warm, dry hand from loss of vasoconstrictor and sudomotor tone.
• Apraclonidine test: apraclonidine is a weak alpha-1 agonist. In Horner syndrome the denervated dilator pupillae develops alpha-1 denervation supersensitivity, so the small pupil dilates and the ptosis lifts, reversing the anisocoria. Supersensitivity takes days, so the test may be negative very early.

Organ effects

• Sympathetic only: most blood vessels, sweat glands, arrector pili, adrenal medulla, juxtaglomerular cells.
• In exercise, sympathetic activity raises heart rate and contractility, constricts skin and gut vessels, and dilates skeletal muscle vessels (mainly by local metabolites).
• Carotid sinus massage stretches baroreceptors (IX afferent), increasing vagal and reducing sympathetic outflow: heart rate and blood pressure fall.
• Loss of sympathetic tone (high spinal injury, spinal anaesthesia) causes neurogenic shock: vasodilatation, hypotension and bradycardia.

Autonomic neuropathy and dysreflexia

• In long-standing diabetes, cardiac vagal fibres are lost early, so the normal rise and fall of heart rate with slow deep breathing (sinus arrhythmia) disappears, and resting tachycardia appears. Postural hypotension follows when sympathetic fibres fail.
• Autonomic dysreflexia: in cord injury above T6, a stimulus below the lesion (blocked catheter, constipation) triggers a massive reflex sympathetic discharge below the injury, constricting the large splanchnic vascular bed. Baroreceptors detect the hypertension and slow the heart through the vagus, but the descending inhibitory signals cannot cross the cord lesion, so the vasoconstriction persists. Above the lesion there is flushing, sweating and headache. Sit the patient up and remove the trigger.

Remember Sympathetic is thoracolumbar with short preganglionic fibres; parasympathetic is craniosacral with long ones; every preganglionic fibre uses acetylcholine.

Exam trap Sweat glands are sympathetic but cholinergic (muscarinic).

Exam trap Horner ptosis is mild with a small pupil; a III palsy gives complete ptosis with a large pupil.

How FCPS asks about this

• Parasympathetic fibres to the lacrimal gland finally travel with: branches of the maxillary nerve.
• Gut keeping parasympathetic supply after truncal vagotomy: sigmoid colon (pelvic splanchnic S2–S4).
• Branches preserved in highly selective vagotomy: crow's foot to the antrum and pylorus.
• After a stellate ganglion block: ipsilateral Horner syndrome with a warm, dry hand.
• Small pupil dilates with apraclonidine: alpha-1 denervation supersensitivity in Horner syndrome.
• Loss of heart rate variation with breathing in diabetes: cardiac vagal fibres damaged.
• Neurone type in dorsal root ganglion: pseudounipolar, with no synapses.