Sacral part of autonomic division

The sacral part of the autonomic nervous system is the caudal outflow of the parasympathetic division. Preganglionic neurons arise from spinal cord segments S2–S4 in the lateral gray horn (intermediolateral cell column; sacral parasympathetic nucleus). Their axons exit via the ventral roots of S2–S4, join the spinal nerves, and form the pelvic splanchnic nerves (historically “nervi erigentes”).

These preganglionic fibers synapse in terminal/intramural ganglia within the inferior hypogastric (pelvic) plexus and in the walls of target organs. Postganglionic fibers are therefore short, producing focused, organ-specific parasympathetic effects.

Primary targets and effects include:

• Urinary bladder: contraction of the detrusor (M3-mediated) and relaxation of the internal urethral sphincter (via inhibition of sympathetic tone) → micturition.

• Distal colon and rectum (from the left colic/splenic flexure): ↑ peristalsis and relaxation of the internal anal sphincter → defecation.

• Reproductive organs: vasodilation of penile and clitoral arteries → erection (not ejaculation, which is sympathetic).

Neurotransmission: Acetylcholine (ACh) acts on nicotinic (N_N) receptors in ganglia and muscarinic (primarily M3; M2 modulatory) receptors on effector cells; NO/VIP co-transmission contributes notably to erectile vasodilation.

Clinical correlations: Lesions can cause neurogenic bladder (urinary retention or incontinence), bowel dysfunction (constipation or incontinence), and erectile dysfunction.