Parasympathetic Nervous System (PSNS) – Study Notes

General Concepts

• Part of the autonomic nervous system (ANS).

• Functions: “Rest and Digest” – promotes energy conservation, digestion, glandular secretion, and reproductive functions.

• Outflow: Craniosacral.

  • Cranial nerves: CN III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus).

  • Sacral spinal cord: S2–S4 via pelvic splanchnic nerves.

• Ganglia: Parasympathetic fibers synapse in intramural (terminal) ganglia, located near or within target organs.

• Neurotransmitters: Both preganglionic and postganglionic release acetylcholine (ACh).

• Control center: Hypothalamus (parasympathetic centers send descending fibers to brainstem nuclei and spinal cord).

Cranial Outflow

1. CN III – Oculomotor (Edinger–Westphal nucleus)

• Preganglionic fibers → ciliary ganglion → postganglionic fibers to:

  • Ciliary muscle: contraction → zonules relax → lens becomes globular → accommodation (near vision).

  • Sphincter pupillae muscle: contraction → pupillary constriction (miosis) → less light enters.

2. CN VII – Facial (Superior Salivatory nucleus)

• Two pathways:

  • Greater petrosal nerve → pterygopalatine ganglion → stimulates lacrimal, nasal, palatine glands → ↑ watery secretions.

  • Chorda tympani nerve → submandibular ganglion → stimulates submandibular & sublingual salivary glands → ↑ saliva (rich in salivary amylase).

3. CN IX – Glossopharyngeal (Inferior Salivatory nucleus)

• Preganglionic fibers via lesser petrosal nerve → otic ganglion → postganglionic fibers → parotid gland → ↑ watery saliva + salivary amylase → aids bolus formation & initial carbohydrate digestion.

4. CN X – Vagus (Dorsal Motor Nucleus of Vagus)

• Controls ~90% of parasympathetic output.

• Major plexuses and effects:

  • Pulmonary plexus: bronchoconstriction + ↑ mucus secretion.

  • Cardiac plexus: acts on SA & AV nodes → ↓ heart rate (negative chronotropy) and ↓ BP.

  • Esophageal plexus: ↑ peristalsis for swallowing.

  • Gastric branches: ↑ stomach motility + ↑ gastric secretions (via enteric plexuses).

  • Celiac plexus (mainly right vagus): supplies kidneys, ureters, pancreas, spleen, small intestine, ascending & proximal 2/3 transverse colon → ↑ motility, ↑ secretions, some absorption; pancreas releases insulin (via β-cells).

  • Hepatic plexus (mainly left vagus):

    • Liver: promotes glycogenesis (glucose storage).

    • Gallbladder: contraction → bile release.

    • Sphincter of Oddi: relaxation (aided by CCK).

Sacral Outflow (S2–S4, pelvic splanchnic nerves)

• Supplies what vagus does not.

• Colon: distal 1/3 transverse colon, descending colon, sigmoid colon → ↑ motility.

• Rectum & internal anal sphincter: ↑ rectal contractions + relax internal sphincter (defecation reflex).

• Bladder (detrusor muscle): contraction + relaxation of internal urethral sphincter (urination reflex).

• Reproductive organs:

  • Male: erection (via NO-mediated vasodilation of corpus cavernosum).

  • Female: clitoral engorgement (similar mechanism).

• Associated plexus: Inferior hypogastric plexus.

Quick Comparison

• Sympathetic: thoracolumbar outflow, short preganglionic/long postganglionic, norepinephrine (NE) mainly.

• Parasympathetic: craniosacral outflow, long preganglionic/short postganglionic, acetylcholine (ACh) for both synapses.

Key Effects

• Eye: Accommodation + miosis.

• Glands: ↑ lacrimal, nasal, palatine, salivary secretions.

• Heart: ↓ HR, ↓ BP.

• Lungs: Bronchoconstriction, ↑ mucus.

• GI tract: ↑ motility, ↑ secretions, ↑ absorption.

• Bladder: Detrusor contraction, sphincter relaxation → urination.

• Rectum: Motility + sphincter relaxation → defecation.

• Reproductive: Erection/clitoral engorgement.