M01.01.010 Parasympathetic Nervous System

The parasympathetic nervous system (PNS) is one of the two main divisions of the autonomic nervous system (ANS), functioning alongside the sympathetic nervous system (SNS) to maintain homeostasis. The PNS is often described as the “rest and digest” system, as it dominates during periods of relaxation and supports restorative processes.

This guide covers the anatomy, actions, and clinical relevance of the PNS, with detailed breakdowns and explanations for USMLE Step 1 preparation.

Anatomical Structure

The PNS consists of the following components:

• Pre-ganglionic fibers: Originate from the brainstem and sacral spinal cord, synapsing with post-ganglionic fibers.
• Peripheral ganglia: Groups of neuronal cell bodies where synapses occur.
• Post-ganglionic fibers: Carry signals from ganglia to effector organs.

Pre-Ganglionic Neurons

• Origin: Brainstem and sacral segments (S2–S4) of the spinal cord.
• Pathways:
  • Cranial nerves III, VII, IX, and X.
  • S2–S4 spinal nerves (forming the pelvic splanchnic nerves).
• Special Note: The PNS is also called the craniosacral division of the ANS due to its origins.

Peripheral Ganglia

• Location: Close to or within the target organs.
• Function: Synaptic site where pre-ganglionic neurons communicate with post-ganglionic neurons.

Post-Ganglionic Neurons

• Origin: Peripheral ganglia.
• Pathway: Transmit signals to effector organs.
• Characteristics: Short, unmyelinated, and release acetylcholine.

Parasympathetic Innervation

Head and Neck

• Cranial nerves involved: III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus).
• Ganglia:
  • Otic.
  • Ciliary.
  • Pterygopalatine.
  • Submandibular.

Trunk and Limbs

Vagus Nerve (CN X)

• Contribution: ~75% of parasympathetic outflow.
• Target organs: Thoracic viscera and most of the gastrointestinal tract.

Sacral Spinal Nerves

• Origin: S2–S4 anterior rami.
• Formation: Pelvic splanchnic nerves.
• Target organs: Distal colon, rectum, bladder, and reproductive organs.

Actions of the Parasympathetic Nervous System

Clinical Relevance

Truncal Vagotomy

• Procedure: Severing the vagal supply to the stomach to reduce acid secretion.
• Indication: Previously used for peptic ulcer disease.
• Complications: Gastroparesis due to impaired smooth muscle contraction.

Points to Remember

1. The PNS is the craniosacral division of the ANS.
2. It uses acetylcholine as its primary neurotransmitter for both pre-and post-ganglionic neurons.
3. The vagus nerve accounts for the majority of parasympathetic outflow.
4. PNS actions include conserving energy, promoting digestion, and facilitating reproductive functions.
5. Clinical conditions like peptic ulcers previously involved interventions targeting the PNS (e.g., truncal vagotomy).

Study Resources

• Anatomy references: Detailed anatomy atlases (e.g., Netter’s or Gray’s).
• Clinical guides: USMLE First Aid, Pathoma.
• 3D Models: Available on platforms like TeachMeSeries or other anatomy apps.