Main Body

21. Agents and Actions of the Autonomic Nervous System: Parasympathetic Nervous System

Parasympathetic Nervous System

Parasympathetic Nervous System [PNS] – “rest and digest”

The PNS can also be thought of as the “D” division – defecation, digestion, and diuresis
Most organs are innervated with parasympathetic nerve ganglions.


EXCEPTION – the ciliary smooth muscle of the eye only has parasympathetic innervation

What does your body need when at rest?

  • Decreased cardiac output (compared to sympathetic) – lower oxygen demand when at rest
  • Energy storage (glycogenesis, lipogenesis) – lower energy demand at rest
  • Increased digestion – increased GI motility and secretions
  • Waste elimination – defecation and urination

Parasympathetic Neurons

Pre-ganglionic and post-ganglionic parasympathetic neurons release acetylcholine [ACh].



Acetylcholine interacts with two types of receptors:

  1. Nicotinic receptors – located on ganglion
  2. Muscarinic receptors – located on effector organ/tissue
    1. M1 –
    2. M2 –
    3. M3 –
    4. M4 –
    5. M5 –

Physiological Effects of PNS on Organs/Tissue & Respective Receptors

Organ/Tissue Receptor Subtype Physiological Effect
BLADDER – detrusor M3 contracts promotes urination
BLADDER – sphincter M3 relaxes promotes urination
EYE – ciliary muscle M3 contracts improves near-sighted vision
EYE – pupil M3 contracts
GASTROINTESTINAL – glands M3 promotes secretion enhances digestion
GASTROINTESTINAL – smooth muscle M3 contracts enhances digestion & motility
GASTROINTESTINAL – sphincter M3 relaxes promotes defecation
HEART – AV node M2 decreases conduction
HEART – cardiac muscle M2 decreases contractility negative ionotropic effect [atria only]
HEART – cardiac output M2 decreases decreases blood delivery
HEART – SV node M2 decreased heart rate negative chronotropic effect – decreases oxygen delivery
LUNG – smooth muscle (bronchioles & trachea) M3 contracts
SALIVARY GLAND M3 stimulates watery secretions enhances digestion
VASCULATURE – smooth muscle no PNS innervation


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Principles of Pharmacology - Study Guide Copyright © by Edited by Dr. Esam El-Fakahany and Becky Merkey, MEd is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.