Structure
The Parasympathetic Nervous System (PNS) originates from the craniosacral region: it arises from the brainstem (cranial nerves) and sacral spinal cord (S2-S4).
Cranial nerves involved include the vagus nerve (cranial nerve X), which innervates most thoracic and abdominal organs.
Sacral nerves primarily innervate the pelvic organs.
Preganglionic neurons: These are longer neurons compared to those in the sympathetic system. They travel from the CNS to ganglia located near or within target organs (e.g., in the heart, lungs, digestive tract).
Postganglionic neurons: These are short neurons that extend from the ganglia to the specific effector organs.
Neurotransmitters:
Both preganglionic and postganglionic neurons in the parasympathetic system release acetylcholine (ACh), which acts on muscarinic receptors in target tissues.
illustration of the Parasympathetic Nervous System (PNS), showing its origin from the craniosacral region with long preganglionic neurons and short postganglionic neurons connecting to target organs
(image above is just for illustration purpose, it might contain incorrect data)
Functions
The parasympathetic nervous system is responsible for the "rest and digest" activities, which help conserve energy and promote bodily functions during restful periods.
Key effects of the parasympathetic nervous system include:
Decreased heart rate and contractility, promoting a relaxed state.
Bronchoconstriction (narrowing of airways), as heavy breathing is not necessary during rest.
Pupil constriction (miosis) to reduce light intake and focus on near objects.
Stimulation of digestive processes, including increased secretion of saliva, digestive enzymes, and gastric acid, as well as enhanced intestinal motility (peristalsis)
Bladder contraction to promote urination (micturition).
Increased blood flow to the digestive organs, aiding nutrient absorption and waste elimination.
Erection of genital tissues in males and females is promoted as part of reproductive functions.