Parasympathetic Nervous System: Structure, Functions and Effects
Introduction
The parasympathetic nervous system is one of the three major components of the autonomic nervous system (ANS). It controls involuntary activities and helps the body maintain normal functions during resting conditions.
It is commonly called the rest-and-digest system because it supports digestion, energy conservation and routine body maintenance.
For example, after eating a meal, the parasympathetic nervous system helps increase digestive secretions and intestinal movements. It also helps regulate heart rate and other involuntary activities.
The parasympathetic nervous system generally works in coordination with the sympathetic nervous system to maintain a balanced internal environment.
[IMG 1 — Horizontal medical infographic showing the rest-and-digest response: reduced heart rate, constricted pupils, increased salivation, increased digestive activity and normal bladder emptying.]
1. Origin of the Parasympathetic Nervous System
The parasympathetic nervous system is known as the craniosacral division because its nerve fibres originate from the brainstem and sacral region of the spinal cord.
Its origin includes:
A. Cranial Region
Parasympathetic fibres arise from four cranial nerves:
Oculomotor nerve (CN III): Helps control pupil constriction and lens accommodation.
Facial nerve (CN VII): Supplies certain salivary and tear glands.
Glossopharyngeal nerve (CN IX): Supplies the parotid salivary gland.
Vagus nerve (CN X): Supplies many organs in the chest and abdomen, including the heart, lungs and digestive tract.
The vagus nerve carries a major portion of the parasympathetic supply to internal organs.
B. Sacral Region
Parasympathetic fibres also arise from spinal cord segments S2–S4.
These fibres supply structures of the pelvic region, including the urinary bladder, distal large intestine and reproductive organs.
[IMG 2 — Horizontal anatomical diagram showing the craniosacral origin of the parasympathetic nervous system, highlighting cranial nerves III, VII, IX and X and spinal cord segments S2–S4.]
2. Pathway of Parasympathetic Nerve Signals
The parasympathetic nervous system generally uses a two-neuron pathway to transmit signals from the central nervous system to target organs.
The two neurons are:
A. Preganglionic Neuron
The preganglionic neuron begins in the brainstem or sacral spinal cord and carries the nerve impulse towards a parasympathetic ganglion.
Its axon is usually long because the ganglia are located close to or within the target organs.
It releases a neurotransmitter called acetylcholine (ACh) at the ganglion.
B. Postganglionic Neuron
The postganglionic neuron begins in the ganglion and carries the signal to the target organ.
Its axon is generally short because the ganglion is located near or inside the organ.
It also releases acetylcholine at the target organ.
Important: Both preganglionic and postganglionic parasympathetic neurons release acetylcholine.
[IMG 3 — Horizontal labelled pathway diagram showing CNS → long preganglionic neuron → parasympathetic ganglion near or inside the target organ → short postganglionic neuron → target organ, with acetylcholine labelled at both synapses.]
3. Neurotransmitters and Receptors
A neurotransmitter is a chemical messenger released by a neuron to communicate with another neuron or target cell.
The main neurotransmitter of the parasympathetic nervous system is acetylcholine (ACh).
It acts through two major types of cholinergic receptors.
A. Nicotinic Receptors
Nicotinic receptors are present on postganglionic neurons in parasympathetic ganglia.
When acetylcholine binds to these receptors, it helps transmit the nerve signal from the preganglionic neuron to the postganglionic neuron.
B. Muscarinic Receptors
Muscarinic receptors are present on target organs supplied by parasympathetic nerves.
When acetylcholine binds to these receptors, it produces responses such as:
Reduced heart rate.
Increased salivary secretion.
Increased intestinal activity.
Contraction of the urinary bladder.
Remember:
Nicotinic receptors: Present at autonomic ganglia.
Muscarinic receptors: Present on parasympathetic target organs.
[IMG 4 — Horizontal comparison diagram showing acetylcholine acting on nicotinic receptors at a parasympathetic ganglion and muscarinic receptors at the target organ.]
4. Effects of the Parasympathetic Nervous System on Different Organs
The parasympathetic nervous system produces different effects depending on the organ and the receptors present in its tissues.
A. Effect on the Heart
Parasympathetic stimulation generally:
Decreases heart rate.
Slows the conduction of electrical impulses through the atrioventricular node.
These effects help maintain a slower heart rate during resting conditions.
B. Effect on the Eyes
Parasympathetic stimulation causes:
Pupil constriction (miosis): Reduces the size of the pupil.
Accommodation for near vision: Helps the lens adjust to focus on nearby objects.
C. Effect on the Digestive System
Parasympathetic stimulation generally increases digestive activity by:
Increasing gastrointestinal movements.
Stimulating digestive secretions.
Supporting the movement of food through the digestive tract.
For example, after eating, increased parasympathetic activity helps the digestive system process food.
D. Effect on Salivary Glands
It stimulates salivary secretion, helping keep the mouth moist and supporting digestion.
E. Effect on the Respiratory System
Parasympathetic stimulation generally causes mild bronchoconstriction and increases bronchial secretions.
This is opposite to the bronchodilation commonly produced by sympathetic stimulation.
F. Effect on the Urinary Bladder
Parasympathetic stimulation helps in urination by:
Contracting the detrusor muscle of the urinary bladder.
Promoting relaxation of the internal urethral sphincter.
These actions help empty the bladder.
G. Effect on the Reproductive System
Parasympathetic activity contributes to erection by promoting increased blood flow to erectile tissues.
[IMG 5 — Horizontal labelled human body infographic showing parasympathetic effects on the eyes, salivary glands, heart, lungs, stomach, intestines, urinary bladder and reproductive organs.]
5. Difference Between Sympathetic and Parasympathetic Nervous Systems
Both systems are parts of the autonomic nervous system, but their effects on many organs are generally opposite.
Feature | Sympathetic System | Parasympathetic System |
|---|---|---|
Main role | Fight or flight | Rest and digest |
Origin | Thoracolumbar (T1–L2) | Craniosacral |
Preganglionic fibres | Generally short | Generally long |
Postganglionic fibres | Generally long | Generally short |
Heart rate | Increases | Decreases |
Pupil | Dilates | Constricts |
Digestion | Generally decreases | Generally increases |
Airways | Dilates | Generally constricts |
Urinary bladder | Promotes urine storage | Promotes urination |
Main neurotransmitter at target organs | Usually noradrenaline | Acetylcholine |
Important: These systems do not simply switch each other off. Their activities are coordinated according to the body's needs.
6. Pharmacological Importance
The parasympathetic nervous system is important in pharmacy because several medicines act on cholinergic receptors or influence acetylcholine activity.
Examples include:
Atropine: Blocks muscarinic receptors. It can increase heart rate and reduce certain glandular secretions. It is also used to produce pupil dilation during eye examinations.
Pilocarpine: Stimulates muscarinic receptors and increases salivary secretion. It may be used to manage dry mouth in selected conditions.
Neostigmine: Inhibits the enzyme acetylcholinesterase, increasing the availability of acetylcholine at cholinergic synapses.
These examples help explain how medicines can increase or decrease parasympathetic effects.
7. Clinical Importance
Abnormal parasympathetic activity or interference with its nerve pathways can affect several body functions.
For example:
Reduced salivary secretion may cause dry mouth.
Excessive parasympathetic stimulation may produce a slow heart rate.
Damage to certain parasympathetic pathways may affect bladder emptying.
Drugs that block muscarinic receptors may cause dry mouth, blurred vision, constipation and urinary retention.
Understanding these effects helps pharmacy students recognize the physiological actions and possible adverse effects of medicines acting on the autonomic nervous system.
Key Takeaways
The parasympathetic nervous system is a division of the autonomic nervous system.
It is called the rest-and-digest system.
It originates from the brainstem and sacral spinal cord (S2–S4).
Its cranial outflow travels through cranial nerves III, VII, IX and X.
It generally has long preganglionic and short postganglionic fibres.
Acetylcholine is released by both preganglionic and postganglionic neurons.
Nicotinic receptors are present in autonomic ganglia, while muscarinic receptors are present on target organs.
It generally decreases heart rate and increases digestive activity.
Atropine, pilocarpine and neostigmine are important examples of drugs related to cholinergic activity.