What Is the Parasympathetic Nervous System?

The parasympathetic nervous system helps the body perform routine maintenance functions such as digesting food, slowing the heart, producing saliva, emptying the bladder, and adjusting the eyes for near vision.

It is often described as the “rest and digest” system. This phrase is useful, but incomplete: the parasympathetic nervous system remains active throughout the day and works continuously with the sympathetic nervous system to maintain internal balance.

The parasympathetic nervous system is one division of the autonomic nervous system.
It regulates involuntary functions involved in digestion, recovery, elimination, and energy conservation.
Its nerve fibers arise from the brainstem and the sacral region of the spinal cord.
The vagus nerve carries parasympathetic signals to the heart, lungs, and much of the digestive tract.
Acetylcholine is the main neurotransmitter used at both synapses in a parasympathetic pathway.

What Does the Parasympathetic Nervous System Do?

The parasympathetic nervous system regulates internal organs without requiring conscious effort. Its effects are especially important during ordinary, non-emergency conditions.

Major parasympathetic functions include:

slowing the heart rate
constricting the pupils
helping the eyes focus on nearby objects
stimulating tear and saliva production
promoting digestive secretions
increasing movement through the gastrointestinal tract
assisting urination and bowel movements
contributing to sexual arousal

These actions help the body process nutrients, eliminate waste, conserve energy, and maintain stable internal conditions.[1,2]

Where Does the Parasympathetic Nervous System Begin?

Parasympathetic nerve signals arise from two main regions:

the brainstem
the sacral spinal cord

For this reason, the parasympathetic system is traditionally called the craniosacral division of the autonomic nervous system.[1]

Parasympathetic Nerves From the Brainstem

Parasympathetic fibers leave the brainstem through four cranial nerves:

Oculomotor nerve (cranial nerve III): constricts the pupil and helps the eye focus on near objects
Facial nerve (cranial nerve VII): stimulates tear glands and some salivary glands
Glossopharyngeal nerve (cranial nerve IX): stimulates the parotid salivary gland
Vagus nerve (cranial nerve X): supplies the heart, lungs, and much of the digestive tract

Among these, the vagus nerve has the widest distribution. It carries parasympathetic signals from the brainstem through the neck and chest into the abdomen.

Parasympathetic Nerves From the Sacral Spinal Cord

Parasympathetic fibers also arise from spinal cord segments S2 to S4. They travel through the pelvic splanchnic nerves to structures in the pelvis and lower abdomen.

These nerves help regulate:

the lower part of the large intestine
the rectum
the bladder
parts of the reproductive system

This sacral pathway is particularly important for urination, defecation, and sexual function.[1,3]

How Does a Parasympathetic Nerve Pathway Work?

A typical parasympathetic pathway uses two neurons to carry a signal from the central nervous system to an organ.

1. Preganglionic neuron

The first neuron begins in the brainstem or sacral spinal cord. Its axon travels toward an autonomic ganglion located close to, or within, the target organ.

2. Postganglionic neuron

The second neuron begins in the ganglion and travels only a short distance to the organ.

Because parasympathetic ganglia lie near their target organs, parasympathetic pathways generally have:

long preganglionic fibers
short postganglionic fibers

This arrangement differs from the sympathetic nervous system, whose ganglia are usually located closer to the spinal cord.[1,2]

Which Neurotransmitter Does the Parasympathetic System Use?

The parasympathetic nervous system uses acetylcholine at both stages of its pathway.

At the autonomic ganglion, acetylcholine released by the preganglionic neuron binds to nicotinic acetylcholine receptors.

At the target organ, acetylcholine released by the postganglionic neuron usually binds to muscarinic acetylcholine receptors.[2,3]

Different muscarinic receptor subtypes produce different effects. For example, M2 receptors in the heart help slow the heart rate, while M3 receptors contribute to glandular secretion and smooth-muscle contraction.

How Does the Parasympathetic System Affect Each Organ?

Organ or system	Typical parasympathetic effect
Eyes	Constricts the pupils and supports near vision
Tear glands	Increases tear production
Salivary glands	Increases watery saliva production
Heart	Slows the heart rate
Airways	Constricts bronchial smooth muscle and increases secretions
Stomach and intestines	Increases motility and digestive secretions
Pancreas	Supports pancreatic secretion during digestion
Gallbladder	Contributes to neural regulation of gallbladder contraction during digestion
Bladder	Contracts the detrusor muscle and promotes relaxation of the bladder outlet, supporting urination
Rectum	Supports defecation
Genitals	Contributes to genital blood flow and erection

These effects are general patterns. The actual response depends on the organ, receptor type, and physiological situation.

What Is the Role of the Vagus Nerve?

The vagus nerve is the major parasympathetic connection between the brainstem and the organs of the chest and abdomen.

Its parasympathetic branches influence:

heart rate
airway tone
digestive secretions
movement of food through the gastrointestinal tract

However, the vagus nerve is not purely a parasympathetic motor nerve. It also contains sensory fibers that carry information from internal organs back to the brain. These signals help the brain monitor the condition of the heart, lungs, and digestive system.[3]

The vagus nerve does not provide parasympathetic control to every organ. Pelvic organs and the lower part of the large intestine receive parasympathetic fibers primarily from the sacral spinal cord.

Is the Parasympathetic System Always the Opposite of the Sympathetic System?

No. The two systems sometimes produce opposing effects, but they are not simple on-and-off switches.

For example:

sympathetic activity tends to accelerate the heart, while parasympathetic activity slows it
sympathetic activity dilates the pupils, while parasympathetic activity constricts them
sympathetic activity can reduce gastrointestinal movement during acute stress, while parasympathetic activity generally promotes digestion

In other organs, the relationship is more complex. Some tissues are controlled mainly by one autonomic division, and some functions require coordinated activity from both systems.

Sweat glands, most blood vessels, and the muscles that raise body hair are controlled mainly by sympathetic pathways rather than by direct parasympathetic innervation.[2,4]

Is “Rest and Digest” an Accurate Description?

“Rest and digest” is a useful memory aid, but it can create two misunderstandings.

First, the parasympathetic nervous system does not become active only when a person is resting. Parasympathetic signals continuously contribute to heart-rate regulation, digestion, secretion, and other normal functions.

Second, parasympathetic activity does not simply indicate that the entire body is calm. Autonomic control is organized by organ and physiological need. Parasympathetic activity may increase in one pathway while other autonomic responses are occurring elsewhere.

A more accurate description is that the parasympathetic nervous system supports routine organ regulation, energy conservation, digestion, secretion, and elimination.

What Happens When Parasympathetic Function Is Disrupted?

Abnormal parasympathetic signaling can affect several organs. Possible manifestations include:

unusually rapid or slow heart rate
dry eyes or dry mouth
difficulty adjusting pupil size
impaired digestion
constipation
difficulty emptying the bladder
sexual dysfunction

These symptoms are not specific to parasympathetic dysfunction. They can also arise from medications, dehydration, endocrine disorders, nerve damage, or other medical conditions. Symptoms alone cannot determine which part of the autonomic nervous system is affected.

Parasympathetic vs. Sympathetic Nervous System

Feature	Parasympathetic system	Sympathetic system
Main anatomical origin	Brainstem and S2–S4 spinal cord	Thoracic and upper lumbar spinal cord
Traditional name	Craniosacral division	Thoracolumbar division
Ganglion location	Near or within the target organ	Usually closer to the spinal cord
Preganglionic fibers	Generally long	Generally short
Postganglionic fibers	Generally short	Generally long
Main postganglionic neurotransmitter	Acetylcholine	Usually norepinephrine
General role	Routine maintenance, digestion, secretion, elimination	Mobilization during physical or emotional demands

This comparison describes the general organization of the two systems. Both divisions remain active and cooperate to regulate the body under changing conditions.

The Main Point to Remember

The parasympathetic nervous system is the craniosacral division of the autonomic nervous system. It uses acetylcholine to regulate functions such as heart rate, pupil constriction, saliva and tear production, digestion, urination, and bowel movements.

Although it is commonly called the “rest and digest” system, its role is broader. It continuously works with sympathetic pathways and central autonomic circuits to maintain homeostasis.

References

Waxenbaum JA, Reddy V, Das JM. Anatomy, Autonomic Nervous System. StatPearls. Updated December 1, 2025. StatPearls Publishing. NCBI Bookshelf
McCorry LK. Physiology of the autonomic nervous system. American Journal of Pharmaceutical Education. 2007;71(4):78. doi:10.5688/aj710478. PubMed Central
Tindle J, Tadi P. Neuroanatomy, Parasympathetic Nervous System. StatPearls. Updated October 31, 2022. StatPearls Publishing.
MSD Manual Professional Edition. Overview of the Autonomic Nervous System. Reviewed May 2025. MSD Manual

Brain Basics Article INDEX

Written by: MedMaru Editorial Team
Reviewed for medical accuracy by: S. Chang, KMD

What Is the Parasympathetic Nervous System?