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Sympathetic Nervous System vs. Parasympathetic Nervous System: What's the Difference?

Edited by Janet White || By Harlon Moss || Updated on October 10, 2023
The sympathetic nervous system activates the "fight or flight" response, while the parasympathetic nervous system promotes "rest and digest."

Key Differences

The sympathetic nervous system is a component of the autonomic nervous system that readies the body for stressful situations. In contrast, the parasympathetic nervous system helps the body relax and conserve energy after a perceived threat has passed.
The sympathetic nervous system, when activated, increases heart rate, dilates pupils, and redirects blood flow to muscles. On the other hand, the parasympathetic nervous system works to slow down the heart rate, constrict pupils, and direct blood flow to internal organs, aiding in digestion and recovery.
By regulating physiological responses, the sympathetic nervous system ensures that the body can rapidly respond to threats or challenges. Simultaneously, the parasympathetic nervous system ensures that the body remains balanced by facilitating recovery and maintaining homeostasis.
Stressors, whether physical or psychological, can activate the sympathetic nervous system, pushing the body into an alert state. In contrast, once the stressor is no longer present, the parasympathetic nervous system assists in returning the body to a state of calm.

Comparison Chart

Primary Function

Activates "fight or flight"
Promotes "rest and digest"

Effect on Heart Rate

Increases heart rate
Decreases heart rate

Effect on Pupils

Dilates pupils
Constricts pupils

Effect on Digestive Activities

Inhibits digestive processes
Stimulates digestive processes

Effect on Energy Consumption

Increases energy consumption
Conserves energy

Sympathetic Nervous System and Parasympathetic Nervous System Definitions

Sympathetic Nervous System

The system is responsible for increasing alertness and metabolic rate.
During an intense workout, the sympathetic nervous system ensures that muscles receive ample blood supply.

Parasympathetic Nervous System

The parasympathetic nervous system is a branch of the autonomic nervous system.
After a meal, the parasympathetic nervous system aids in digestion.

Sympathetic Nervous System

It prepares the body for the "fight or flight" response.
The sympathetic nervous system increases heart rate and adrenaline production during stress.

Parasympathetic Nervous System

This system is often referred to as the "rest and digest" system.
After a stressful event, the parasympathetic nervous system assists in calming the body down.

Sympathetic Nervous System

The sympathetic nervous system is part of the autonomic nervous system.
When faced with danger, the sympathetic nervous system prepares the body for quick action.

Parasympathetic Nervous System

It counterbalances the effects of the sympathetic nervous system.
To maintain equilibrium, the parasympathetic nervous system acts opposite to the sympathetic system.

Sympathetic Nervous System

This system regulates immediate and short-term reactions to stress.
The rapid breathing experienced during fear is a result of the sympathetic nervous system's activation.

Parasympathetic Nervous System

It promotes body relaxation and energy conservation.
Post-exercise, the parasympathetic nervous system helps slow down the heart rate.

Sympathetic Nervous System

It responds to external stressors by activating various body systems.
Upon hearing a loud noise, the sympathetic nervous system triggers an immediate response.

Parasympathetic Nervous System

The system helps maintain homeostasis in the body.
The parasympathetic nervous system ensures that our internal environment remains stable.


Do the sympathetic and parasympathetic systems ever work together?

Yes, they often work in tandem to maintain a balanced state in the body.

How does the parasympathetic nervous system help after stress?

It promotes relaxation and recovery, often termed "rest and digest."

When is the sympathetic nervous system typically activated?

During stressful, challenging, or threatening situations.

What activities does the parasympathetic nervous system support?

It supports activities like digestion, energy conservation, and relaxation.

Which system helps in promoting digestion?

The parasympathetic nervous system.

What effect does the parasympathetic nervous system have on the pupils?

It constricts the pupils.

What happens to our energy levels during sympathetic nervous system activation?

Energy consumption increases to handle stress or threats.

How does the sympathetic nervous system affect heart rate?

It increases heart rate and prepares the body for action.

How does meditation influence the parasympathetic system?

Meditation stimulates the parasympathetic system, encouraging relaxation.

How do relaxation techniques impact the parasympathetic nervous system?

They stimulate the parasympathetic nervous system, promoting calmness and recovery.

How do the two systems impact metabolic rate?

The sympathetic system increases metabolic rate, while the parasympathetic system reduces it.

Which system predominates during a relaxation massage?

The parasympathetic nervous system.

Can these systems be consciously controlled?

While largely involuntary, certain practices like deep breathing can influence their activity.

What is the main function of the sympathetic nervous system?

It activates the "fight or flight" response during stress or danger.

Can chronic stress harm the sympathetic nervous system?

Chronic activation can lead to health issues like hypertension and decreased immunity.

How does breathing change under the influence of the sympathetic system?

Breathing becomes rapid and shallow.

Are both systems always active?

Both systems are always active to some degree, maintaining balance in the body.

Which system is more active during sleep?

The parasympathetic nervous system.

What role does the parasympathetic system play post-meal?

It aids in digestion and nutrient absorption.

Which system prepares the body for immediate action?

The sympathetic nervous system.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.

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