The parasympathetic nervous system (PNS) controls homeostasis and the body at rest and is responsible for the body's "rest and digest" function. The sympathetic nervous system (SNS) controls the body's responses to a perceived threat and is responsible for the "fight or flight" response. The PNS and SNS are part of the ANS, or autonomic nervous system which is responsible for the involuntary functions of the human body.

Comparison chart

Parasympathetic nervous system versus Sympathetic nervous system comparison chart
Edit this comparison chartParasympathetic nervous systemSympathetic nervous system
Introduction The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system (ANS). Its general function is to control homeostasis and the body's rest-and-digest response. The sympathetic nervous system (SNS) is one of two main divisions of the autonomic nervous system (ANS). Its general action is to mobilize the body's fight-or-flight response.
Function Control the body's response while at rest. Control the body's response during perceived threat.
Originates in Sacral region of spinal cord, medulla, cranial nerves 3, 7, 9, and 10 Thoracic and lumbar regions of spinal cord
Activates response of Rest and digest Fight-or-flight
Neuron Pathways Longer pathways, slower system Very short neurons, faster system
General Body Response Counterbalance; restores body to state of calm. Body speeds up, tenses up, becomes more alert. Functions not critical to survival shut down.
Cardiovascular System (heart rate) Decreases heart rate Increases contraction, heart rate
Pulmonary System (lungs) Bronchial tubes constrict Bronchial tubes dilate
Musculoskeletal System Muscles relax Muscles contract
Pupils Constrict Dilate
Gastrointestinal System Increases stomach movement and secretions Decreases stomach movement and secretions
Salivary Glands Saliva production increases Saliva production decreases
Adrenal Gland No involvement Releases adrenaline
Glycogen to Glucose Conversion No involvement Increases; converts glycogen to glucose for muscle energy
Urinary Response Increase in urinary output Decrease in urinary output
Neurotransmitters neurons are cholinergic: acetylcholine neurons are mostly adrenergic: epinephrine / norepinephrine (acetylcholine)

The Autonomic Nervous System

The autonomic nervous system (ANS) regulates visceral functions, i.e. functions of the internal organs such as the heart, stomach and intestines. The ANS is part of the peripheral nervous system and also has control over some muscles within the body. The functions of the ANS are involuntary and reflexive, e.g. the beating of the heart, expansion or contraction of blood vessels or pupils, etc. — which is why we are seldom conscious of it. The parasympathetic and sympathetic nervous systems, along with the enteric nervous system make up the ANS.

What is the parasympathetic nervous system?

The parasympathetic nervous system is part of the autonomic nervous system. It originates in the spinal cord and the medulla and controls homeostasis, or the maintenance of the body's systems. The parasympathetic nervous system controls the "rest and digest" functions of the body.

What is the sympathetic nervous system?

The sympathetic nervous system, also part of the autonomic nervous system, originates in the spinal cord; specifically in the thoracic and lumbar regions. It controls the body's "fight or flight" responses, or how the body reacts to perceived danger.

Sympathetic vs Parasympathetic Responses

With sympathetic nervous responses, the body speeds up, tenses up and becomes more alert. Functions that are not essential for survival are shut down. Following are the specific reactions of sympathetic nervous system:

The parasympathetic nervous system counterbalances the sympathetic nervous system. It restores the body to a state of calm. The specific responses are:

A diagram of parasympathetic and sympathetic effects. Click to enlarge.
A diagram of parasympathetic and sympathetic effects. Click to enlarge.

How it Works

The parasympathetic nervous system is a slower system and moves along longer pathways. Preganglionic fibers from the medulla or spinal cord project ganglia close to the target organ. They create a synapse, which eventually creates the desired response.

The sympathetic nervous system is a faster system as it moves along very short neurons. When the system is activated, it activates the adrenal medulla to release hormones and chemical receptors into the bloodstreams. The target glands and muscles get activated. Once the perceived danger is gone, the parasympathetic nervous system takes over to counterbalance the effects of the sympathetic nervous system's responses.


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