parasympathetic nervous system vs sympathetic-nervous-system

The main difference between the parasympathetic and sympathetic nervous system is the “Response”. The parasympathetic nervous system is the one part of the autonomic nervous system that originates in the spinal cord and medulla and is mainly responsible for the body’s rest and digestion response when the body is relaxed, resting, or feeding while the sympathetic nervous system is another part of the autonomous nervous system that originates from the spinal cord of the thoracic and lumbar region and is mainly responsible for controlling the fight and flight response when the body is in stress such as threat or injury.

In general, the parasympathetic nervous system is opposite in its function to the sympathetic nervous system.

Comparison Chart              

 Basis Parasympathetic Nervous System Sympathetic Nervous System
Definition It is a part of the autonomic nervous system that controls the functions of the body at rest It is a part of the autonomic nervous system that prepares the body during fight or flight response
Function It causes muscles to relax and heart rate to decrease and also involved in maintaining homeostasis in the body It causes muscles to contract and heart rate to increase and prepare the body for intense physical activity
Response Time Due to the longer neuronal pathway, it has a slower response time It gives comparatively quicker response due to shorter neuronal pathway
Ganglion Location Ganglions of the parasympathetic nervous system are found away from the central nervous system but close to the effector.  Ganglions are found close to the central nervous system but away from the effector
Size of Pre-Ganglionic Fibers Long Short
Size of Post-Ganglionic Fibers Short Long
Numbers of Post-Ganglionic Fibers A small number of post-ganglionic fibers are found in the parasympathetic nervous system Large number of post-ganglionic fibers are found in the sympathetic nervous system
Neurotransmitter Released Acetylcholine released at the effector to produce a cholinergic response Nor adrenaline released at the effector
Effect on Metabolic Rate Decreases metabolic rate Increases metabolic rate
Effect on Heart Rate Decrease Increase
Effect on Saliva Secretion It stimulates saliva secretions Saliva secretion is inhibited
Effect on Pupil Constricts pupil Dilates pupil
Effect on Urinary Output Increases the urinary output and relaxes the rectum Decreases the urinary output and constricts the rectum
Effect on Adrenaline Gland It has no action on the adrenaline gland Stimulates the production of adrenaline from the adrenal gland
Effect on Pulmonary System Constricts bronchial tubes Dilates bronchial tubes
Effect on Target Area Parasympathetic nervous system generates a localized effect at the target area It generates a diffused effect at its target area

What is Parasympathetic Nervous System?

The parasympathetic nervous system is one of the three divisions of the autonomic nervous system which is sometimes also called as, “The Rest & Digest System”.

It is generally involved in controlling the unconscious actions of the body such as digestion, respiration, and heart rate when the body is feeding, resting, or relaxed. It basically undoes the work of Sympathetic Nervous System.

parasympathetic nervous system vs sympathetic-nervous-system

For example, during a fight and flight response, (during stress or injury) the Sympathetic Nervous System performs the following functions,

  • Inhibits digestion.
  • Constricts blood vessels and diverts blood flow away from the gastrointestinal (GI) tract and skin
  • Blood flow to skeletal muscles and the lungs is enhanced (by as much as 1,200% in the case of skeletal muscles).
  • Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange at rest.
  • Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
  • Dilates pupils and relaxes the ciliary muscle by allowing more light to enter the eye and far vision.
  • Enhance vasodilation for the coronary vessels of the heart.
  • Constricts all the intestinal sphincters and the urinary sphincter.
  • Inhibits peristalsis.
  • Stimulates orgasm.

Conversely, the parasympathetic nervous system is involved to bring the body back into homeostasis to conserve precious energy and do the following functions,

  • Dilates blood vessels connecting to the GI tract and increasing blood flow.
  • It constricts the bronchioles when the need for oxygen has reduced.
  • It causes constriction of the pupil and contraction of the ciliary muscle.
  • Stimulates salivary gland secretion, and accelerates peristalsis.
  • Stimulates sexual arousal.

What is Sympathetic Nervous System?

Another division of the autonomic nervous system is the sympathetic nervous system which is especially known for its “fight-or-flight” response.

The sympathetic nervous system prepares your body to either run from danger or fight back. It’s also activated in response to mental or physical stress. During the fight-or-flight response, the following occurs:

  • Blood pressure increases
  • Blood flow increases to muscles, lungs, and other areas essential for moving away from perceived danger
  • Blood flow decreases to the digestive and reproductive systems
  • Stress hormones, such as cortisol, and neurotransmitters, like epinephrine, increase to make us stronger and faster
  • Glucose is rapidly released to be burned for quick energy

The SNS is perhaps best known for mediating the neuronal and hormonal stress response commonly known as the fight-or-flight response, also known as sympathoadrenal response of the body. This occurs as the preganglionic sympathetic fibers that end in the adrenal medulla secrete acetylcholine, which activates the secretion of adrenaline (epinephrine), and to a lesser extent nor adrenaline (nor epinephrine).

Therefore, this response is mediated directly via impulses transmitted through the sympathetic nervous system, and also indirectly via catecholamines that are secreted from the adrenal medulla, and acts primarily on the cardiovascular system.

Messages travel through the sympathetic nervous system in a bidirectional flow. Efferent messages can trigger simultaneous changes in different parts of the body.

For example, the sympathetic nervous system can accelerate heart rate, widen bronchial passages, decrease motility of the large intestine, constrict blood vessels, increase peristalsis in the esophagus, cause pupillary dilation, piloerection (goosebumps) and perspiration (sweating), and raise blood pressure.

Some evolutionary theorists suggest that the sympathetic nervous system operated in early organisms to maintain survival since the sympathetic nervous system is responsible for priming the body for action. One example of this priming is in the moments before waking, in which sympathetic outflow spontaneously increases in preparation for activity.

Key Differences between Parasympathetic and Sympathetic Nervous System

  1. The parasympathetic nervous system restores the body to a calm and composed state and prevents it from overworking whereas the sympathetic nervous system prepares the body for fight and flight response.
  2. The sympathetic nervous system releases the hormones epinephrine and nor epinephrine that accelerate the heart rate while the parasympathetic nervous system releases acetylcholine, the hormone that slows down the heart rate.
  3. The parasympathetic nervous system is composed of cranial and spinal nerves. The sympathetic nervous system comprises cell bodies that lie within the gray column of the spinal cord.

Comparison Video


The autonomic nervous system comprises of two parts- the sympathetic and parasympathetic nervous system. The sympathetic nervous system activates the fight or flight response during a threat or perceived danger, and the parasympathetic nervous system restores the body to a state of calm.