Nervous system
SNS
Definition
The sympathetic nervous system is a major division of the autonomic nervous system that helps mobilise resources and coordinate responses to changing demands.
Aliases and abbreviations
SNS · sympathetic division · sympathetic branch
Editorial record
Published: August 25, 2026 · Reviewed: August 25, 2026
Expanded definition
Sympathetic pathways influence functions such as heart rate, vascular tone, sweating, pupil size, and the distribution of energy during activity or challenge. Their effects depend on the organ, signal pattern, and wider physiological context.
The system is often discussed through the phrase “fight or flight”, but that phrase is only a shorthand. Sympathetic activity also contributes to ordinary movement, attention, temperature regulation, standing, exercise, and moment-to-moment adaptation.
Autonomic regulation is not a simple contest in which sympathetic activity is bad and parasympathetic activity is good. Healthy regulation involves coordinated, flexible shifts among multiple neural and bodily systems.
What the term does not establish
Why the distinction matters
The distinction prevents normal mobilisation and adaptation from being reduced to a single “stress mode” or treated as something the body should eliminate.
Main guide
A plain-language guide to mobilisation, protective responses, and why nervous-system states are more complex than one switch.
Read the response-state guideSources and further reading
Related terms
Nervous system
Nervous system
Measurement and interpretation
Measurement and interpretation
Editorial scope
This glossary entry supports general education. It does not diagnose, recommend treatment, or transfer findings from one VNS method, protocol, or device to another.