Sympathetic nervous system: definition and function

The sympathetic nervous system is one of the two main branches of your autonomic nervous system, which runs body functions you don’t consciously control. It originates in your spinal cord and activates your body’s stress response: a faster heart rate, quicker breathing, and blood redirected toward your muscles. Its counterpart, the parasympathetic nervous system, brings your body back down afterward.

Where does the term “sympathetic” come from?

The word predates modern neuroscience by centuries. Greek physician Galen first used “sympathy” to describe how the body’s organs seemed connected, each reacting to what happened elsewhere. Danish anatomist Jacobus Winslow picked up the idea in his influential 1732 textbook, popularizing “sympathetic nerve” for the chain of ganglia running along the spine.

Physiologist John Langley later narrowed the term. A 2008 historical review in European Neurology explains that Langley applied “sympathetic” specifically to the division rooted in your thoracic and lumbar spinal cord. He then coined “autonomic nervous system” as the broader umbrella term.

What does the sympathetic nervous system actually do?

Your sympathetic nervous system starts in the middle of your spinal cord, in segments running from your upper back to your lower back. Short nerve fibers travel from there to a chain of ganglia alongside your spine, handing off the signal with a chemical messenger called acetylcholine. Longer fibers then carry it onward, releasing norepinephrine at your organs to speed things up, according to a 2023 anatomy overview in StatPearls.

One target works differently. Sympathetic fibers reach your adrenal glands directly, triggering a rush of adrenaline straight into your bloodstream that amplifies the effect body-wide. Per a 2018 review in Frontiers in Behavioral Neuroscience, the combined result can include a faster heart rate, open airways, dilated pupils, and slowed digestion. Your liver also releases stored energy for quick use, all within seconds.

How is it different from the parasympathetic nervous system?

The sympathetic and parasympathetic nervous systems work as a pair, not a simple on/off switch. Sympathetic activity spends energy and readies you for action; parasympathetic activity conserves it and supports rest, digestion, and recovery. Most moments involve some blend of both, adjusting continuously rather than flipping between two extremes.

This pairing is also central to polyvagal theory, a widely referenced framework that places sympathetic activation between a calmer, connected state and a deeper shutdown response. What people commonly call “fight or flight” is really just your sympathetic nervous system running at full activation.

Can you calm an overactive sympathetic nervous system?

To some degree, yes. Slower exhales, movement, and grounding through your senses all cue your body toward its calmer, parasympathetic side. Many people also build bilateral stimulation, the left-right rhythm at the center of EMDR, into that routine; see how bilateral stimulation affects the nervous system for the research behind it.

EmEase, a self-guided EMDR app, offers a guided version of this practice at app.emease.com for everyday stress. For a step-by-step way to settle an activated stress response right now, see how to turn off fight-or-flight.

Frequently asked questions

What is the sympathetic nervous system in simple terms?

It's one of two main branches of your autonomic nervous system, the network running functions you don't consciously control. It starts in your spinal cord and activates your body's stress response, raising heart rate and breathing while redirecting energy toward your muscles. Its counterpart, the parasympathetic nervous system, calms things back down.

Where does the term 'sympathetic nervous system' come from?

Greek physician Galen first used 'sympathy' to describe connections between body organs. Danish anatomist Jacobus Winslow popularized 'sympathetic nerve' in his 1732 textbook. Physiologist John Langley later narrowed the term to the thoracic-lumbar division specifically, per a 2008 historical review in European Neurology, and coined 'autonomic nervous system' as the umbrella term.

What does the sympathetic nervous system control?

It affects most of your organs: raising heart rate, opening airways, dilating pupils, slowing digestion, and releasing stored energy from your liver. Signals travel from your spinal cord through a chain of ganglia along your spine, and directly to your adrenal glands, which release adrenaline into your bloodstream for a body-wide effect.

Is the sympathetic nervous system the same thing as fight-or-flight?

Almost. 'Fight or flight' is the popular name for what happens when your sympathetic nervous system activates strongly under threat. The sympathetic nervous system itself is the underlying anatomy and physiology, active at low levels all the time, not just during a dramatic stress reaction.

How is the sympathetic nervous system different from the parasympathetic nervous system?

They're partners, not opposites on a switch. The sympathetic branch spends energy to ready you for action; the parasympathetic branch conserves it, supporting rest and digestion. Most moments involve a shifting blend of both, adjusting continuously rather than flipping between two extremes.

Can you calm an overactive sympathetic nervous system?

To some degree, yes. Slower exhales, movement, and grounding through your senses cue your body toward its calmer parasympathetic side. Many people also use bilateral stimulation, the rhythmic technique at the center of EMDR, as a regular practice for everyday stress.

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