Parasympathetic nervous system: rest-and-digest mode

The parasympathetic nervous system is the “rest-and-digest” half of your autonomic nervous system: the branch that calms your body after stress and runs baseline functions like digestion and heart rate. Physiologist John Langley named it in 1921. Its main channel, the vagus nerve, carries roughly 75% of its fibers to your heart, lungs, and gut.

Where does the term “parasympathetic” come from?

By the early 1900s, physiologists already had a name for the nerve chain that speeds the body up: “sympathetic.” Physiologist John Langley spent years mapping a separate set of nerves, running from the brainstem and lower spine, that consistently did the opposite: slowing the heart, narrowing pupils, and stimulating digestion. Per a 2008 historical review in European Neurology, Langley named this counterpart “parasympathetic” (“para,” meaning alongside) in his 1921 monograph, grouping it with the sympathetic and enteric divisions under “autonomic,” the umbrella term he had already introduced.

What does the parasympathetic nervous system actually do?

Per a 2022 anatomy overview in StatPearls, the parasympathetic nervous system runs on a “craniosacral” pathway: fibers exit your brainstem through four cranial nerves, including the vagus, and your lower spinal cord through sacral nerves, rather than the mid-spine chain sympathetic fibers use. Acetylcholine carries the signal at each handoff. Most of the workload falls to one nerve: Cleveland Clinic notes the vagus nerve alone carries about 75% of the body’s parasympathetic fibers, reaching your heart, lungs, and most of your digestive tract. The net effect is a slower heart rate, narrowed pupils, and more active digestion.

How does it relate to the sympathetic nervous system?

The parasympathetic and sympathetic nervous systems are partners, not a simple on-off switch. Per a 2018 review in Frontiers in Behavioral Neuroscience, sympathetic activity spends energy to ready you for action, while parasympathetic activity conserves it and gradually returns your body to baseline once a stressor passes. Polyvagal theory further splits the parasympathetic side in two: a calm, socially engaged “ventral vagal” state, and an older “dorsal vagal” shutdown reserved for when a threat feels inescapable. Both still run through the same vagus nerve, just different branches of it.

Can you activate your parasympathetic nervous system on purpose?

To a modest degree. A 2018 randomized trial in JMIR Formative Research found brief cold water on the face measurably increases vagal activity, though the boost is small and short-lived. Slow, exhale-focused breathing has a stronger evidence base: a 2023 Stanford trial in Cell Reports Medicine linked five minutes a day to improved mood and a lower resting breathing rate over a month. Many people pair either practice with bilateral stimulation, the left-right rhythm at the center of EMDR, as a steadier daily habit; 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. Our five-minute nervous-system reset is a good place to start.