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    Autonomic nervous system

    自主神经系统

    Concept
    Also called
    ANS, sympathetic nervous system, parasympathetic nervous system, rest and digest

    What is the autonomic nervous system?

    The autonomic nervous system is the part of the peripheral nervous system that controls internal organs and glands, mostly outside voluntary control.[1] Its sympathetic division readies the body for stress and high arousal; its parasympathetic division, nicknamed “rest and digest,” returns it to routine.[1][2]

    Sympathetic Mobilize for stress and threat From the thoracic and lumbar spinal cord
    Parasympathetic Restore: rest and digest From the brainstem and sacral spinal cord

    Heart rate (beats/min)

    • At rest, parasympathetic dominant: 60–80
    • No parasympathetic input: about 100
    • Exercise, sympathetic up: 120–140

    Typical textbook values

    Select a row for details.
    Schematic of what the sympathetic (left) and parasympathetic (right) divisions do to the main organs. Most organs get both; sweat glands and most blood vessels get only sympathetic input. Select a row for details; the heart-rate strip below compares three states. Select a part for details
    Schematic of what the sympathetic (left) and parasympathetic (right) divisions do to the main organs. Most organs get both; sweat glands and most blood vessels get only sympathetic input. Select a row for details; the heart-rate strip below compares three states. Save as image (PNG)

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    Key points

    • Sympathetic activity dilates the pupils, speeds the heart, widens the airways, slows digestion and makes the liver release glucose. Parasympathetic activity mostly does the reverse and also boosts salivation.[1][2]
    • Most organs receive both. Left to itself the heart beats about 100 times a minute; the resting 60–80 reflects parasympathetic slowing, and sympathetic input speeds it up during exercise.[3]
    • There are exceptions: sweat glands and most blood vessels are run by the sympathetic side alone, and the parasympathetic side has little effect on them.[2][3]
    • In freezing animals both divisions are active at once, so heart rate may barely rise, hold steady or fall; in fight or flight, the parasympathetic brake on the heart is released.[4]

    An example

    As Ms. Wang steps up to present to her team, her palms sweat, her heart races, her mouth goes dry and her stomach knots. Back at her desk afterward, her heart slows and her stomach settles. Sympathetic activity dominates the first phase; in the second, the parasympathetic side takes over again.[1][2]

    Misreadings and limits

    A common picture treats the two divisions as a switch: sympathetic for stress and “bad,” parasympathetic for calm and “good.” In fact most organs receive both at once,[3] and the two can be active together, as in freezing.[4] Sympathetic arousal is a normal adaptation; constant, repeated stress is what is linked to health problems.[1]

    Sources

    1. 1 Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020). 3.3 Parts of the nervous system. Psychology 2e. OpenStax. View source
    2. 2 LeBouef, T., Yaker, Z., & Whited, L. (2023). Physiology, autonomic nervous system. StatPearls. StatPearls Publishing (last update May 1, 2023). View source
    3. 3 Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022). 15.2 Autonomic reflexes and homeostasis. Anatomy and Physiology 2e. OpenStax. View source
    4. 4 Kozlowska, K., Walker, P., McLean, L., & Carrive, P. (2015). Fear and the defense cascade: Clinical implications and management. Harvard Review of Psychiatry, 23(4), 263–287. doi:10.1097/HRP.0000000000000065

    Last checked against sources: October 27, 2026