the autonomic nervous system

the short answer

the autonomic nervous system is the part of your nervous system that runs your organs without asking you. it sets your heart rate, narrows and widens blood vessels, moves food through your gut, makes you sweat and changes the size of your pupils (Hall and Hall, 2026, ch 61). it can act fast: within 3 to 5 seconds it can double the heart rate, and sweating can start within seconds (Hall and Hall, 2026, ch 61). the foreword to a standard reference book defines it as the system that carries signals from the brain and spinal cord to all the organs and tissues of the body except skeletal muscle fibres (Jänig, 2006).

it has three divisions: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system in the wall of the gut (Jänig, 2006). the first two are usually drawn as opposites, an accelerator and a brake. the research gives a more precise picture.

autonomic nervous system function: what it actually does

think of the jobs your body does that you never decide to do. keeping blood pressure steady when you stand up. pushing food along your intestines. adjusting heart rate to what you are doing. sweating when you are hot. widening your pupils in the dark. all of that is autonomic nervous system function (Hall and Hall, 2026, ch 61).

it does not do these jobs with one switch. one standard reference book describes it as many separate pathways, each serving a particular target for a particular job, and its foreword warns that attempts to generalise about it have often proved wrong (Jänig, 2006). that point comes back several times below.

autonomic and somatic nervous system

your nervous system reaches the body in two ways.

1. the somatic nervous system drives the skeletal muscles you move on purpose, and carries touch, pressure and position from your skin, muscles and joints.
2. the autonomic nervous system drives the heart, smooth muscle in blood vessels and organs, and glands (Hall and Hall, 2026, ch 61).

breathing sits on the border. the textbook does not count it as an autonomic function, but as one of the body's involuntary functions, controlled from centres in the brainstem (Hall and Hall, 2026, ch 61). a respiratory centre in the brainstem generates the rhythm and keeps you breathing with almost no conscious effort (Hall and Hall, 2026, ch 42). a small region within it, the pre-Bötzinger complex, is thought to be a key part of the rhythm generator, although how the rhythm is produced is still debated (Hall and Hall, 2026, ch 42). you can also take over for short periods and breathe faster, slower or not at all (Hall and Hall, 2026, ch 42). that is one reason breathing comes up so often in how to regulate your nervous system.

how it is wired

the outgoing autonomic pathways share one basic design. a first neuron sits in the brainstem or spinal cord. its fibre runs out to a cluster of nerve cells called a ganglion, where it connects to a second neuron, and the second neuron runs on to the organ (Jänig, 2006; Hall and Hall, 2026, ch 61). the details below come from the same two sources.

• sympathetic pathways leave the spinal cord from the segments at chest level and the top of the lower back. their ganglia sit some distance from the organs, mostly in chains running alongside the spine.
• parasympathetic pathways leave from the brainstem and from the lowest part of the spinal cord. their ganglia sit close to, or inside, the organs they supply.
• the chemical messengers are mainly acetylcholine and noradrenaline. every first neuron, in both branches, releases acetylcholine. the second neuron releases acetylcholine in the parasympathetic branch and, in most cases, noradrenaline in the sympathetic branch. the sympathetic nerves to sweat glands are an exception: they use acetylcholine.
• the adrenal medulla, the inner part of the adrenal gland, is an exception to the two-neuron design. sympathetic fibres reach it without a relay in a ganglion, and its cells release adrenaline and noradrenaline into the blood, on average about 80 percent adrenaline. their effects resemble direct sympathetic stimulation but last 5 to 10 times as long.

the sympathetic nervous system

the sympathetic nervous system speeds and strengthens the heartbeat, narrows blood vessels in the skin and gut, drives sweating, widens the pupils and, through the adrenal medulla, releases adrenaline into the blood (Hall and Hall, 2026, ch 61).

it is often summed up as "fight or flight". that nickname describes an emergency. the reference books describe something more ordinary. each pathway has its own pattern of activity, one for blood vessels in the skin, another for sweating, another for the heart, and they change separately (Jänig, 2006; Hall and Hall, 2026, ch 61). both branches are also active all the time at a background level, called tone. sympathetic tone normally keeps almost all the body's arterioles, the smallest arteries, narrowed to about half their maximum width, and turning it up or down narrows or widens them (Hall and Hall, 2026, ch 61). the whole sympathetic system firing at once, which the textbook calls a mass discharge, happens in particular situations, such as fright or severe pain: that is the alarm or stress response (Hall and Hall, 2026, ch 61). the sympathetic nervous system works all day, not only when you are under threat. what happens when demand does arrive is covered in the stress response in the body.

the parasympathetic nervous system

the parasympathetic nervous system slows the heart, increases the secretions and movement that digest food, narrows the pupils, and empties the bladder (Hall and Hall, 2026, ch 61). about 75 percent of all parasympathetic fibres run in the vagus nerve, which reaches the heart, the lungs and most of the gut (Hall and Hall, 2026, ch 61). the vagus nerve has its own article.

its nickname is "rest and digest". like "fight or flight", it is a fair summary of one end of the range and a poor description of how the system works from moment to moment.

sympathetic and parasympathetic nervous system: not a seesaw

the common picture of the sympathetic and parasympathetic nervous system is a seesaw: when one goes up, the other goes down, and your state is whichever one is winning. three findings complicate it.

1. many targets answer to one branch, not two. most blood vessels and the sweat glands are run by the sympathetic branch: parasympathetic stimulation has little or no effect on them, so for them there is no opposing branch to balance against (Jänig, 2006; Hall and Hall, 2026, ch 61).
2. where both branches do reach the same organ, as in the heart and the pupil, their effects can oppose each other. that is the case the seesaw picture is built on. the textbook itself notes that no general rule predicts which branch excites or inhibits a given organ, and that most organs are mainly controlled by one branch or the other (Hall and Hall, 2026, ch 61).
3. the sympathetic nervous system and parasympathetic nervous system can be active at once. in freezing, an animal goes still and its heart rate slows, which is a parasympathetic effect, while sympathetic arousal is also raised. a review of the research describes freezing as a parasympathetic brake on movement, not a passive state: the body stays primed to act (Roelofs, 2017). the review describes this as well documented in animals. in humans, the same review says, it is only starting to be studied. the freeze response goes into it further.

so "am i in sympathetic or parasympathetic?" is usually the wrong question. a better one is: which parts of me are doing what, right now?

the third division: the enteric nervous system

the enteric nervous system is a network of neurons in the wall of the gut. it was named as a separate division about a century ago, and it can run much of digestion on its own, without instructions from the brain (Jänig, 2006; Hall and Hall, 2026, ch 61). the brain still adjusts it, through both the sympathetic and parasympathetic pathways (Hall and Hall, 2026, ch 61).

it listens as well as it commands

the autonomic nervous system is usually described by what it sends out. it also depends on what comes in. nerve fibres from the organs report stretch, chemistry and other changes inside the body back to the brainstem and spinal cord, and the vagus nerve carries signals in that direction as well as out to the organs (Jänig, 2006; Hall and Hall, 2026, ch 61 and ch 42). many of these signals set off reflexes that adjust the organs without your awareness, such as the baroreceptor reflex that steadies blood pressure (Hall and Hall, 2026, ch 61).

some of those signals travel up to the insula, a region of the cortex. the neuroscientist A. D. Craig proposed that this pathway is the basis of how you feel the state of your own body (Craig, 2002). what it means for feeling and for the sense of self is still emerging. this sense of the body from the inside is called interoception.

the traffic also runs from the higher brain down. Julian Thayer and Richard Lane described a network linking brain regions involved in attention and emotion to the circuits that set heart rate, and proposed that heart rate variability, the beat-to-beat change in the timing of your heart, gives a window onto how well that network is working (Thayer and Lane, 2000). it is a model, and some of its links are still being tested. vagal tone covers what heart rate variability does and does not measure.

can you control your autonomic nervous system?

mostly not directly: you cannot simply decide to speed up your gut or narrow the blood vessels in your hands. you can reach it indirectly.

• breathing is one route in. in a review of fifteen studies of healthy people, none of them randomised trials, breathing slower than about ten breaths a minute went with higher heart rate variability while people were doing it, and in some of the studies with lower anxiety (Zaccaro et al, 2018). the evidence is emerging, not settled. there is more in breathing patterns and your nervous system.
• there may be a direct wire between breathing and arousal. in mice, a small group of neurons in the brainstem's breathing rhythm generator connects to an arousal centre. removing them left breathing intact and made the mice calmer (Yackle et al, 2017). that is one study in mice. it shows a possible route, not that the same holds in people.
• a popular model, polyvagal theory, divides the parasympathetic side into two vagal circuits and ties them to social behaviour (Porges, 2007). its basic premises have been challenged in a detailed critique (Grossman, 2023). the vagus nerve article sets out both sides.

for coming back down after a surge of activation, see how to get out of fight or flight mode.

where the evidence stops

• the basic anatomy is established and comes from two standard references (Jänig, 2006; Hall and Hall, 2026). the breathing findings, freezing in humans, and the idea that heart rate variability shows how well the brain's regulating network is working are emerging, and the article says so where each appears.
• what you notice is not a direct measurement of your autonomic nervous system. in one study of 80 people, how accurately they could detect their own heartbeat was only partly predicted by how they rated their own body awareness (Garfinkel et al, 2015), and how best to measure interoception is still an open problem in the field (Khalsa et al, 2018).
• autonomic disorders are medical problems with their own research and treatment. this article does not cover them.

try this

next time you notice a change in your body, skip "stressed" or "calm" and name the parts. heart: fast or slow. breath: shallow or deep. hands: warm or cold. stomach: tight or settled. two of those can point in different directions at the same time. that is the autonomic nervous system working as separate pathways, not as one dial.

where the aware system fits

the aware system does not measure your autonomic nervous system. it records what you notice of it, in five lanes: breath, body, mind, command (what you deliberately did) and regulate (what brought you back). read back over weeks, that record shows which parts of your system tend to move together and which do not. it is not a medical device, and there is no diagnosis. it opens on iOS and Android in february 2027.

read next

• 09 how to regulate your nervous system
• 02 interoception
• 03 the stress response in the body coming soon
• 06 the vagus nerve coming soon
• 05 the freeze response coming soon

references

1. Jänig W (2006). The Integrative Action of the Autonomic Nervous System: Neurobiology of Homeostasis. Cambridge University Press. doi:10.1017/CBO9780511541667
2. Yackle K et al (2017). Breathing control center neurons that promote arousal in mice. Science 355(6332):1411-1415. doi:10.1126/science.aai7984
3. Roelofs K (2017). Freeze for action: neurobiological mechanisms in animal and human freezing. Philosophical Transactions of the Royal Society B 372(1718):20160206. doi:10.1098/rstb.2016.0206
4. Craig AD (2002). How do you feel? Interoception: the sense of the physiological condition of the body. Nature Reviews Neuroscience 3(8):655-666. doi:10.1038/nrn894
5. Thayer JF, Lane RD (2000). A model of neurovisceral integration in emotion regulation and dysregulation. Journal of Affective Disorders 61(3):201-216. doi:10.1016/S0165-0327(00)00338-4
6. Zaccaro A et al (2018). How breath-control can change your life: a systematic review on psycho-physiological correlates of slow breathing. Frontiers in Human Neuroscience 12:353. doi:10.3389/fnhum.2018.00353
7. Porges SW (2007). The polyvagal perspective. Biological Psychology 74(2):116-143. doi:10.1016/j.biopsycho.2006.06.009
8. Grossman P (2023). Fundamental challenges and likely refutations of the five basic premises of the polyvagal theory. Biological Psychology 180:108589. doi:10.1016/j.biopsycho.2023.108589
9. Garfinkel SN, Seth AK, Barrett AB, Suzuki K, Critchley HD (2015). Knowing your own heart: distinguishing interoceptive accuracy from interoceptive awareness. Biological Psychology 104:65-74. doi:10.1016/j.biopsycho.2014.11.004
10. Khalsa SS et al (2018). Interoception and mental health: a roadmap. Biological Psychiatry: Cognitive Neuroscience and Neuroimaging 3(6):501-513. doi:10.1016/j.bpsc.2017.12.004
11. Hall JE, Hall ME (2026). Guyton and Hall Textbook of Medical Physiology, 15th edition. Elsevier. chapters 42 (Regulation of Respiration) and 61 (The Autonomic Nervous System and the Adrenal Medulla). ISBN 9780443111013