The sympathetic nervous system (SNS) is one of two main divisions of the autonomic nervous system, which controls involuntary body processes like heart rate and digestion.
The SNS activates what is often termed the fight or flight response. When the body perceives a threat, the SNS stimulates bodily responses that serve to prepare the body for action in response to danger rapidly.
Key Takeaways
- The sympathetic nervous system helps you respond quickly to stress or danger.
- It triggers the fight-or-flight response, the rapid, whole-body reaction that prepares you to face or escape danger.
- The SNS works with neurotransmitters like norepinephrine and epinephrine to signal organs.
- Besides stress response, the SNS also helps regulate temperature and cardiovascular function.
- Chronic overactivation of the SNS can harm your health, but relaxation techniques can help restore balance.
Sympathetic vs. parasympathetic nervous system
The SNS works in opposition to the parasympathetic nervous system (PNS), which governs “rest-and-digest” functions to conserve energy. The parasympathetic branch stimulates digestion and the urinary system when relaxed. The SNS does the opposite, slowing them, because they are not needed during a stress response.
The two systems also differ in their nerve pathways. Both relay signals through a chain of two neurons.
The SNS reverses the parasympathetic pattern: its first neuron is short, and its second is long. A short first neuron can branch onto many long second neurons nearby. This lets one signal reach far.
The SNS therefore tends to fire as one broad, coordinated burst rather than a single pinpoint signal. The reaction can feel instant.
It often arrives before a person is even aware of it. Together, the SNS and PNS work to maintain homeostasis, the body’s stable internal balance, by pulling the same organs in opposite directions.
Fight-or-Flight: Your Body’s Stress Response
The main role of the SNS is to trigger the “fight-or-flight” response. This reaction is your body’s way of preparing to face or escape a threat.
Cannon’s Discovery of Fight-or-Flight (1929)
Aim: American physiologist Walter Cannon wanted to know what happens in the body during strong emotion. Did the SNS and adrenal medulla act as one?
Method: Across many animal studies, Cannon provoked fear and rage in animals, for example a cat facing a barking dog. He measured adrenal secretion, blood sugar, blood flow, and heart and digestive changes, comparing animals with an intact sympathetic-adrenal system to those without one.
Results: Emotional arousal produced a coordinated, whole-body pattern. Adrenaline surged and blood sugar rose, the heart quickened, blood shifted toward the muscles, and digestion shut down. This pattern appeared as one unit, only when the sympathetic-adrenal system was intact.
Conclusion: Cannon named this the fight-or-flight response. He argued its purpose is to defend the body’s homeostasis in an emergency. The work founded the modern physiology of stress.
When activated, the SNS causes several automatic changes:
- Increases heart rate to pump more blood to muscles
- Dilates pupils to improve vision
- Slows digestion to conserve energy
- Stimulates sweat glands to cool the body
- Relaxes the bladder
- Heightens alertness for quicker reactions
Example: Imagine walking alone at night and hearing footsteps behind you. Your SNS immediately kicks in. You feel alert, your heart races, and your body gets ready to respond. Even non-life-threatening events like exams or public speaking can trigger this response.
Nerves of the SNS
The SNS uses a chain of neurons to send signals throughout the body. Here’s how it works:
- Preganglionic neurons start in the spinal cord (specifically in the thoracic and lumbar regions).
- They connect to ganglia, which are clusters of nerve cells outside the spinal cord.
- From there, postganglionic neurons carry the message to organs like the heart, lungs, and sweat glands.
Neurotransmitters in the SNS
The SNS uses neurotransmitters to communicate. Neurotransmitters are the chemical messengers that are transmitted through neurons.
These neurotransmitters are:
- Norepinephrine: also known as noradrenaline. It helps in activating the body and brain to act during the fight-or-flight response, aiding in alertness.
- Epinephrine: also known as adrenaline. Increases heart rate, dilates air passages and contracts blood vessels to help the body prepare for stressful or dangerous situations.
- Acetylcholine: the signal-relay chemical at the ganglion, passing the message from the first neuron to the second. It also activates the sweat glands directly, an exception to the sympathetic system’s usual reliance on noradrenaline.
As a result, these neurotransmitters encourage the body’s organs involved in the SNS to respond to a threat.
SNS Functions Beyond Fight-or-Flight
The SNS isn’t just about emergencies. It helps regulate other important body functions:
1. Body Temperature
- Sweating: the SNS switches on the sweat glands directly, using acetylcholine rather than its usual noradrenaline, so sweat can evaporate and cool the body.
- Skin blood flow: it narrows the blood vessels in the skin, pulling blood toward the core and muscles and limiting heat loss in the cold.
- Piloerection: it also raises goosebumps by contracting tiny muscles at each hair follicle, a reflex that fluffs fur for insulation in other animals but does little in humans.
2. Cardiovascular Support
- Heart rate and force: the SNS raises the heart’s rate and the force of each beat, which helps push blood pressure up when it is needed.
- Blood vessel tone: it narrows some blood vessels and widens others, redirecting blood toward the muscles and brain rather than the gut and skin.
- Readiness: together these changes prime the body for sudden physical activity and help maintain a steady blood supply to the brain.
What Happens When the SNS Is Overactive or Underactive?
Sometimes, the SNS can become overactive or underactive. Either extreme can affect health.
Overactivation
- Chronic activation: ongoing stress can keep the SNS switched on long after a threat has passed, instead of letting it settle back down.
- Cardiovascular strain: repeatedly raised heart rate and blood pressure can raise the risk of heart disease and damage the arteries over time.
- Anxiety and panic: overactivation is linked to anxiety disorders. In panic disorder, it can fire from internal cues alone and be mistaken for a heart attack.
Underactivation
- Slower responses: an underactive SNS can leave a person less alert, with poorer judgment and slower physical reactions to danger.
- Long-term risks: a 2020 review of 47 studies covering 32,866 people found that a blunted, under-responsive stress system predicts health problems later, including obesity and depression (Turner et al., 2020).
How to Calm an Overactive SNS
If your body feels stuck in fight-or-flight mode, these strategies can help activate your parasympathetic system and promote relaxation:
- Deep breathing: Try square breathing (inhale for 4 counts, hold, exhale, hold)
- Mindfulness meditation: Focus on the present moment
- Progressive muscle relaxation: Tense and release muscle groups
- Regular exercise: Boosts long-term stress resilience
- Sleep: Aim for 7–9 hours a night
- Professional support: Seek therapy or medical help for chronic anxiety
Remember, everyone responds to stress differently, so it may take some experimentation to find the strategies that work best for you.
Critical Evaluation
The physiology of the sympathetic nervous system is well established, but the classic model has real limits. Four criticisms are worth understanding.
- Two Opposing Switches Is a Simplification: the SNS and its partner rarely act as simple on/off switches; they often work together and by degrees, not just against each other.
- Not Every Organ Has Two Switches: some organs, like the adrenal medulla and sweat glands, receive sympathetic input only, with no parasympathetic opposite.
- An Androcentric Behavioural Model: the classic fight-or-flight response was based mainly on male subjects, and may not fully capture how women respond to stress.
- Individual Differences in Reactivity: people differ widely in how strongly their SNS reacts to stress, and both too much and too little reactivity carry health risks.
Two Systems, Not Always Opposed
Textbooks often draw the sympathetic and parasympathetic systems as a simple see-saw, one purely activating, one purely calming. That picture is too tidy. Sexual arousal is a clear counter-example. It depends on both systems acting together, not one overriding the other.
Many of their “opposite” effects are better described as complementary tuning than a strict switch. The heart makes the point well.
It is braked by the parasympathetic vagus nerve and accelerated by sympathetic nerves at the same moment. Its rate reflects the balance between the two, not which one is switched on.
The result is a dial, not a switch. Treating the sympathetic system as a single on/off button understates how graded and subtle autonomic control really is.
The model remains a useful teaching device. It simplifies a more layered reality.
Some Organs Have No Parasympathetic Opposite
The antagonism model assumes both divisions reach every organ. Several important sympathetic targets break that rule.
The adrenal medulla, the sweat glands, the piloerector muscles, and most blood vessels receive sympathetic input only. They have no parasympathetic opponent.
For these organs, there is only one input to control, and it simply goes up or down. The tidy “balance of two systems” story does not apply to them at all.
This matters because it is easy to over-generalise.
A theorist can move from organs like the heart, where both systems clearly compete, to the sympathetic system as a whole.
A meaningful share of its targets work on a single dial, not a double one. That is a real limit on the antagonism model as a general description of autonomic control.
An Androcentric Behavioural Model
Shelley Taylor and colleagues raised a specific challenge to the classic account in 2000. They looked at who the research was based on.
Most of the foundational stress research, including much of Cannon’s own work, had used male animals or participants. Taylor’s team reviewed the human and animal literature.
Women under stress often show a different pattern, they found. They called it “tend-and-befriend”: protecting and nurturing offspring, and seeking out social support, rather than fighting or fleeing. Oxytocin may be the reason.
They proposed this pattern is underpinned by oxytocin and other affiliative systems that modulate the sympathetic response.
The critique does not challenge the underlying sympathetic physiology. Taylor’s team accepted that as accurate. What it corrects is the assumption that “fight or flight” exhausts how organisms behave when that physiology fires (Taylor et al., 2000).
Individual Differences in Reactivity
Two people can face an identical stressor. They can produce very different sympathetic responses. This difference is stable over time, too. Averaged accounts of “the” fight-or-flight response can obscure this.
They treat a population average as if it described everybody. That matters because the variation is not neutral.
Large prospective studies show that both an exaggerated and a blunted sympathetic response are linked to worse long-term health (Turner et al., 2020).
The two paths differ. Exaggerated responses wear down the heart and arteries over years. Blunted responses instead predict later obesity, depression, and anxiety.
A one-size-fits-all model of sympathetic arousal misses this. The difference between people may turn out to be clinically decisive. Researchers increasingly measure it directly, using heart-rate and blood-pressure responses to a standardised laboratory stressor.
Contemporary Research
Recent work has shifted focus. It used to ask only what the sympathetic system does in an emergency. Now it asks something else.
Aim: How do individual differences in reactivity affect long-term health? Turner and colleagues (2020) tested this question. They searched the whole prospective literature. Their question was simple: does the size of a person’s stress response predict future health?
Method: Their systematic review searched several databases. It covered prospective studies of healthy adults. Each study measured sympathetic (SAM) or HPA-axis reactivity to a lab stressor. Participants were then followed for at least a year.
Forty-seven studies met the criteria. Together they covered 32,866 participants, followed for one to 23 years.
Results: Both exaggerated and blunted reactivity predicted later problems, but of different kinds. High reactivity predicted future cardiovascular risk factors and shorter telomeres. Blunted reactivity predicted later obesity, depression, anxiety, and poorer overall health. Only about a third of the associations were significant.
The effect is real, but modest.
Conclusion: Dysregulation in either direction is the marker that matters, not simply “too much” arousal. A healthy sympathetic system responds appropriately, not maximally.
References
Alshak, M. N. (2019). Neuroanatomy, sympathetic nervous system.
Biology Dictionary. (October 4, 2019). Sympathetic Nervous System. https://biologydictionary.net/sympathetic-nervous-system/
Britannica, T. Editors of Encyclopaedia (2019, September 13). Sympathetic nervous system. Encyclopedia Britannica. https://www.britannica.com/science/sympathetic-nervous-system
Lumen. (n.d.). Functions of the Autonomic Nervous System. Retrieved May 5, 2021 from https://courses.lumenlearning.com/boundless-ap/chapter/functions-of-the-autonomic-nervous-system/
Turner, A. I., Smyth, N., Hall, S. J., Torres, S. J., Hussein, M., Jayasinghe, S. U., Ball, K., & Clow, A. J. (2020). Psychological stress reactivity and future health and disease outcomes: A systematic review of prospective evidence. Psychoneuroendocrinology, 114, 104599. https://doi.org/10.1016/j.psyneuen.2020.104599
