Sympathetic Nervous System: Functions & Examples

The sympathetic nervous system (SNS) is a subdivision of the autonomic nervous system that regulates autonomic processes. The sympathetic nervous system is involved in preparing the body for stress-related activities, and it slows bodily processes that are less important in emergencies, such as digestion

These processes are not under direct conscious control, occurring automatically without conscious thought.

The sympathetic nervous system typically functions in actions requiring quick responses.

  • Increase heart rate.
  • Dilation of the pupils
  • Secretion of sweat glands
  • Dilated muscles
  • Increased alertness
  • Slowing down or stopping digestion
  • Relaxation of the bladder

The SNS can maintain homeostasis through actions such as sweating to cool down the body or in regulating heart rate. In contrast to the parasympathetic nervous system, which slows down physiological processes, the SNS typically stimulates organs.

The parasympathetic branch, however, stimulates digestion and the urinary system when relaxed, whereas the SNS slows them down as these processes are not required during heightened stress.

The SNS also works alongside the parasympathetic nervous system to maintain homeostasis – the balance of internal physiological mechanisms essential for all living organisms.

Essentially, the parasympathetic branch is the antagonist of the SNS. Also, the neurons of the SNS have shorter axons in comparison to those of the parasympathetic nervous system. Thus they act a lot quicker, sometimes the responses happen before a person is consciously aware of them.

sympathetic vs parasympathetic nervous systems


Fight-or-flight Response

The primary function of the SNS is to activate the fight-or-flight response in threatening situations. For instance, if you are walking alone down a dark street at night and a stranger approaches you, your body responds in a way to enable you to either fight or run away from the situation.

In this situation, the SNS would trigger responses such as causing the eyes to dilate and the heart to beat faster. These autonomic responses to a threatening situation are, therefore, essential for survival. In evolutionary terms, the SNS would have been used in order to fight or escape prey and for hunting to eat and survive.

More modern-day stressors can also stimulate the SNS such as financial pressures, stresses at work, or anything that can cause high anxiety for individuals.

When a stressful or anxiety-provoking situation arises, the amygdala (an area of the brain associated with fear and emotions) will send a distress signal to the hypothalamus (a command structure of the brain associated with maintaining homeostasis).

Impulses are then transmitted through the SNS to the adrenal glands, which then pump adrenaline into the bloodstream.

This will then bring about the physiological changes needed to be prepared to either fight or flight.

The reactions brought about by the SNS result in heightened awareness and preparation for combat or running. Fundamentally, the fight-or-flight response is mediated via impulses transmitted throughout the SNS to the adrenal glands.

The adrenal glands facilitate both short-term responses to stress as well as long-term responses.

Once the threat has been resolved, the parasympathetic nervous system takes over and returns bodily
functions to a relaxed state.

Regulating Body Temperature

For homeostasis to be achieved, the SNS can control the body temperature of organisms through the use of fat reserves in the body.

The SNS uses these reserves to increase the production of heat and through changing the flow of blood to the skin.

The SNS is also able to stimulate the sweat glands to enable the body to cool down, as well as being able to stimulate fatty acid release to instigate long-term responses to persistent periods of cold.

Cardiovascular Effects

The SNS can have effects on the cardiovascular system within the body. This comes into play when exercising (when heart rate needs to increase), changing posture (e.g., going from a sitting to a standing position), and when transitioning from sleep to being aware.

These changes via the SNS are necessary, especially when changing positions, otherwise this can cause dizziness and fainting.

Nerves of the SNS

The SNS consists of neurons found within the peripheral nervous system and the central nervous system, which usually works in stimulating the body’s organs in response to fear or stress.

There are two types of neurons within the sympathetic nervous system: preganglionic and postganglionic neurons, or ganglion cells.

The word ‘ganglia’ refers to clusters of neurons outside the brain and spinal cord. Instead, they are part of the autonomic nervous system and run alongside the spinal cord.

The preganglionic neurons originate in the brain stem or spinal cord and will always leave the spinal cord through areas called the thoracic and lumbar regions.

The preganglionic neurons will then synapse with the postganglionic neurons which sit outside of the spinal cord.

The postganglionic neurons will then extend to target organs of the SNS (e.g., heart, sweat glands, and stomach) in order to trigger certain effects when activated.

Neurotransmitters within the SNS

Neurotransmitters are the chemical messengers which are transmitted through neurons. The preganglionic neuron’s primary neurotransmitter is acetylcholine.

Acetylcholine is a neurotransmitter found in both the central nervous system and the peripheral nervous system and plays a role in brain and muscle function.

The preganglionic neurons within the thoracic and lumbar regions in the spinal cord carry acetylcholine and release it at synapses within the ganglia.

Acetylcholine is then taken up by the receptors on the postganglionic neurons outside of the spinal cord. Activation of this process results in signals being extended to target areas of the sympathetic nervous system and the release of another neurotransmitter called norepinephrine.

Norepinephrine (also known as noradrenaline) is an excitatory neurotransmitter as it stimulates the body. This chemical helps in activating the body and brain to act during the fight-or-flight response, aiding in alertness.

Norepinephrine is released from the adrenal medulla after prolonged activation from postganglionic neurons. Epinephrine (also known as adrenaline) is also released from the adrenal medulla after increased levels of activation.

Epinephrine is also an excitatory neurotransmitter which is released into the blood stream and enhances the neuronal effects of the SNS. As a result, these neurotransmitters encourage the organs involved in the SNS to respond to a threat and causing blood vessels to open up to allow more blood flow in order for the muscles to fight or flight.

In other words, a perceived threat results in the secretion of epinephrine and norepinephrine from the adrenal medulla, which then mediates the fight-or-flight response.

Problems with the SNS

Although most modern-day stressors which trigger the SNS may appear small, they may be interpreted by our nervous system as a potential life threat.

If the SNS is activated too frequently, this can have long-lasting effects on the body, resulting in chronic stress. Similarly, constant surges of epinephrine can damage blood vessels and arteries, which in turn can increase blood pressure and increase the risk of strokes and heart attacks.

Alternatively, if the SNS is under-functioning, this can also cause issues. If someone’s SNS is not functioning, they may not have appropriate responses in times of stress.

They may not recognize that there is a danger, and they may take more risks as they are not being alerted by their SNS that they are in life-threatening situations.

As their organs are not receiving signals to fight-or-flight, this may result in being under-prepared in these situations, due to lack of blood being pumped around the body or pupils not dilating.

Autonomic dysfunction is a condition whereby the autonomic nervous system and its divisions do not work properly.

This may lead to altered functioning of the heart, sweat glands, pupils, and blood vessels depending on the condition. Autonomic dysfunction can develop when nerves of the autonomic nervous system are damaged and can range from mild to life-threatening.

The most common cause of autonomic dysfunction is diabetes, but there could be hereditary reasons, as well as aging, Parkinson’s disease or chronic fatigue syndrome being some of the possible causes.

If someone believes they may be suffering from autonomic dysfunction, they may be experiencing one or more of the following symptoms:

  • Feeling dizziness or actually fainting.
  • Inability to alter heart rate in response to exercise.
  • Abnormally fast heart rate.
  • Digestive issues.
  • Visual problems, e.g. blurriness.
  • Abnormal sweating – either too much sweating or not sweating enough.
  • Lack of pupillary response.

Autonomic dysfunction can be treated depending on the symptoms being experienced. For instance, if the cause of dysfunction is due to diabetes, controlling blood sugars will be the primary treatment.

In many cases, treatment of the underlying disease (if applicable) can allow damaged nerves within the ANS to repair and regenerate.

Autonomic dysfunction can be diagnosed through a doctor taking their time in order to understand what exactly the issue is. For example, using blood pressure monitors to test high or low blood pressure or using an electrocardiogram to measure heart rate.

How to calm an overactive sympathetic nervous system

If an individual has an overactive SNS in times that are not considered dangerous, there are quick methods which can somewhat aid in calming down the SNS.

Taking deep breaths at a slow and steady pace, as well as various breathing exercises, are ways to encourage our parasympathetic nervous system to antagonize the SNS. This can be a quick way to help manage stress responses and decrease anxiety.

Similarly, practicing mindfulness is another method to actively prompt the body to rest, rejuvenate and regenerate, allowing a return to homeostasis.

For more serious cases of chronic stress, deep breathing may not be useful, so it is recommended to seek a doctor’s advice, who may recommend medical treatment or talking therapies to be able to combat the cause of the stress.


Biology Dictionary. (October 4, 2019). Sympathetic Nervous System.

Britannica, T. Editors of Encyclopaedia (2019, September 13). Sympathetic nervous system. Encyclopedia Britannica.

Lumen. (n.d.). Functions of the Autonomic Nervous System. Retrieved May 5, 2021 from

Saul Mcleod, PhD

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Educator, Researcher

Saul Mcleod, Ph.D., is a qualified psychology teacher with over 18 years experience of working in further and higher education.

Olivia Guy-Evans

Associate Editor for Simply Psychology

BSc (Hons), Psychology, MSc, Psychology of Education

Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.