Sympathetic Nervous System: What to Know

Medically Reviewed by Mahammad Juber, MD on September 29, 2022
5 min read

In order for your body to work properly, your nervous system needs to be working properly. Your nervous system is responsible for helping your brain communicate with your body and helping your body communicate with your brain. Unfortunately, today’s fast-paced world can sometimes affect our SNS (sympathetic nervous system).

The sympathetic nervous system is a sub-system of your peripheral nervous system.

Whenever you touch, taste, or see something, it’s because nerves are sending impulses to your brain to help you interpret the world around you. Nerves are like wires sending inputs from all over your body to your brain.

Your nervous system is the network of these nerves. Your body has two primary nervous systems: the central nervous system and the peripheral nervous system. Your central nervous system is made up of your brain and spinal cord. It’s the system responsible for processing input from the peripheral nervous system and sending directions to the peripheral nervous system.

Your peripheral nervous system is made up of all the rest of the nerves in your body. They carry input from your central nervous system to the rest of your body and vice versa. Your peripheral nervous system is itself divided into two systems: your somatic nervous system and your autonomic nervous system.

The somatic nervous system is in charge of voluntary movement, such as walking or using your hands. It’s also in charge of relaying information from your skin, ears, and eyes to your central nervous system.

The autonomic nervous system is responsible for all the things your body does without your control, like breathing, heart rate, and digestion. The autonomic nervous system is further divided into two categories: the parasympathetic nervous system and the sympathetic nervous system.

The parasympathetic nervous system is responsible for keeping your body running during normal situations. It keeps your heart rate regular, keeps your digestion going, ensures that your internal organs are doing their jobs, and sees that your glands are properly releasing hormones.

The sympathetic nervous system, on the other hand, takes over when your body is in a stressful or dangerous situation. 

Your sympathetic nervous system is responsible for your “fight or flight” response. It’s activated when your brain senses that you’re in a stressful situation. The sympathetic nervous system function includes:

  • Dilating (enlarging) your pupils, the dark hole in the center of your eye, so it can take in more light and you can see better
  • Increasing your heart rate to deliver oxygen throughout your body more quickly
  • Relaxing your airway muscles so your lungs can take in more oxygen
  • Stimulating sweat production
  • Encouraging the production and release of glucose to provide your body with more energy
  • Slowing your digestion so that energy can be diverted elsewhere

The goal of the fight or flight response is to increase your eyesight, strength, and speed in times of distress.

The sympathetic nervous system has other functions as well. It’s responsible for keeping your body at a safe temperature by using fat reserves to increase heat and stimulating the sweat glands to cool you down. On top of that, your sympathetic nervous system also accelerates your heart rate for other necessary reasons, like when you exercise or when you go from lying down or sitting to a standing position.

Your sympathetic nervous system communicates through chemicals called neurotransmitters. Neurotransmitters are the messengers that pass on chemical signals from the neuron to another cell or a gland. The neurotransmitters used by the sympathetic nervous system are acetylcholine, epinephrine, and norepinephrine.

Acetylcholine. Acetylcholine is a neurotransmitter that plays a role in regulating heart rate, blood flow, and digestion.

Epinephrine. Also known as adrenaline, epinephrine is both a hormone and neurotransmitter. Only a small amount of epinephrine is produced by the nerves, but it still plays a role in:

  • Attention and focus
  • Excitement
  • Metabolism
  • Panic

Norepinephrine. Norepinephrine, also called noradrenaline, is also both a hormone and a neurotransmitter. As a neurotransmitter, norepinephrine is made from nerve cells out of dopamine, the pleasure and reward neurotransmitter. Norepinephrine’s job is to:

  • Increase your alertness, arousal levels, and/or attention
  • Constrict your blood vessels to maintain blood pressure during stressful situations
  • Affect mood and memory
  • Affect your sleep-wake cycle

Your sympathetic nervous system is spread throughout your body. It retrieves signals from the brain via the central nervous system and, from there, carries the signals to the appropriate parts of your body, like your eyes, heart, digestive system, liver, lungs, and sweat glands.

When your sympathetic nervous system isn’t working properly, it may cause health issues or put you in danger.

Before modern times, the fight or flight response helped us avoid predators. In modern-day society, that’s not much of a concern. Instead, our fight or flight may go off over everyday stressors like work or parenting. When your sympathetic nervous system is activated too often, it can lead to chronic stress, which may raise your blood pressure and increase your risk of heart attacks or strokes.

On the other hand, an under-functioning sympathetic nervous system can cause problems if you do ever find yourself in a life-threatening situation.

Autonomic dysfunction is the term used to describe a malfunctioning autonomic nervous system, including the sympathetic nervous system. Symptoms may include:

  • Digestive issues
  • Dizziness or fainting
  • The inability of your heart rate to adapt to exercise
  • Rapid heart rate
  • Sweating too much or not enough
  • Vision problems such as blurry vision

Often, autonomic dysfunction occurs due to damage to the nerves: autonomic neuropathy. Many things can cause autonomic neuropathy, including:

  • Diabetes, the most common cause
  • Autoimmune diseases like Guillain-Barre syndrome and celiac disease
  • Genetic conditions
  • Irregular protein buildup
  • Nutrient deficiencies
  • Parkinson’s disease
  • Physical trauma
  • Some medications, including chemotherapy
  • Some bacteria and viruses, including HIV and the bacteria that cause Lyme disease
  • Toxicity due to drug or alcohol abuse or heavy metal poisoning

When the nerves in your autonomic nervous system don’t function properly, that may lead to a condition called dysautonomia. Dysautonomia may lead to issues with your blood pressure, body temperature, breathing, digestion, heart rate, kidney function, and sexual function.