Sympathetic Nervous System (SNS)

What is the sympathetic nervous system (SNS)?

Your sympathetic nervous system is part of your autonomic nervous system. It could be called your “automatic” nervous system, as it is responsible for many functions that you don’t have to think about to control. This can include control of your heart rate, blood pressure, digestion, urination and sweating, among other functions.

Your sympathetic nervous system is best known for its role in responding to dangerous or stressful situations. In these situations, your sympathetic nervous system activates to speed up your heart rate, deliver more blood to areas of your body that need more oxygen or other responses to help your get out of danger.

What does the sympathetic nervous system do?

Your sympathetic nervous system controls your “fight-or-flight” response. Danger or stress activates your sympathetic nervous system, which can cause several things to happen in your body. In response to danger or stress, your sympathetic nervous system may affect your:

• Eyes. Enlarge your pupils to let more light in and improve your vision.
• Heart. Increase your heart rate to improve the delivery of oxygen to other parts of your body.
• Lungs. Relax your airway muscles to improve oxygen delivery to your lungs.
• Digestive tract. Slow down your digestion so its energy is diverted to other areas of your body.
• Liver. Activate energy stores in your liver to an energy that can be used quickly.

These effects help you in situations where you might need to think or act quickly. They improve your eyesight, reflexes, endurance and strength. Your sympathetic nervous system also activates at times when your body’s under strain, like when you’re exercising or are sick.

Your sympathetic nervous system activity also affects your immune system and your body’s repair processes. These effects can help your body start repairs on an injury quickly if you get hurt.

Neurotransmitters

Your sympathetic nervous system uses chemicals called neurotransmitters to communicate. Specifically, these chemicals are norepinephrine, epinephrine and acetylcholine.

What’s the difference between the sympathetic and parasympathetic nervous systems?

Your sympathetic and parasympathetic nervous systems have opposite roles. While your sympathetic nervous system carries signals that put your body’s systems on alert, your parasympathetic carries signals that relax those systems.

The two systems work together to keep your body in balance. Your sympathetic nervous system takes the lead for as long as is necessary to get you through a period of danger. Then, your parasympathetic nervous system steps in and returns things to normal.

Where is your sympathetic nervous system located?

Most of the signals that your sympathetic nervous system sends start in your spinal cord. The signals leave your spinal cord and activate structures called ganglia. Your sympathetic ganglia then send the necessary signals far and wide to different parts of your body. This could include your heart, lungs, arteries, sweat glands and digestive system.

What is it made of?

The components of your sympathetic nervous system are similar to those found in other parts of your nervous system. The main type of cell is a neuron, which can generate and receive signals.

What are the common conditions and disorders that affect the sympathetic nervous system?

There are many conditions and causes of sympathetic nervous system problems. Common examples include:

• Type 2 diabetes. Uncontrolled Type 2 diabetes can damage your autonomic nervous system, including your sympathetic nervous system. An example of this is orthostatic hypotension, where your blood pressure drops when you stand up. Diabetes-related neuropathy can damage the nerves that normally trigger a blood pressure increase reflex when you stand.
• Anxiety disorders and chronic stress. Anxiety and chronic stress can strain your sympathetic nervous system. Over time, that can increase your risk of obesity and other metabolic problems.
• Cancer. Pheochromocytomas are a type of cancer that affects your adrenal glands, which are at the top of your kidneys. Your adrenal glands produce neurotransmitters, like adrenaline and norepinephrine. This kind of cancer makes these glands release too much adrenaline and norepinephrine, which keeps your sympathetic nervous system far more active than needed.
• Genetic conditions. Genetic conditions like amyloidosis can affect your sympathetic nervous system.
• Horner’s syndrome. This condition affects a small part of the sympathetic nervous connections in your face. This can cause a small pupil, a lack of facial sweating and eyelid drooping.
• Infections. Nerve damage can happen because of viruses and certain bacteria.
• Multiple system atrophy. This severe condition is similar to Parkinson’s disease, damaging autonomic nerves over time.
• Sexual dysfunction. Your sympathetic and parasympathetic nervous systems play a role in sexual function. Individuals with a penis with sympathetic nervous system damage may experience priapism, an erection lasting at least four hours. Without treatment, priapism can lead to permanent erectile dysfunction.
• Trauma. Injuries can cause nerve damage, which may be long-term or even permanent. This is especially the case when you have injuries to your spinal cord that damage or cut off sympathetic nervous system connections farther down.

What are some common signs and symptoms of sympathetic nervous system problems?

Several potential symptoms can happen with sympathetic nervous system conditions, including:

• Heart rhythm issues (including arrhythmias).
• Constipation.
• Dizziness or passing out when standing up.
• Eyelid droop (ptosis).
• Fast heart rate (tachycardia), even when resting.
• Sexual dysfunction, including priapism.
• Sweating too much (hyperhidrosis) or not sweating enough (anhidrosis).
• Trouble digesting food (including gastroparesis).
• Trouble swallowing (dysphagia).

What are some common tests to check sympathetic nervous system functions?

Potential tests include:

• Blood tests (these can detect many problems, ranging from immune system issues to excessive amounts of neurotransmitters in your blood).
• Blood pressure tests (including ones where your position changes, like the tilt table test).
• Electrocardiogram (EKG).
• Electroencephalogram (EEG).
• Electromyogram (nerve conduction test).
• Genetic testing.
• Magnetic resonance imaging (MRI).
• Autonomic nervous system testing (this series of tests checks your body’s ability to control heart rate, blood pressure and sweating).

What are the common treatments for sympathetic nervous system conditions?

Sympathetic nervous system condition treatments can take many different forms, and ultimately, there’s no one type of treatment or approach to these conditions. Some might be as simple as medication or lifestyle changes. Others can be more complicated.

Some treatments focus on an underlying cause, which may resolve your sympathetic nervous system problem at least partially. If a condition isn’t curable, treatments would likely focus on the symptoms and minimizing the condition’s effects and progress.

How can I prevent conditions and problems that affect the sympathetic nervous system?

Prevention can make a big difference with conditions that affect your sympathetic nervous system. In some cases, there’s a chance to delay conditions that aren’t otherwise preventable. Unfortunately, some conditions aren’t preventable, such as those you inherit or have when you’re born.

The best preventive measures include:

• Eat a variety of healthy foods. Vitamin deficiencies, especially vitamin B12, can damage your nervous system. You should also avoid overusing vitamins (especially B6, which has toxic effects on your nervous systems at high levels).
• Avoid using drugs and alcohol. Prescription and recreational drug abuse, as well as frequent heavy drinking, can have toxic effects and damage your sympathetic nervous system.
• Stay physically active and a weight weight that's healthy for you. Type 2 diabetes damages your autonomic nerves over time. That’s why preventing it, or at least delaying when it starts, can make a big difference in your overall well-being.
• Wear safety equipment as needed. Safety equipment can be a big help if you want to prevent nerve damage from injuries, regardless of whether you use the equipment during work or play activities.
• Manage chronic conditions as recommended. If you have a chronic condition that can damage your nervous system, you should take steps to manage this condition. Your healthcare provider can help guide you on how to do that. That guidance can make a big difference in helping you limit the condition’s progress or how it affects your life.

A note from Cleveland Clinic

Your sympathetic nervous system is the part of your nervous system that carries signals related to your “fight-or-flight” response. That makes it a key part of your response to stressful situations. Unfortunately, like the rest of your nervous system, this subdivision of your autonomic nervous system is prone to damage from other conditions and injuries. That means taking care of your nervous system overall, including your sympathetic nervous system, is something that’ll go a long way to helping you live a long, healthy life.