The sympathetic and parasympathetic nervous systems are two branches of the autonomic nervous system, which controls involuntary bodily functions such as heart rate, blood pressure, respiration and digestion. The sympathetic nervous system is responsible for the “fight or flight” response, while the parasympathetic nervous system is responsible for the “rest and digest” response.
Sympathetic Nervous System
The sympathetic nervous system is one of the two branches of the autonomic nervous system, which is responsible for controlling involuntary bodily functions. The sympathetic nervous system is responsible for the “fight or flight” response, which is a physiological state that prepares the body to either fight or flee a perceived threat.
The sympathetic nervous system is activated by the hypothalamus, which is a region of the brain that is responsible for regulating a variety of bodily functions, including heart rate, blood pressure and body temperature. When the hypothalamus perceives a threat, it sends signals to the sympathetic nervous system, which in turn activates a cascade of physiological changes.
These changes include:
- Increased heart rate and blood pressure: This allows the body to deliver more blood and oxygen to the muscles and other tissues that need it most during a fight or flight response.
- Dilated pupils: This allows more light to enter the eye, which improves vision.
- Bronchodilation: This widens the airways, which allows the body to breathe in more oxygen.
- Increased sweating: This helps to cool the body and prevent overheating.
- Decreased digestion: This diverts blood and energy away from the digestive system and towards other tissues that need it more during a fight or flight response.
The sympathetic nervous system also plays a role in regulating a number of other bodily functions, including blood glucose levels, metabolism and sexual function.
Parasympathetic Nervous System
The parasympathetic nervous system is the other branch of the autonomic nervous system. It is responsible for the “rest and digest” response, which is a physiological state that prepares the body for rest and relaxation.
The parasympathetic nervous system is activated by the vagus nerve, which is the longest nerve in the body. The vagus nerve connects the brain to many different organs, including the heart, lungs, stomach and intestines.
When the parasympathetic nervous system is activated, it causes a number of physiological changes, including:
- Decreased heart rate and blood pressure
- Constriction of pupils
- Bronchoconstriction: This narrows the airways, which helps to conserve heat and prevent dehydration.
- Increased salivation and digestion
- Decreased sweating
The parasympathetic nervous system also plays a role in regulating a number of other bodily functions, including urination, defecation and sexual function.
Sympathetic vs Parasympathetic Nervous System
The following table compares the sympathetic and parasympathetic nervous systems:
| Organ | Sympathetic Nervous System | Parasympathetic Nervous System |
| Heart | Increased rate and force of contraction | Decreased rate and force of contraction |
| Blood Vessels | Constricted | Dilated |
| Bronchi | Dilated | Constricted |
| Eyes | Dilated pupils | Constricted pupils |
| Sweat Glands | Increased sweating | Decreased sweating |
| Digestive System | Decreased activity | Increased activity |
Paramedic Scenarios
Trauma Response
In a patient experiencing a trauma, such as a car accident or a fall, the sympathetic nervous system is activated. This is known as the “fight or flight” response. The sympathetic nervous system causes a number of physiological changes, including:
- Increased heart rate and blood pressure
- Dilated pupils
- Bronchodilation (widening of the airways)
- Increased sweating
- Decreased digestion
These changes prepare the body to either fight or flee the perceived threat.
In a trauma response, the sympathetic nervous system can also cause a number of pathophysiological changes, including:
- Increased blood pressure can lead to hypertension, which can damage the heart and blood vessels.
- Increased heart rate can lead to tachycardia, which can increase the risk of arrhythmias.
- Dilated pupils can lead to blurred vision and light sensitivity.
- Bronchodilation can make it difficult to breathe if the patient has a chest injury.
- Increased sweating can lead to dehydration, which may worsen the patient’s condition.
- Decreased digestion can lead to nausea and vomiting.
Cardiac Emergencies
In a cardiac emergency, such as a heart attack or cardiac arrest, the sympathetic nervous system is activated in an attempt to increase blood flow to the heart. This can lead to increased heart rate and blood pressure.
In a heart attack, the sympathetic nervous system can also cause a number of pathophysiological changes, including:
- Increased heart rate can lead to tachycardia, which can increase the risk of arrhythmias.
- Increased blood pressure can lead to hypertension, which can damage the heart and blood vessels.
- Decreased blood flow to the heart muscle can lead to myocardial ischemia, which can damage the heart muscle.
In cardiac arrest, the sympathetic nervous system can also cause a number of pathophysiological changes, including:
- Cessation of blood flow to the brain can lead to loss of consciousness and seizures.
- Cessation of blood flow to the heart can lead to ventricular fibrillation, which is a fatal arrhythmia.
Respiratory Distress
In respiratory distress, the parasympathetic nervous system may be activated in an attempt to slow down the respiratory rate. This can lead to bronchoconstriction and increased airway resistance.
In respiratory distress, the sympathetic nervous system can also cause a number of pathophysiological changes, including:
- Bronchoconstriction can make it difficult to breathe.
- Increased airway resistance can lead to respiratory acidosis, which is a condition in which the blood becomes too acidic.
- Decreased oxygen levels in the blood can lead to hypoxia, which can damage the brain and other organs.
Infection Response
In response to an infection, the sympathetic nervous system can be activated to increase the heart rate and blood pressure. This helps to deliver more blood and immune cells to the site of infection. The parasympathetic nervous system can also be activated to conserve energy and promote healing.
In an infection, the sympathetic nervous system can also cause a number of pathophysiological changes, including:
- Increased heart rate can lead to tachycardia, which can increase the risk of arrhythmias.
- Increased blood pressure can lead to hypertension, which can damage the heart and blood vessels.
- Fever can lead to dehydration and electrolyte imbalances.
- Sepsis, a severe infection, can lead to multiple organ failure.