Sympathetic Nervous System

Last Updated: April 28, 2024

Sympathetic Nervous System

The sympathetic nervous system (SNS) is a key part of the autonomic nervous system, primarily responsible for triggering the body’s fight-or-flight response. It functions continuously to maintain homeostasis—keeping internal balance—by regulating critical functions such as blood glucose levels, body temperature, and cardiac output. Acting in opposition to the parasympathetic nervous system, which promotes rest and digestion, the SNS is essential for both immediate survival and long-term health. Its development starts in the embryonic stage and plays a crucial role throughout life, underscoring its importance in health and disease.

Definition of the Sympathetic Nervous System

The sympathetic nervous system is a part of the autonomic nervous system that activates what is often termed the “fight or flight” response. This system primarily functions to mobilize the body’s resources under stress or in emergency situations, preparing the body to act quickly. It increases heart rate, dilates the pupils, and diverts blood flow away from the digestive system and towards the muscles, enabling a swift response to stressful or dangerous situations.

The Function of the Sympathetic Nervous System

The sympathetic nervous system (SNS) is a key part of our body’s response mechanism to stress and danger, famously known as the “fight-or-flight” response. This system prepares your body to either face a threat or escape from it by making several quick adjustments in various organs. Here’s a straightforward breakdown of how the SNS impacts different parts of your body:


  • Action: Pupil dilation
  • Purpose: Allows more light to enter, improving vision, which is crucial for quickly assessing the surroundings during dangerous situations.


  • Action: Increases heart rate and strength of heart contractions
  • Purpose: Boosts blood circulation, speeding up the delivery of oxygen and essential nutrients to vital organs and muscles.


  • Action: Expands the airways
  • Purpose: Enhances the flow of air into the lungs, increasing oxygen intake, which is vital for heightened physical activity.

Digestive System

  • Action: Reduces digestive activity
  • Purpose: Conserves energy by diverting it from digestion to more critical functions needed during a crisis.


  • Action: Stimulates the release of glucose
  • Purpose: Provides a quick energy boost by converting glycogen (a form of stored energy) to glucose.

Immune System and Healing

  • Action: Prioritizes body repair processes
  • Purpose: Helps initiate rapid healing after injury, ensuring quick recovery to maintain defense capabilities.

The SNS functions through a complex network of nerve cells that start from the spinal cord and extend throughout the body. These nerve cells release chemicals like adrenaline and noradrenaline, which trigger these changes in the body organs.

Key Points on How SNS Operates

  • Eye Muscles: The SNS causes the muscles in the iris to contract, allowing you to see more clearly and sharply.
  • Heart and Lungs: It increases heart rate and opens up lung airways to boost energy and oxygen supply, so your body can act swiftly.
  • Digestive and Excretory Systems: Slows down digestion and reduces urine output, so all energy is focused on immediate survival needs.
  • Liver and Muscles: Increases glucose release for immediate energy and prepares muscles for quick, powerful movements.
  • Immune Response: Enhances the body’s ability to start repairs immediately in case of injury.

Examples of the Sympathetic Nervous System

The sympathetic nervous system (SNS) is a key component of the autonomic nervous system, which plays a crucial role in the body’s response to stress or perceived threats. Here are examples of how the sympathetic nervous system influences various bodily functions:

Fight Response

When faced with danger, the sympathetic nervous system triggers the “fight or flight” response. This reaction prepares the body to either confront the threat or flee from it. It involves a surge of adrenaline, increased heart rate, and heightened alertness.

Blood Pressure

The sympathetic nervous system increases blood pressure to ensure that all body parts receive adequate blood supply, which is essential during emergencies. This is achieved by constricting blood vessels, which raises the pressure of the blood flowing through them.

Cause Pupillary Dilation

During a sympathetic response, the pupils dilate (widen). This reaction, known as mydriasis, allows more light to enter the eye, which improves vision in low-light conditions, helping an individual to better assess and respond to a situation.


The sympathetic nervous system plays a critical role in regulating the heartbeat. During stressful situations, it increases the heart rate to boost blood circulation, ensuring that oxygen and nutrients are quickly distributed to vital organs and muscles.

Human Body

Overall, the sympathetic nervous system primes the human body for rapid, intense activity. It diverts blood from the digestive system and other non-essential functions to the muscles and brain, enhancing physical and mental capacity to handle the stressor.


The sympathetic nervous system inhibits digestion during acute stress. It directs the body’s energy and resources towards more critical functions necessary for immediate survival, such as muscle activity and alertness.


Apart from increasing the heart rate, the sympathetic nervous system also increases the force of each heartbeat. This enhances cardiac output, which is the volume of blood the heart pumps per minute.

Relaxation of Bladder

Under the influence of the sympathetic nervous system, the muscles of the bladder relax while the sphincters contract. This prevents urination, allowing the body to focus on handling the emergent situation.

Slowing Down or Stopping Digestion

During a sympathetic response, the process of digestion slows down or stops. This is because the body prioritizes other functions that are more immediately necessary for dealing with stress.

Sympathetic Nervous System Anatomy

Sympathetic Nervous System Anatomy

The sympathetic nervous system (SNS) is a branch of the autonomic nervous system that is activated during stress or emergencies, preparing the body for rapid action. Here is an overview of the anatomy of the sympathetic nervous system, accompanied by a diagram to illustrate its main components.

  • Spinal Cord: The sympathetic nervous system originates in the spinal cord, specifically from the thoracic and lumbar regions (T1 to L2). These nerves are known as thoracolumbar outflow.
  • Sympathetic Chain Ganglia: Parallel to the spinal column on either side, the sympathetic chain ganglia extend from the neck down to the pelvis. These ganglia are interconnected and form the main pathway for sympathetic fibers.
  • Preganglionic Neurons: These neurons have their cell bodies in the spinal cord and send their axons to the sympathetic chain ganglia, where they either synapse or pass through to connect with other ganglia or reach prevertebral ganglia.
  • Postganglionic Neurons: After synapsing in the ganglia, the postganglionic neurons extend to various target organs, including the heart, lungs, blood vessels, and glands, conveying the sympathetic response.
  • Splanchnic Nerves: These nerves arise from the lower thoracic and upper lumbar spinal nerves and pass through the sympathetic chain without synapsing until they reach the prevertebral ganglia (e.g., celiac and mesenteric ganglia), where they innervate organs in the abdominal cavity.
  • Adrenal Medulla: Unique in its direct innervation by preganglionic fibers, the adrenal medulla releases adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream, amplifying the sympathetic response throughout the body.

Symptoms and Effects of Sympathetic Nervous System Disorders

Disorders of the sympathetic nervous system can manifest a range of symptoms that affect various body functions, primarily those related to the body’s response to stress or emergency situations. Here are the key symptoms and effects associated with such disorders:

  • Heart Rhythm Issues: This includes arrhythmias, where the heart beats irregularly, either too fast or too slow.
  • Constipation: Reduced gut movement can lead to difficulty in bowel movements.
  • Dizziness or Lightheadedness: This often occurs when standing up quickly, due to a sudden drop in blood pressure.
  • Eyelid Droop (Ptosis): A partial or complete sagging of the eyelids, which can affect one or both eyes.
  • Increased Heart Rate (Tachycardia): This can occur even while resting, causing the heart to beat faster than normal.
  • Sexual Dysfunction: This may include conditions like priapism, where an erection lasts longer than normal without sexual arousal.
  • Abnormal Sweating: This can range from excessive sweating (hyperhidrosis) to insufficient sweating (anhidrosis), affecting body temperature regulation.
  • Digestive Issues: Disorders such as gastroparesis slow down the digestive process, causing discomfort and other complications.
  • Swallowing Difficulties (Dysphagia): This can make swallowing food and liquids challenging, leading to further health issues.


What is the Difference Between Sympathetic and Parasympathetic?

The sympathetic nervous system triggers the “fight or flight” response, while the parasympathetic restores the body to a state of calm.

What are 5 Actions of the Sympathetic Nervous System?

Actions include increasing heart rate, dilating pupils, slowing digestion, expanding airways, and releasing adrenaline.

What Causes an Overactive Sympathetic Nervous System?

Causes include stress, anxiety disorders, neurological issues, and certain medications that stimulate nervous system activity.

What Happens When the Sympathetic Nervous System is Stimulated?

Stimulation leads to increased alertness, energy, and readiness for physical activity, enhancing the body’s ability to respond to stress.

AI Generator

Text prompt

Add Tone

10 Examples of Public speaking

20 Examples of Gas lighting