What Is the Sympathetic Nervous System?
What Is the Sympathetic Nervous System?
Introduction
When you exercise, stand up quickly, enter a hot environment, or face a sudden threat, your body must adjust automatically. Your heart may beat faster, blood vessels change their diameter, and sweat production increases.
Many of these responses are coordinated by the sympathetic nervous system, one division of the autonomic nervous system. It is commonly called the “fight-or-flight” system, but its role is broader than responding to danger. Sympathetic nerves are continuously active at varying levels and help regulate circulation, body temperature, energy use, and other essential functions.
The sympathetic nervous system is a division of the autonomic nervous system that prepares and adjusts the body for increased physical or mental demands.
It can:
- Increase heart rate and the force of heart contractions
- Help maintain or raise blood pressure
- Redirect blood flow among different tissues
- Dilate the pupils
- Promote sweating
- Mobilize stored energy
- Reduce some digestive and urinary activity temporarily
The sympathetic nervous system is not simply an emergency switch. It also contributes to everyday functions such as maintaining blood pressure when standing and regulating body temperature.
What Is the Sympathetic Nervous System?
The sympathetic nervous system is a network of neurons that helps regulate organs, glands, blood vessels, and other structures without requiring conscious control.
It is one of the major divisions of the autonomic nervous system, which also includes the parasympathetic and enteric nervous systems. Sympathetic and parasympathetic pathways often produce different effects on the same organ, but they do not function as simple on-and-off opposites. Instead, their activity is adjusted according to the needs of each organ and situation.
Sympathetic activity is coordinated by several parts of the central nervous system, including the hypothalamus, brainstem, and spinal cord.
Where Is the Sympathetic Nervous System Located?
The cell bodies of sympathetic preganglionic neurons are located mainly in the intermediolateral cell column of the spinal cord from T1 to L2. For this reason, the sympathetic system is often described as having a thoracolumbar outflow.
Most sympathetic signals reach their targets through a two-neuron pathway:
- A preganglionic neuron begins in the spinal cord.
- Its axon travels to a sympathetic ganglion.
- It communicates with a postganglionic neuron.
- The postganglionic neuron carries the signal to the target tissue.
Sympathetic ganglia are organized primarily into two groups:
- Paravertebral ganglia: form paired sympathetic chains along the vertebral column
- Prevertebral ganglia: lie in front of the vertebral column, near major abdominal arteries
Fibers can synapse at the level where they enter the sympathetic chain, travel upward or downward within the chain, or pass through it to reach a prevertebral ganglion. This organization allows sympathetic signals to reach structures throughout the head, chest, abdomen, pelvis, and limbs.
How Does the Sympathetic Nervous System Work?
All sympathetic preganglionic neurons release acetylcholine. Acetylcholine binds primarily to nicotinic receptors on neurons within sympathetic ganglia.
Most sympathetic postganglionic neurons then release norepinephrine, which acts on adrenergic receptors in target tissues. Different receptor types—including alpha and beta receptors—produce different effects depending on the organ.
There are important exceptions:
- Sympathetic nerves supplying most sweat glands release acetylcholine rather than norepinephrine.
- The adrenal medulla acts like a modified sympathetic ganglion.
- Preganglionic neurons stimulate adrenal chromaffin cells to release mainly epinephrine, along with some norepinephrine, into the bloodstream.
Neural signals can therefore create rapid, targeted responses, while hormones released by the adrenal medulla can produce more widespread effects.
What Does the Sympathetic Nervous System Do?
Sympathetic effects vary among organs and depend on which receptors are activated.
Heart and Blood Vessels
Sympathetic stimulation generally increases heart rate, cardiac conduction, and the force of contraction. It also changes the diameter of selected blood vessels.
These actions help maintain blood pressure and distribute blood flow according to the body’s needs. For example, sympathetic reflexes help prevent an excessive fall in blood pressure when a person stands up.
Eyes
Sympathetic pathways contract the radial muscle of the iris, causing the pupil to dilate. This is called mydriasis.
They also help elevate the upper eyelid slightly through the superior tarsal muscle.
Lungs
Sympathetic signaling can promote relaxation of airway smooth muscle through beta-2 adrenergic receptors. In humans, circulating epinephrine contributes importantly to this effect.
Skin and Body Temperature
Sympathetic nerves regulate:
- Sweat production
- Blood flow through the skin
- Contraction of the tiny muscles that raise body hairs
Sweating and changes in skin blood flow are important parts of thermoregulation.
Digestive System
Sympathetic activity can reduce gastrointestinal movement and secretion while increasing the tone of some sphincters. These effects temporarily shift resources away from digestion during periods of increased physical demand.
Digestion is not controlled by a single autonomic division, however. It also depends on parasympathetic pathways, the enteric nervous system, hormones, and local reflexes.
Bladder
During urine storage, sympathetic pathways help relax the bladder wall and contract the internal outlet region. Urination itself requires coordinated activity among autonomic, somatic, and central nervous system pathways.
Metabolism
Sympathetic activation helps make energy available by promoting processes such as:
- Glycogen breakdown
- Glucose release
- Lipolysis in adipose tissue
The exact response depends on the tissue, receptor type, hormones, and the body’s metabolic condition.
Why Is the Sympathetic Nervous System Important?
The sympathetic nervous system helps the body respond to rapidly changing internal and external conditions.
Its everyday functions include:
- Stabilizing blood pressure during changes in posture
- Adjusting circulation during exercise
- Supporting temperature regulation
- Controlling sweating
- Regulating energy availability
- Coordinating responses to pain, stress, and environmental challenges
Without adequate sympathetic control, simply moving from lying down to standing could cause a substantial drop in blood pressure and reduced blood flow to the brain.
What Is the Fight-or-Flight Response?
The fight-or-flight response describes a coordinated physiological reaction to a perceived threat or major challenge.
Sympathetic activation during this response may:
- Accelerate the heartbeat
- Increase cardiac contraction strength
- Dilate the pupils
- Increase sweating
- Mobilize stored energy
- Temporarily reduce digestive activity
However, “fight or flight” is only a simplified description. Sympathetic activity also rises during ordinary exercise, exposure to cold, changes in posture, and the period around awakening.
The response is not identical throughout the body. Different sympathetic nerve pathways can be regulated separately rather than always activating as one uniform system.
Sympathetic vs. Parasympathetic Nervous System
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Main outflow | Thoracic and upper lumbar spinal cord | Brainstem and sacral spinal cord |
| Ganglia | Near the spinal cord or major abdominal arteries | Near or within target organs |
| Preganglionic transmitter | Acetylcholine | Acetylcholine |
| Main postganglionic transmitter | Usually norepinephrine | Acetylcholine |
| General role | Mobilization, vascular control and adaptation to demand | Maintenance, digestion and restoration |
This comparison is useful, but it should not be interpreted as a perfect opposition. Some organs receive stronger control from one division, and many physiological responses require cooperation among multiple systems.
Clinical Relevance
Problems affecting sympathetic pathways can produce different symptoms depending on the location and cause of the dysfunction.
Orthostatic Hypotension
If sympathetic reflexes do not constrict blood vessels adequately when a person stands, blood pressure may fall. This can cause lightheadedness, blurred vision, weakness, or fainting.
Horner Syndrome
Damage to the sympathetic pathway supplying one side of the face can cause:
- A smaller pupil
- Mild drooping of the upper eyelid
- Reduced sweating on the affected side in some cases
This group of findings is called Horner syndrome. Its cause must be investigated because the damage can occur at different points between the brain and the eye.
Disorders of Sweating
Abnormal sympathetic control may result in excessive sweating, reduced sweating, or uneven sweating. These symptoms can have neurological, endocrine, medication-related, or other causes.
Autonomic Disorders
Sympathetic dysfunction may occur in disorders such as autonomic neuropathies, pure autonomic failure, and some forms of dysautonomia. Symptoms alone cannot determine whether sympathetic activity is excessive or deficient; clinical evaluation and, in selected cases, autonomic testing are needed.
Common Misunderstandings
“The sympathetic nervous system is active only during danger.”
This is incorrect. Sympathetic nerves maintain varying levels of activity during ordinary life. They help control blood pressure, circulation, sweating, and temperature even when no threat is present.
“Sympathetic and parasympathetic systems are always opposites.”
They sometimes have opposing effects, particularly on heart rate, but their relationship is more complex. Some tissues receive predominantly sympathetic control, and both divisions may work together during certain functions.
“Norepinephrine is used by every sympathetic nerve.”
Most sympathetic postganglionic neurons release norepinephrine, but sympathetic fibers supplying most sweat glands release acetylcholine. Preganglionic sympathetic neurons also release acetylcholine.
“A fast heartbeat proves that the sympathetic system is overactive.”
A fast heartbeat can have many causes, including exercise, fever, dehydration, pain, medication effects, anemia, endocrine conditions, or an abnormal heart rhythm. It cannot be attributed to sympathetic overactivity without proper evaluation.
Related Articles
- What Is the Autonomic Nervous System?
- What Is the Parasympathetic Nervous System?
- What Is the Peripheral Nervous System?
- What Is a Neurotransmitter?
- How Does the Brain Control the Autonomic Nervous System?
References
- Waxenbaum JA, Reddy V, Das JM. Anatomy, Autonomic Nervous System. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026. Updated December 1, 2025. NCBI Bookshelf
- McCorry LK. Physiology of the autonomic nervous system. American Journal of Pharmaceutical Education. 2007;71(4):78. PubMed Central
- LeBouef T, Yaker Z, Whited L. Physiology, Autonomic Nervous System. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026. Updated May 1, 2023. NCBI Bookshelf
- Coon EA. Overview of the Autonomic Nervous System. MSD Manual Professional Edition. Full review May 2025; updated November 2025. MSD Manual
Written by: MedMaru Editorial Team
Reviewed for medical accuracy by:
S. Chang, KMD