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Sympathetic Nervous System: Definition, Function, Disorders

Sympathetic Nervous System: Definition, Function, Disorders

Irshad AnwarUpdated on 02 Jul 2025, 06:51 PM IST

The sympathetic nervous system is part of the autonomic nervous system and is involved in the body's involuntary "fight or flight" response. The structure of the sympathetic nervous system consists of neurons of the spinal cord connected to organs through ganglia. Its function allows for rapid adjustments, such as increased heart rate and blood flow when one experiences stress. Disorders may interrupt these responses and disturb the cardiovascular and metabolic balance. This is one of the important topics from the Neural Control and Coordination chapter of Biology.

This Story also Contains

  1. Sympathetic Nervous System (SNS)
  2. Sympathetic Nervous System Functions
  3. Anatomy of the Sympathetic Nervous System
  4. Physiology of the Sympathetic Nervous System
  5. Sympathetic Nervous System and Homeostasis
  6. Common Disorders and Diseases
Sympathetic Nervous System: Definition, Function, Disorders
Sympathetic Nervous System

Sympathetic Nervous System (SNS)

The sympathetic nervous system consists of one of the two main divisions in the autonomic nervous system the other being the parasympathetic nervous system. It prepares the body for responding to stress or emergencies, which one commonly calls the "fight-or-flight" response. This involuntary action is essential in maintaining homeostasis in the body.

It is important because the SNS can rapidly marshal all the resources of the body during stressful situations to ensure survival. Through its regulation of heart rate, blood pressure, energy mobilisation, and other activities, the SNS allows the body to cope with external challenges while maintaining internal balance.

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Sympathetic Nervous System Functions

Read about the functions of Sympathetic Nervous system functions:

Fight-or-Flight Response:

  • Increases blood pressure

  • Dilates airways

  • Glucose released from energy stores

Effects on Other Organs and Systems:

  • Cardiovascular System: Increases heart rate and blood pressure

  • Respiratory System: Bronchodilators

  • Digestive System: Slows digestion

  • Metabolic Processes: Glucose metabolism

Commonly Asked Questions

Q: How does the sympathetic nervous system affect the skin?
A:
The sympathetic nervous system causes piloerection (goosebumps) by contracting arrector pili muscles attached to hair follicles. It also increases sweat production and can cause pallor (paleness) due to vasoconstriction of blood vessels in the skin.
Q: How does the sympathetic nervous system affect the bladder?
A:
The sympathetic nervous system relaxes the detrusor muscle of the bladder wall and contracts the internal urethral sphincter. This inhibits urination, allowing the body to retain urine during stressful situations when emptying the bladder might be inconvenient or dangerous.
Q: How does the sympathetic nervous system affect the liver?
A:
The sympathetic nervous system stimulates the liver to release glucose into the bloodstream through glycogenolysis (breakdown of glycogen) and gluconeogenesis (production of new glucose). This provides readily available energy for the body during stressful situations.
Q: What is the role of the sympathetic nervous system in thermoregulation?
A:
The sympathetic nervous system plays a crucial role in thermoregulation by controlling sweat glands, blood vessel dilation/constriction, and piloerection. These mechanisms help the body maintain its core temperature in response to environmental changes or internal heat production.
Q: What is sympathetic tone, and why is it important?
A:
Sympathetic tone refers to the baseline level of activity in the sympathetic nervous system. It's important because it maintains a constant, low-level stimulation of various organs and tissues, allowing for rapid adjustments in response to changing physiological needs or environmental conditions.

Anatomy of the Sympathetic Nervous System

The SNS anatomy consists of neurons and ganglia that run from the spinal cord to various organs and systems of the body.

Neurons And Neurotransmitters Involved:

  • The SNS is a system composed of preganglionic neurons originating in the spinal cord and postganglionic neurons extending to target organs.

  • Principal neurotransmitters include norepinephrine and epinephrine.

Preganglionic And Postganglionic Neurons:

  • The preganglionic neurons reside within the thoracic and lumbar regions of the spinal cord.

  • The postganglionic neurons reside within the ganglia near or in target organs.

Sympathetic Chain And Collateral Ganglia:

  • The sympathetic chain refers to the row of ganglia that runs down along the vertebral column parallel to the spinal cord.

  • The visceral efferent collateral ganglia are more anterior and associated with major abdominal arteries.

Pathways Of Sympathetic Fibers:

  • There are specific pathways of sympathetic fibres from the spinal cord to their destination organs in connecting either through the ganglia of the sympathetic chain or bypassing it for the collateral ganglia.

Physiology of the Sympathetic Nervous System

The SNS physiology represents the intricate mechanisms of such quick and effective responses.

Mechanisms of Action

  • Activation starts with a stimulus, which causes the hypothalamus to send signals to the spinal cord for the activation of preganglionic neurons.

  • Neurotransmitters: Norepinephrine and Epinephrine these neurotransmitters play a very vital role in transmitting the signal across synapses and in binding with adrenergic receptors on target cells.

  • Receptor Types: Adrenergic Receptors (Alpha and Beta Receptors): Alpha receptors generally cause constriction of blood vessels, while beta receptors increase heart rate and relax airway muscles.

Effects on Different Organs and Systems

  • Cardiovascular System: Increases heart rate and raises blood pressure.

  • Respiratory System: Bronchodilation, thus it eases breathing.

  • Digestive System: Inhibits digestive processes.

  • Urinary System: Affects bladder control.

  • Metabolic Effects: Increases glucose metabolism, and mobilizes energy.

Sympathetic Nervous System and Homeostasis

It is essential in homeostasis because it controls various activities in the body.

Role in Homeostatic Regulation

  • Body Temperature: Maintains body temperature through the control of blood flow to the skin surface and activity of sweat glands.

  • Stress Response and Adaptation: Help the body adjust to stress by liberating cortisol and other stress hormones.

Interactions with the Parasympathetic Nervous System

  • SNS is responsible for the "fight or flight" response, the parasympathetic nervous system is responsible for the "rest and digest" response.

  • The autonomic nervous system has two divisions, sympathetic and parasympathetic, that balance each other's effects to maintain homeostasis.

Commonly Asked Questions

Q: What role does the sympathetic nervous system play in digestion?
A:
The sympathetic nervous system generally inhibits digestion. It decreases blood flow to the digestive organs, reduces the production of digestive enzymes, and slows down peristalsis (the movement of food through the digestive tract). This allows the body to redirect energy and resources to more critical functions during stress or emergencies.
Q: How does the sympathetic nervous system affect sweat glands?
A:
The sympathetic nervous system stimulates sweat glands to increase sweat production. This helps regulate body temperature during stress or physical exertion by promoting heat loss through evaporation.
Q: How does the sympathetic nervous system differ from the parasympathetic nervous system?
A:
The sympathetic nervous system prepares the body for action and stress, while the parasympathetic nervous system promotes rest and digestion. The sympathetic system increases heart rate and blood pressure, while the parasympathetic system decreases them. They work in opposition to maintain balance in the body.
Q: What is the role of the adrenal glands in the sympathetic nervous system?
A:
The adrenal glands, particularly the adrenal medulla, play a crucial role in the sympathetic nervous system. When stimulated by sympathetic nerves, the adrenal medulla releases epinephrine and norepinephrine directly into the bloodstream, amplifying and prolonging the sympathetic response throughout the body.
Q: How does the sympathetic nervous system affect breathing?
A:
The sympathetic nervous system increases the rate and depth of breathing. It does this by stimulating the bronchi to dilate, which allows more air to enter the lungs. This ensures that the body receives more oxygen to support increased physical activity during stressful situations.

Common Disorders and Diseases

  • Hypertension: High blood pressure, continued over time, can be influenced by overactivity of the SNS.

  • Heart Failure: The SNS may result in heart failure because of the excess load on it through stress signals.

  • Anxiety Disorders: Overactive activity of SNS is associated with anxiety and panic disorders.

  • Pheochromocytoma: A rare tumour of the adrenal gland that might lead to excess production of adrenaline.

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Frequently Asked Questions (FAQs)

Q: How does the sympathetic nervous system affect cognitive function?
A:
The sympathetic nervous system can influence cognitive function through its effects on arousal and attention. Moderate sympathetic activation can enhance alertness and cognitive performance. However, excessive activation (as in severe stress) can impair certain cognitive functions, particularly those involving complex decision-making or memory recall.
Q: How does the sympathetic nervous system affect metabolism?
A:
The sympathetic nervous system generally increases metabolic rate. It stimulates the breakdown of glycogen and fat, increases glucose production, and enhances cellular energy expenditure. This metabolic boost helps provide energy for the "fight or flight" response.
Q: What is sympathetic nervous system pruning?
A:
Sympathetic nervous system pruning refers to the natural process of refining sympathetic innervation during development. Excess sympathetic nerve connections are eliminated, leaving only the most appropriate and efficient connections. This process is crucial for the proper functioning of the mature sympathetic nervous system.
Q: How does the sympathetic nervous system affect muscle tension?
A:
The sympathetic nervous system can increase muscle tension by enhancing the excitability of motor neurons. This prepares the muscles for rapid action in response to stress or threats. Chronic sympathetic activation can contribute to muscle tension and associated pain in conditions like tension headaches or fibromyalgia.
Q: What is the role of the sympathetic nervous system in circadian rhythms?
A:
The sympathetic nervous system plays a role in maintaining circadian rhythms, particularly in regulating the sleep-wake cycle. Sympathetic activity tends to be higher during waking hours and lower during sleep, helping to coordinate various physiological processes with the body's internal clock.
Q: How does the sympathetic nervous system affect the spleen?
A:
The sympathetic nervous system causes contraction of the spleen, which releases stored red blood cells into circulation. This increases oxygen-carrying capacity in the blood, supporting the increased oxygen demands during stress or physical activity.
Q: What is sympathetic nervous system-mediated inflammation?
A:
Sympathetic nervous system-mediated inflammation refers to the influence of sympathetic activity on inflammatory processes. While acute sympathetic activation can have anti-inflammatory effects, chronic sympathetic activation may promote inflammation through the release of neurotransmitters that interact with immune cells.
Q: How does the sympathetic nervous system affect saliva production?
A:
The sympathetic nervous system generally reduces saliva production and makes saliva more viscous (thicker). This is part of the body's response to redirect resources away from digestive functions during stress or emergencies.
Q: What is the relationship between the sympathetic nervous system and stress?
A:
The sympathetic nervous system is a key mediator of the body's stress response. When the brain perceives stress, it activates the sympathetic system, triggering the "fight or flight" response. Chronic stress can lead to persistent sympathetic activation, which may contribute to various health problems.
Q: How does the sympathetic nervous system affect the lungs?
A:
The sympathetic nervous system causes bronchodilation (widening of the airways) in the lungs. This allows for increased airflow, supporting the increased oxygen demands during stress or physical activity. It also increases the rate and depth of breathing.
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