Sympathetic Nervous System and Parasympathetic Nervous System: Functions, Uses & Examples
The nervous system controls everything in the human body — from heartbeat to digestion, breathing, movement, emotions, memory, and thinking. Among its major divisions, the Autonomic Nervous System (ANS) plays a vital role in involuntary bodily actions such as heart rate, digestion, gland secretions, blood pressure, and metabolism.
The ANS further divides into two major systems:
Sympathetic Nervous System (SNS) – Fight and Flight
Parasympathetic Nervous System (PNS) – Rest and Digest
These two systems work together to maintain homeostasis, meaning internal body balance during different conditions.
Understanding the Autonomic Nervous System (ANS)
The Autonomic Nervous System functions unconsciously and is responsible for:
• Heartbeat regulation
• Breathing control
• Digestive processes
• Smooth muscle movement
• Hormone secretion
• Temperature regulation
• Pupil size control
Main Divisions of ANS
System Nickname Role Primary Response
Sympathetic Nervous System Fight or Flight Prepares body for danger or stress Increases alertness & energy
Parasympathetic Nervous System Rest and Digest Conserves energy Promotes healing & relaxation
Both systems constantly balance each other for survival and wellness.
Sympathetic Nervous System (SNS)
• Definition
The Sympathetic Nervous System activates during stress, fear, excitement, exercise, or emergencies — preparing the body to fight or flee.
• Origin
• Thoracolumbar region of spinal cord
• T1 to L2 segments
Main Neurotransmitters
Neurotransmitter Function
Norepinephrine (Noradrenaline) Increases heart rate, alertness
Epinephrine (Adrenaline) Released by adrenal gland — boosts energy
Acetylcholine (ACh) Pre-ganglionic transmission
Functions of Sympathetic Nervous System (SNS)
Function Effect on Body
Heart Rate ⬆ Increases
Blood Pressure ⬆ Increases
Pupils Dilate (for better vision)
Digestion ⬇ Slows down
Breathing ⬆ Rapid & deep
Glucose Level ⬆ Increases (more energy)
Sweat Glands ⬆ Sweating
Bladder Relaxes (delay urination)
SNS prioritizes survival by redirecting blood flow to important organs like heart, lungs, and muscles.
Why SNS is Important?
• Improves physical strength during danger
• Enhances speed, attention, and alertness
• Prepares body for injuries (blood clotting increases)
• Real-Life Examples of SNS Activation
Situation Reaction
Seeing a dangerous animal Heart races, muscles tighten
Public speaking anxiety Sweaty palms, dry mouth
Running or Gym workout Fast heartbeat, increased breathing
Hearing sudden loud noise Startle response, dilated pupils
Parasympathetic Nervous System (PNS)
• Definition
The Parasympathetic Nervous System maintains resting state — recovery, digestion, and conservation of energy.
Nickname: Rest and Digest
• Origin
Craniosacral outflow
• Cranial nerves: III, VII, IX, X (Vagus nerve – 75% role)
• Sacral nerves: S2–S4
Main Neurotransmitter
Neurotransmitter Function
Acetylcholine Stimulates relaxation and digestion
Functions of Parasympathetic Nervous System
Function Effect on Body
Heart Rate ⬇ Slows down
Breathing ⬇ Slower and calm
Digestion ⬆ Stimulates (saliva, bile, enzymes)
Pupils Constrict
Urination & Defecation Stimulated
Energy Storage ⬆ Increases glycogen storage
Immune System ⬆ Boosted
PNS promotes healing, growth, and rejuvenation.
Real-Life Examples of PNS Activation
Situation Reaction
Eating meal Increased saliva & digestion
Sleeping Lower heart rate, relaxed muscles
Meditation or Yoga Calm breathing, reduced stress
After stress reduction Warm skin, normal blood pressure
Sympathetic vs Parasympathetic Nervous System — Key Differences
Feature Sympathetic NS (SNS) Parasympathetic NS (PNS)
Main Function Fight or Flight Rest and Digest
Heart Rate Increases Decreases
Digestion Inhibits Activates
Pupils Dilates Constricts
Breathing Rapid Slow
Blood Pressure Increases Decreases
Neurotransmitter Norepinephrine Acetylcholine
Energy Uses stored energy Stores/Energizes
Body State Stress, danger Safety, calm
Balance = Health
Too much SNS → stress, anxiety, hypertension
Too much PNS → low alertness, depression
The body must constantly balance both.
Dual Innervation
Both SNS and PNS supply the same organs but have opposite effects.
Organ Sympathetic Parasympathetic
Heart Faster Slower
Lungs Bronchodilation (more air) Bronchoconstriction
GI Tract Slows movement Speeds movement
Urinary Bladder Prevent urination Stimulate urination
This push-pull system keeps organs functioning normally.
Hormonal Role: Adrenal Glands & Vagus Nerve
Adrenal Glands (SNS)
• Located above kidneys
• Release adrenaline during stress
• Increases:
• Glucose
• Strength
• Alertness
Vagus Nerve (PNS)
Longest autonomic nerve
• Promotes:
• Digestion
• Relaxation
• Hormone balance
Supporting vagus nerve improves mental and physical health.
How Lifestyle Affects SNS/PNS Balance
Activity System Stimulated
Exercise Sympathetic
Anger/Stress Sympathetic
Deep breathing Parasympathetic
Meditation Parasympathetic
Cold exposure Sympathetic
Outdoor nature time Parasympathetic
To stay healthy → balance stress with relaxation.
Disorders Associated with ANS Imbalance
Condition Cause Effects
Chronic Stress Excess SNS Anxiety, high BP
IBS (Irritable Bowel Syndrome) Poor PNS activity Digestive issues
POTS SNS dysfunction Dizziness
Sleep disorders Overactive SNS Insomnia
Depression Low PNS Low mood
Strengthening PNS improves emotional and gut health.
Clinical Applications
Medical Use System Targeted
Beta-blockers → lower HR SNS inhibition
Inhalers for asthma SNS stimulation
Meditation therapy PNS activation
Digestion medicines PNS support
Healthcare relies on ANS regulation.
Summary Table
Feature SNS PNS
Role Fight/Flight Rest/Digest
Alertness High Low
Digestion Decreased Increased
Breathing Faster Slower
Energy Used Stored
Mental State Stress Relaxed
Dominates When Emergency Safe
Both are essential – survival + recovery.
FAQs
What triggers the Sympathetic Nervous System?
Fear, excitement, exercise, emergency, or stress.
What triggers the Parasympathetic Nervous System?
Eating, rest, relaxation, sleep, meditation.
Which nerve is most important in PNS?
Vagus nerve — controls heart, lungs, digestion.
What happens if the SNS is overactive?
High BP, stress, anxiety, poor digestion, headaches.
How can we activate PNS naturally?
Deep breathing
Yoga & meditation
Laughing & social bonding
Good sleep
Is sweating sympathetic or parasympathetic?
Sympathetic response.
Conclusion
The Sympathetic Nervous System and Parasympathetic Nervous System work together to protect the body and help it recover. One prepares us for stress (fight or flight), while the other relaxes and heals us (rest and digest).
The balance between these two systems is the key to healthy physical and mental functioning.
I hope that you liked this article.
Thanks!! 🙏 😊
Writer: Vandita Singh, Lucknow (GS India Nursing Group)