Sympathetic and Parasympathetic Functions: Comparison Table & Explained
The autonomic nervous system (ANS) is a crucial part of the human nervous system that manages a wide range of involuntary body functions.
These processes include heartbeat regulation, blood circulation, digestion, respiration, urination, and more.
The ANS operates without conscious effort, ensuring the body maintains homeostasis—a stable internal environment.
The ANS is divided into two main branches—the sympathetic and parasympathetic nervous systems.
Each branch plays important and often opposing roles in responding to different situations.
The sympathetic nervous system is often associated with the "fight-or-flight" response.
It activates the body to respond quickly to stress, fear, or danger by increasing heart rate, dilating pupils, inhibiting digestion, and releasing adrenaline.
This branch is located near the thoracic and lumbar regions of the spinal cord.
In contrast, the parasympathetic nervous system is known for promoting "rest-and-digest" or "feed-and-breed" activities.
It slows the heart rate, stimulates digestion, enhances saliva secretion, and supports recovery processes.
These nerves arise from the brainstem and sacral areas of the spinal cord.
Both branches work together to maintain balance; while one prepares the body for action, the other restores calmness and encourages restful activities.
| Aspect | Sympathetic Nervous System | Parasympathetic Nervous System |
|---|---|---|
| Primary Function | Fight-or-flight (stress or danger) | Rest-and-digest (relaxation, recovery) |
| Origin | Thoracic and lumbar spinal cord | Brainstem and sacral spinal cord |
| Main Neurotransmitter | Norepinephrine (adrenaline/noradrenaline released) | Acetylcholine |
| Heart Rate | Increases | Decreases |
| Digestive Activity | Inhibits | Enhances |
| Pupil Response | Dilates pupils | Constricts pupils |
| Saliva Production | Decreases | Increases |
| Energy Utilisation | Mobilizes glucose for quick energy | Conserves and restores energy |
How the Sympathetic and Parasympathetic Systems Work:
-
Sympathetic Activation: When you face a stressful event (like public speaking), the sympathetic system triggers.
- Heart rate increases
- Breathing quickens
- Pupils dilate for better vision
- Digestion slows, redirecting energy to muscles
-
Parasympathetic Recovery: Once the stress passes, the parasympathetic system takes over.
- Heartbeat slows
- Muscle tension decreases
- Digestion resumes and saliva increases
- Body returns to a calm state
Everyday Examples:
- After a meal, your parasympathetic system stimulates digestion and energy storage.
- When startled or afraid, your sympathetic system speeds up your heart and breathing.
- During sleep or relaxation, the parasympathetic system supports recovery and healing.
- Before a competition, the sympathetic system increases alertness and mobilizes energy.
The opposing actions of these two systems help regulate critical functions automatically, adaptively, and rapidly.
This balance maintains internal stability, helping the body quickly adjust to changing environments.
Their coordination is essential for survival and health—too much dominance of either system can disrupt normal body functioning.
Practice Questions
- Which branch of the ANS is active during relaxation after eating?
- What happens to your pupils when the sympathetic nervous system is activated?
- Explain why your heart rate increases before an exam or performance.
- How does the parasympathetic nervous system promote energy storage?
Explore Related Concepts:
- Nervous System
- Autonomic Nervous System
- Sympathetic Nervous System
- Central Nervous System
- Neurons and Nerves
- Neurotransmitters
Understanding the difference between the sympathetic and parasympathetic nervous systems helps explain how our bodies react to stress, danger, rest, and recovery. These systems work together for overall balance and proper functioning, supporting everything from rapid response in emergencies to digestive comfort after meals. Mastering this concept is essential for exploring further topics in human physiology and biology.
FAQs on Difference Between Sympathetic and Parasympathetic Nervous System
1. What is the main difference between the sympathetic and parasympathetic nervous systems?
The main difference is in their function: The sympathetic nervous system prepares the body for 'fight or flight' in stressful situations by increasing alertness, heart rate, and energy availability. The parasympathetic nervous system promotes 'rest and digest,' restoring the body to a calm state by conserving energy, slowing the heart rate, and stimulating digestion.
2. What are the key neurotransmitters for the sympathetic and parasympathetic systems?
Sympathetic nervous system: Norepinephrine (noradrenaline) is mainly released by postganglionic neurons.
Parasympathetic nervous system: Acetylcholine is released by both preganglionic and postganglionic neurons.
These neurotransmitters act on organs to create rapid, opposite physiological effects.
3. From which parts of the central nervous system do the sympathetic and parasympathetic nerves arise?
Sympathetic nerves arise from the thoracic and lumbar regions of the spinal cord (T1-L2), while parasympathetic nerves originate in the brainstem (cranial nerves III, VII, IX, X) and the sacral spinal cord (S2-S4). This distinction is called thoracolumbar (sympathetic) and craniosacral (parasympathetic) outflow.
4. How does the location of ganglia differ between the two systems and why is this important?
Sympathetic ganglia are located close to the spinal cord in a paravertebral chain, enabling rapid, widespread responses. Parasympathetic ganglia are near or within target organs, providing targeted, slower actions. This arrangement supports their specific functions during stress or rest.
5. How do the sympathetic and parasympathetic systems affect the heart?
The sympathetic system increases heart rate and force of contraction to prepare the body for action. The parasympathetic system decreases heart rate to conserve energy and promote relaxation.
6. Can you give real-life examples when the sympathetic nervous system is active?
Examples include:
- Increased heart rate and sweating before exams or public speaking
- Pupil dilation in a dark or frightening environment
- Rapid heartbeat and energy boost during emergencies
- Faster breathing during intense physical activity or stress
7. How does the parasympathetic nervous system help after a meal?
The parasympathetic system:
- Stimulates digestive enzyme secretion
- Increases peristalsis for food movement
- Reduces heart rate
- Promotes nutrient absorption and energy storage.
This helps the body in 'rest and digest' mode.
8. Is the autonomic nervous system the same as the somatic nervous system?
No, they are different:
- Autonomic nervous system (sympathetic and parasympathetic) controls involuntary functions like heartbeat, digestion, and breathing.
- Somatic nervous system controls voluntary movements like walking and writing using skeletal muscles.
9. Can both sympathetic and parasympathetic systems be active at the same time?
Yes, both systems can be active simultaneously.
This dynamic balance is called autonomic tone. For example, the heart maintains a resting rate through constant input from both systems, with one becoming more dominant as needed.
10. What are some mnemonic aids to remember sympathetic vs parasympathetic functions?
Common mnemonics include:
- Sympathetic – "S for Stress" or "Fight or Flight"
- Parasympathetic – "P for Peace" or "Rest and Digest"
These help recall their main actions in exams.
11. How do the two systems affect the pupils of the eyes?
Sympathetic stimulation dilates (enlarges) pupils for better vision in emergencies. Parasympathetic stimulation constricts (shrinks) pupils to reduce light entry in restful states.
12. Which system predominates during sleep and relaxation?
The parasympathetic nervous system predominates during sleep and relaxation. It slows the heart, enhances digestion, conserves energy, and restores body functions while you rest.
