The sympathetic nervous system prepares the body for activity or stress, whereas the parasympathetic nervous system promotes recovery, energy conservation and routine maintenance. They are two divisions of the autonomic nervous system, which regulates involuntary body functions.
The Reasoning and Mechanism
During exercise or a threatening situation, sympathetic activity produces the fight-or-flight response. Signals from the central nervous system increase cardiac output and redirect resources toward tissues that support movement, especially skeletal muscles.
After exercise or when the body is resting, parasympathetic activity produces the rest-and-digest response. It lowers physiological arousal and supports digestion, recovery and restoration of energy stores.
| Body function | Sympathetic effect | Parasympathetic effect |
|---|---|---|
| Heart | Increases heart rate and force of contraction | Decreases heart rate |
| Airways | Dilates bronchioles, increasing airflow | Constricts bronchioles toward resting diameter |
| Pupils | Dilates pupils | Constricts pupils |
| Digestion | Reduces digestive activity | Stimulates digestive activity |
| Energy availability | Promotes mobilization of energy stores | Supports energy storage and recovery |
These branches often provide antagonistic control: they act on the same organ but produce opposite effects. For example, sympathetic stimulation increases heart rate during exercise, while parasympathetic stimulation helps return heart rate toward its resting level during recovery.
Most sympathetic postganglionic neurons release noradrenaline, while parasympathetic postganglionic neurons release . Both systems interact with the cardiovascular, respiratory and digestive systems, making them an important example of in SEHS A.1.1.