Autonomic nervous system: sympathetic and parasympathetic
The autonomic nervous system controls organs that work on their own, like the heart and gut. Sympathetic nerves prepare for action; parasympathetic nerves calm the body.
What the ANS does
Some organs work without your control. Your heart beats, food moves along the gut and the pupil changes size on its own. The autonomic nervous system (ANS) controls these involuntary actions. The nervous system is often divided into three parts: the central, peripheral and autonomic nervous systems. The ANS has two divisions: sympathetic and parasympathetic. Most organs get nerves from both. The two have opposite effects, so together they keep the body balanced.
Effects on the body
| Organ | Sympathetic (fight or flight) | Parasympathetic (rest and digest) |
|---|---|---|
| Heartbeat | Speeds up | Slows down |
| Pupil of the eye | Dilates (widens) | Constricts (narrows) |
| Breathing | Faster; air tubes widen | Slower; air tubes narrow |
| Saliva | Less saliva, so the mouth goes dry | More saliva |
| Digestion | Slows down | Speeds up |
| Urinary bladder | Relaxes (holds urine) | Contracts (passes urine) |
| Blood pressure | Rises | Falls |
Sympathetic vs parasympathetic: where they start and when they act
Sympathetic is for Stress; Parasympathetic is for Peace.
Everyday examples
- A street dog chases you: your heart races and your mouth goes dry (sympathetic).
- Just before an exam, your palms sweat and your heart beats fast (sympathetic).
- In a scary film scene, your pupils widen (sympathetic).
- After a calm lunch of sambar rice, your heart slows and the food digests (parasympathetic).
Eat slowly and calmly: the parasympathetic system increases saliva and digestion when you are relaxed. A few slow, deep breaths before an exam help calm the sympathetic 'fight or flight' rush.
1 marks: “Which division of the ANS dilates the pupil?” Sympathetic nervous system.
2 marks: “Name the two divisions of the autonomic nervous system.” Sympathetic and parasympathetic nervous systems.
4 marks: “Differentiate between the sympathetic and parasympathetic nervous systems.” Origin: thoracic and lumbar vs brain and sacral; fight or flight vs rest and digest; heart speeds up vs slows; pupil dilates vs constricts; digestion and saliva decrease vs increase.
Sympathetic = Stress (fight or flight); Parasympathetic = Peace (rest and digest).