Biology · Neural Control and Coordination · NEET
| Common name | Fight or flight | Rest and digest |
| When active | Stress, danger, hard effort | Calm, relaxed, after a meal |
| Heart rate | Increases | Decreases |
| Pupils | Dilate (widen) | Constrict (narrow) |
| Digestion | Slows down | Speeds up |
| Overall goal | Prepare body for action | Conserve energy, calm body |
Sympathetic = fight or flight. It prepares the body for an emergency or hard effort. So it speeds up the heart, widens the pupils, and sends more blood to muscles. The parasympathetic system is the opposite: 'rest and digest', which calms the body after the emergency is over. Do not swap these two labels — that is the most common mistake in NEET questions.
Sympathetic INCREASES heart rate. NCERT (in the heartbeat chapter) states that sympathetic nerves increase the rate of heart beat, the strength of ventricular contraction, and the cardiac output. Parasympathetic nerves do the opposite — they DECREASE the heart rate and cardiac output. Both act on the same heart but pull in opposite directions.
Both are involuntary. They are the two divisions of the autonomic neural system, and NCERT says the autonomic system carries impulses from the CNS to involuntary organs and smooth muscles. You do not consciously decide your heart rate or your digestion — this is why it is 'autonomic' (self-controlling). The voluntary control of skeletal muscle is done by the somatic neural system, which is a separate branch.
The parasympathetic system boosts digestion (part of 'rest and digest'). It increases gut movement and the release of digestive juices. The sympathetic system slows digestion down during stress, because blood is redirected to muscles instead of the stomach. So during an exam or a fright, digestion is paused by the sympathetic side.
Sympathetic DILATES (widens) the pupils, so more light enters and you can see danger clearly. Parasympathetic CONSTRICTS (narrows) the pupils back to normal when you are relaxed. Use the hook: Sympathetic = Sight ready = wide pupils.
Both are part of the peripheral nervous system's autonomic division, and both carry efferent (motor) signals out from the CNS to involuntary organs. NEET keeps the mechanism simple at NCERT level: you mainly need the CLASSIFICATION (PNS to somatic + autonomic; autonomic to sympathetic + parasympathetic) and the OPPOSITE effects on organs, not detailed outflow tracts.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They are the two branches of the autonomic (involuntary) neural system with opposite jobs. Sympathetic prepares the body for action (fight or flight); parasympathetic calms the body down (rest and digest). On most organs their effects are opposite.
Yes. NCERT places both under the autonomic neural system, which is one of the two divisions of the peripheral nervous system (the other being the somatic neural system).
The sympathetic system. It raises heart rate, widens pupils, and pauses digestion so the body is ready to react. This is the 'fight or flight' response.
Yes, and usually in opposite directions. For example, on the heart, sympathetic increases the rate while parasympathetic decreases it. This balance keeps the body stable (homeostasis).
Yes, but mainly at the classification and 'opposite effect' level. Remember the branch diagram (PNS to somatic + autonomic; autonomic to sympathetic + parasympathetic) and the sympathetic-vs-parasympathetic effects on heart, pupils and digestion.
