(i) Define a reflex arc. Draw a labelled diagram of a reflex arc and trace the complete sequence of events that occur when you accidentally step on a sharp object.
(ii) Explain why reflex arcs evolved in animals, and why they continue to be useful even in animals that possess highly developed brains.
Generated by claude-sonnet-4-6 · 2026-06-26 01:01 · grounding rag
Model Answer
(i) Reflex Arc — Definition, Diagram & Sequence of Events
A reflex arc is the nervous pathway through which a reflex action occurs — connecting the receptor to the effector via the spinal cord, without involving the brain.
Labelled Diagram:
```
Receptor (skin) → Sensory neuron → Spinal cord (relay neuron)
↓
Effector (muscle) ← Motor neuron ←────────
```
(Label: Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector/Muscle)
Sequence of events on stepping on a sharp object:
- Receptor (pain receptors in foot) detects the stimulus.
- Electrical impulse travels along sensory neuron to the spinal cord.
- Relay neuron in spinal cord processes the signal and passes it on.
- Impulse travels along motor neuron to the leg muscles (effector).
- Muscle contracts — foot is pulled away instantly.
- Simultaneously, information is sent to the brain, which registers pain after the action.
(ii) Why Reflex Arcs Evolved
Reflex arcs evolved because the brain's thinking process is too slow for urgent responses — by the time a conscious decision is made, serious harm could occur (e.g., getting burnt). Many simpler animals lack complex neuron networks, so reflex arcs provided efficient survival responses.
Even in animals with highly developed brains, reflex arcs continue to be useful because they allow much quicker responses than brain-directed actions, as the signal need only travel to the spinal cord and back, not all the way to the brain.
Source: Chapter 6, Section 6.1.1 — What happens in Reflex Actions?
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Explanation
- Definition must mention: pathway, receptor to effector, spinal cord involvement — examiners check all three.
- The diagram must be labelled with at least: receptor, sensory neuron, spinal cord/relay neuron, motor neuron, effector/muscle. Even a simple block-arrow diagram earns marks if correctly labelled.
- For the sequence, using numbered steps is cleaner and easier to mark. The key detail examiners look for is that the brain is informed after the action, not before.
- For part (ii), directly use the textbook reasoning: brain is slow → reflex arc bypasses it → faster response. Also note: simpler animals with no thinking tissue rely entirely on reflex arcs.
- Avoid writing an essay — tight, numbered points score better in board exams.