A REFLEX is a rapid, automatic response to a stimulus that does not require conscious thought.
Reflexes are protective — they allow the body to respond to potentially harmful stimuli BEFORE the conscious brain can process the situation.
Key features of a reflex:
AUTOMATIC — happens without thinking.
RAPID — much faster than a voluntary response.
INVOLUNTARY — you cannot choose not to do it.
STEREOTYPED — always the same response to the same stimulus.
Examples:
Withdrawing your hand from a hot surface.
Knee-jerk reflex (tapping the patellar tendon).
Pupil constricting in bright light (pupil reflex).
Sneezing and coughing.
Blinking when an object moves towards the eye.
Baby's grasp reflex.
The Reflex Arc
A reflex arc is the specific nerve pathway along which a reflex signal travels. Crucially, it passes through the SPINAL CORD rather than up to the conscious brain — this is what makes it so fast.
The pathway of a reflex arc:
1. STIMULUS — a change detected by a sense organ (e.g. heat on the skin).
2. RECEPTOR — specialised cells detect the stimulus and generate an electrical impulse.
3. SENSORY NEURONE — carries the impulse to the spinal cord (part of the CNS).
4. RELAY NEURONE — in the spinal cord, receives the impulse and passes it on. Also sends a signal UP to the brain — so the brain becomes aware AFTER the reflex has already occurred.
5. MOTOR NEURONE — carries impulse from spinal cord to the effector.
6. EFFECTOR — the muscle contracts (or gland secretes) to produce the response.
7. RESPONSE — e.g. the hand moves away from the heat source.
The brain is aware of the reflex — but AFTER the response has already happened.
Why Reflexes are Faster than Voluntary Responses
VOLUNTARY response pathway:
Receptor → sensory neurone → spinal cord → UP to brain → conscious processing → DOWN from brain → motor neurone → effector.
The reflex bypasses the conscious brain (cerebral cortex) — this eliminates the time needed for:
Signal travelling up to the brain.
Conscious processing and decision-making.
Signal travelling back down from the brain.
Typical voluntary reaction time: 0.2–0.3 seconds.
Typical reflex reaction time: 0.04–0.1 seconds.
This 3–5× speed advantage can prevent serious injury — for example, moving your hand away from a sharp object or flame before the pain signal has even reached consciousness.
⚠️ Common Mistake
A reflex bypasses the CONSCIOUS BRAIN (cerebral cortex) — it is processed in the SPINAL CORD. The brain does receive the signal (that's why you become aware of it), but it does NOT initiate the reflex. Students often say 'the brain controls reflexes' — the brain is only aware of them afterwards.
📌 Key Note
Reflex arc: stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response. Faster than voluntary because it bypasses the conscious brain. Involuntary and automatic.
🎯 Matching Activity — Match Each Step of the Reflex Arc
Match each component to its role in the reflex arc. — drag the symbols on the right to match the component names on the left.
Stimulus
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Receptor
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Sensory neurone
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Relay neurone
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Motor neurone
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Effector
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Detects the stimulus and generates an electrical impulse
Carries the impulse from the receptor to the spinal cord
Carries impulse from spinal cord to the effector
The change that triggers the reflex — e.g. touching a sharp object
In the spinal cord — connects sensory to motor neurone
Muscle contracts or gland secretes — produces the response
🎯 Test Yourself
Question 1 of 3
1. Why are reflexes faster than voluntary responses?
2. You touch a hot surface and pull your hand away before you feel pain. What does this demonstrate?
3. In which part of the CNS is a spinal reflex arc processed?
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