Everything on the GCSE Biology The Reflex Arc poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
What Is a Reflex?
A reflex is a rapid, automatic and involuntary response to a stimulus. Reflexes are protective – they happen before you have time to think. The pathway bypasses the conscious parts of the brain; the signal is processed in the spinal cord, which makes the response very fast.
The Reflex Arc
A reflex arc is the nerve pathway from stimulus to response. The example below is pulling your hand away from a hot object.
1. Stimulus
Heat from the hot object.
2. Receptor
A temperature receptor in the skin detects the heat.
3. Sensory neurone
Carries an electrical impulse from the receptor towards the central nervous system (CNS).
4. Relay neurone
- Found in the spinal cord.
- Passes the impulse directly to a motor neurone – no decision is made by the conscious brain.
5. Motor neurone
Carries the impulse from the spinal cord to the effector.
6. Effector
A muscle (or gland) that carries out the response – for example, the biceps muscle in the arm.
7. Response
The muscle contracts and the hand pulls away from the hot object.
Synapses
Each junction between neurones is a synapse. The electrical impulse cannot jump the gap, so the first neurone releases a neurotransmitter into the synaptic gap. The chemical diffuses across and triggers an impulse in the next neurone.