A reflex is a fast, automatic response to a stimulus that does not involve conscious thought, and it protects the body from harm. It follows a reflex arc: receptor, sensory neurone, relay neurone, motor neurone and effector. It is fast because the impulses do not have to travel to the conscious part of the brain first.
A reflex is a fast, automatic response to a stimulus that does not involve conscious thought. The pathway it follows is called a reflex arc. Reflexes protect the body from harm.
The stimulus is detected by a receptor (for example, a pain receptor in the skin).
A sensory neurone carries impulses to the CNS.
At a synapse the impulses pass to a relay neurone in the CNS.
At another synapse the impulses pass to a motor neurone.
The motor neurone carries the impulses to an effector, such as a muscle.
The effector produces a response, such as the muscle contracting to pull the hand away.
The impulses do not have to travel to the conscious part of the brain before the response, so the reflex is very fast.
Tap to enlarge (opens full size in a new tab)A reflex arc: receptor, sensory neurone, relay neurone, motor neurone, effector. Impulses cross a synapse between each pair of neurones.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026
Frequently asked questions
What is a reflex arc?
A reflex arc is the pathway followed in a reflex, a fast, automatic response that does not involve conscious thought. A receptor detects a stimulus, a sensory neurone carries impulses to the CNS, a relay neurone passes them to a motor neurone, and the motor neurone carries them to an effector, which responds.
Why are reflexes faster than normal responses?
Reflexes are faster because the impulses do not have to travel to the conscious part of the brain before the response. The pathway goes from the sensory neurone through a relay neurone in the CNS straight to the motor neurone, so the effector responds automatically. This speed protects the body from harm.
How does a nerve impulse cross the synapse?
A nerve impulse cannot cross the synapse as an electrical impulse. Instead, when it reaches the end of the first neurone, a chemical called a neurotransmitter is released into the gap. The neurotransmitter diffuses across and binds to receptors on the next neurone, which triggers a new electrical impulse in that neurone.
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