A synapse is a gap between two neurones. The electrical impulse cannot cross the gap, so the first neurone releases a chemical called a neurotransmitter. The neurotransmitter diffuses across the synapse and binds to receptors on the next neurone, which triggers a new electrical impulse. The impulse itself never crosses the gap.
A synapse is a gap between two neurones. The electrical impulse cannot cross the gap, so a chemical is used instead.
An electrical impulse arrives at the end of the first neurone.
The neurone releases a chemical called a neurotransmitter into the synapse.
The neurotransmitter diffuses across the gap.
It binds to receptors on the membrane of the next neurone.
This triggers a new electrical impulse in the next neurone.
The impulse itself does not cross the synapse. A neurotransmitter diffuses across and starts a new impulse in the next neurone.
Written and checked against the Edexcel GCSE Biology (1BI0) 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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