A synapse is a gap between two neurones that an electrical impulse cannot cross. When an impulse arrives at the end of the first neurone, it releases a neurotransmitter into the synapse. The neurotransmitter diffuses across the gap and binds to receptors on the membrane of the next neurone, triggering a new electrical impulse there.
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 IGCSE Science Double Award (4SD0) specification · Updated October 2026
Frequently asked questions
What is a reflex arc?
A reflex arc is the pathway of a reflex, a fast, automatic response that does not involve conscious thought. A receptor detects the stimulus, a sensory neurone carries impulses to the spinal cord, a relay neurone passes them to a motor neurone, and the motor neurone carries them to an effector, such as a muscle that contracts.
Why are reflexes faster than normal responses?
Reflexes are faster because the impulses do not have to reach the conscious part of the brain before the response happens. The pathway is short: sensory neurone, relay neurone in the spinal cord, then motor neurone to the effector. Because no conscious thought is involved, the response is automatic, which helps protect the body from harm.
How does a nerve impulse cross the synapse?
A nerve impulse crosses a synapse using a chemical, because the electrical impulse cannot cross the gap. The first neurone releases a neurotransmitter into the synapse. The neurotransmitter diffuses across the gap and binds to receptors on the membrane of the next neurone, which triggers a new electrical impulse in that neurone.
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