IGCSE Biology glossary
Synapse
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Definition
A tiny gap between two neurones across which nerve impulses are transmitted by chemical transmitter substances (neurotransmitters). The neurotransmitter is released from the presynaptic neurone, diffuses across the gap, and binds to receptors on the postsynaptic neurone, triggering a new impulse.
What to include in an exam answer
The signal crosses the synapse as a CHEMICAL (neurotransmitter), not as an electrical impulse. Synapses ensure impulses travel in one direction only because neurotransmitter is only released from the presynaptic side.
Example in context
Explain how a nerve impulse crosses a synapse (3 marks): The impulse arrives at the presynaptic neurone and causes neurotransmitter molecules to be released into the synaptic gap. The neurotransmitter diffuses across the gap and binds to receptors on the postsynaptic neurone. This triggers a new electrical impulse in the postsynaptic neurone.
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