Cambridge O Level Biology · Syllabus 5090 · Coordination and Control
Neurotransmitter
What is Neurotransmitter?
A chemical stored in vesicles at the end of a neurone and released into the synaptic gap when an impulse arrives. It diffuses across the gap and binds to complementary receptor proteins on the next neurone, where it stimulates a new electrical impulse. It acts only across the width of one synapse, unlike a hormone, which is carried around the body in the blood.
This definition is part of the Coordination and Control chapter in Cambridge O Level Biology.
Common mistakes with Neurotransmitter
- Three things a hormone is not. It is not an enzyme — it is not a catalyst and it is not used to speed up a specific reaction. It is not an electrical impulse — it is a chemical, and it travels in blood, not along a neurone. And it is not a neurotransmitter — that acts across a single synapse, not around the whole body.
Questions students ask about Neurotransmitter
What happens at a synapse?
An electrical impulse arrives at the end of the first neurone. Vesicles release a neurotransmitter into the synaptic gap. The neurotransmitter diffuses across the gap and binds to complementary receptor proteins on the next neurone, where it stimulates a new electrical impulse. No electrical impulse crosses the gap itself.
Why can an impulse cross a synapse in only one direction?
Because the synapse is structurally asymmetrical. Vesicles containing neurotransmitter are present only in the neurone on one side of the gap, and complementary receptor proteins are present only on the membrane of the neurone on the other side. Neurotransmitter can therefore be released from one side only and detected on the other side only.