The big idea: A nerve impulse travels along a neuron as an electrical signal — but neurons do not actually touch.
Crossing the gap
The gap
Where one neuron meets the next there is a tiny gap called a synapse.
A chemical carries it
The signal cannot jump the gap electrically, so it is carried across by a chemical instead.
The neurotransmitter
That chemical is a neurotransmitter: it is released by the first neuron, drifts across the gap, and triggers a brand-new impulse in the second neuron.
Electrical → chemical → electrical
An impulse reaches the presynaptic neuron; vesicles release neurotransmitter (amber dots) into the synaptic cleft; it diffuses across, binds receptors on the postsynaptic neuron and triggers a new impulse.
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- Synapse
- The junction (a tiny gap) between two neurons, where a signal is passed from one to the next using a chemical.
- Synaptic cleft
- The narrow gap between the two neurons that the neurotransmitter diffuses across.
- Presynaptic neuron
- The neuron BEFORE the synapse — the one that releases the neurotransmitter (the sender).
- Postsynaptic neuron
- The neuron AFTER the synapse — the one that receives the neurotransmitter (the receiver).
- Neurotransmitter
- The chemical messenger released into the synaptic cleft; it carries the signal across the gap.
- Synaptic vesicle
- A small membrane-bound sac inside the presynaptic neuron that stores neurotransmitter ready for release.
- Receptor
- A protein on the postsynaptic membrane that the neurotransmitter binds to, passing the signal on.
Why a chemical, not just electricity: The impulse is electrical INSIDE each neuron, but the two neurons are separated by the synaptic cleft.
Electricity cannot cross that gap, so the signal is converted into a chemical (the neurotransmitter) to bridge it — then turned back into an electrical impulse on the other side.
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The big idea: A synapse is a junction between a neuron and the next cell: another neuron, or an effector such as a muscle fibre or a gland cell. At a chemical synapse a neurotransmitter carries the signal across a narrow gap, and the signal can only pass one way.
The points to remember
- Presynaptic terminal: vesicles of neurotransmitter.
- Synaptic cleft: a very narrow gap.
- Postsynaptic membrane: receptors.
- The next cell can be a neuron or an effector (muscle, gland).
- Vesicles on one side, receptors on the other: one way only.
Remember it as: Vesicles here, receptors there, so one way.
Real example: the neuromuscular junction. Where a motor neuron meets a skeletal muscle fibre there is a synapse; the fibre is the effector, and the neurotransmitter is acetylcholine.
Trap: The impulse does not jump across the gap as electricity: at a chemical synapse it is carried by a neurotransmitter.
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How it is asked: Expect a diagram of a synapse (draw an arrow for the direction of the impulse, identify the region where chemical transmission happens, distinguish the two membranes) or a written question on why transmission is one way.
Explain why a nerve impulse can pass across a synapse in one direction only.
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