Synapse Structure & Transmission IGCSE Biology 0610

Synapses: How Nerve Impulses Cross the Gap

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

A synapse is the junction between two neurones. When an impulse reaches the end of the pre-synaptic neurone, vesicles release a chemical transmitter substance into the synaptic cleft. The transmitter diffuses across and binds to receptors on the post-synaptic neurone, triggering a new impulse. Synapses ensure one-way transmission.

Synapse transmission is a Supplement topic. The sequence is: impulse arrives → vesicles fuse with membrane → transmitter released → diffuses across gap → binds to receptors → new impulse generated. Each step is a mark point.

Structure of a synapse

A synapse consists of three parts: the pre-synaptic neurone (the neurone sending the signal), the synaptic cleft (the small gap between the two neurones), and the post-synaptic neurone (the neurone receiving the signal).

The end of the pre-synaptic neurone contains many vesicles filled with chemical transmitter substance (also called neurotransmitter). The post-synaptic membrane has specific receptor molecules that the transmitter binds to. The transmitter is only in vesicles on one side, which is why the synapse enforces one-directional transmission.

How transmission occurs

1. A nerve impulse arrives at the end of the pre-synaptic neurone.

2. The impulse causes vesicles containing transmitter substance to move toward and fuse with the pre-synaptic membrane.

3. Transmitter substance is released into the synaptic cleft by exocytosis.

4. The transmitter substance diffuses across the narrow synaptic cleft (this takes a very short time because the gap is tiny).

5. The transmitter binds to receptor molecules on the post-synaptic membrane.

6. This triggers a new nerve impulse in the post-synaptic neurone.

7. The transmitter is then broken down by enzymes or reabsorbed, resetting the synapse for the next impulse.

Why synapses matter

Synapses serve several important functions beyond one-way transmission:

One-way transmission: impulses can only cross from pre-synaptic to post-synaptic, ensuring signals follow the correct pathway
Integration: multiple pre-synaptic neurones can converge on one post-synaptic neurone, allowing the nervous system to combine signals
Amplification: one pre-synaptic neurone can connect to many post-synaptic neurones, spreading the signal

Some drugs and toxins affect synaptic transmission. For example, they may mimic the transmitter substance, block its receptors, or prevent its breakdown: which is why they can speed up, slow down or completely disrupt nervous system function.

Try a focused question (4 marks)

Describe how a nerve impulse is transmitted across a synapse.

Show the answer and marking guidance

The impulse arrives at the pre-synaptic neurone and causes vesicles to release transmitter substance into the synaptic cleft.
The transmitter substance diffuses across the synaptic cleft.
It binds to receptor molecules on the post-synaptic membrane.
This triggers a new nerve impulse in the post-synaptic neurone.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying the impulse jumps across the gap: the impulse does not cross; a chemical transmitter does.
Forgetting that the transmitter diffuses across the cleft: it is not an electrical process.
Describing transmission as bidirectional: synapses enforce one-way transmission.
Not mentioning that the transmitter is broken down or reabsorbed after use.