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Synapses

Coordination and response · Nervous control · note 3 of 3

Spec 14.1.7, 14.1.8, 14.1.9, 14.1.10Download all Nervous control notes (PDF)Download all notes (PDF)7 pages

SynapsesSpec 14.1.7, 14.1.8, 14.1.9, 14.1.10

In short

A synapse is a junction between two neurones. The electrical impulse cannot cross the synaptic gap, so an impulse arriving at the first neurone releases neurotransmitter molecules from vesicles. These diffuse across the gap and bind with receptor proteins on the next neurone, stimulating a new impulse. Synapses ensure that impulses travel in one direction only.

Synapse
A junction between two neurones.

There is a tiny gap between one neurone and the next. The electrical impulse cannot cross this gap, so a chemical is used instead.

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Structure of a synapse

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
  • Vesicles in the first neurone contain neurotransmitter molecules.
  • The synaptic gap separates the two neurones.
  • The membrane of the next neurone has receptor proteins.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Events at a synapse

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
  1. An impulse arriving at the end of the first neurone stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap.
  2. The neurotransmitter molecules diffuse across the gap.
  3. The neurotransmitter molecules bind with receptor proteins on the next neurone.
  4. An impulse is then stimulated in the next neurone.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
Diagram of a synapse: an impulse arrives at the end of the first neurone, vesicles containing neurotransmitter molecules release them into the synaptic gap, the molecules diffuse across and bind to receptor proteins on the next neurone, and an impulse is stimulated in the next neurone; an arrow shows the impulse travels in one direction only. (opens full size in a new tab)
At a synapse, neurotransmitter molecules diffuse across the synaptic gap and bind to receptor proteins on the next neurone.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Synapses ensure that impulses travel in one direction only. Neurotransmitter is released from only one side of the gap, and the receptor proteins are only on the next neurone.

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
Common mistake:

The impulse itself does not cross the gap. The neurotransmitter diffuses across and starts a new impulse in the next neurone.

Quick check

  1. What does the central nervous system consist of?

    Show answer

    The brain and the spinal cord.

  2. Which neurone is found entirely within the CNS?

    Show answer

    The relay neurone.

  3. Name the five parts of a simple reflex arc in order.

    Show answer

    Receptor, sensory neurone, relay neurone, motor neurone, effector.

  4. What is a synapse?

    Show answer

    A junction between two neurones.

  5. Supplement What diffuses across the synaptic gap?

    Show answer

    Neurotransmitter molecules.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is a reflex arc?

A reflex arc is the pathway that impulses follow in a reflex action. It has five parts in order: a receptor detects the stimulus, a sensory neurone carries impulses to the CNS, a relay neurone passes them on within the CNS, and a motor neurone carries them to an effector, a muscle or gland, which responds.

Why are reflexes faster than normal responses?

Reflexes are fast because they are automatic, so you do not have to think about the response first. Impulses pass straight from the receptor through the sensory, relay and motor neurones to the effector. This rapid, automatic response protects the body from harm, such as pulling your hand away from something hot.

How does a nerve impulse cross the synapse?

Supplement A nerve impulse crosses a synapse by chemical means, because the electrical impulse cannot cross the gap. The impulse makes vesicles release neurotransmitter molecules into the synaptic gap. These diffuse across the gap and bind with receptor proteins on the next neurone, which stimulates a new impulse in that neurone.

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