An impulse travels from one neuron to the next over a small gap called a synapse. A synapse consists of:

• a presynaptic ending which contains neurotransmitters along with mitochondria and other organelles
• a postsynaptic ending which contains the receptor sites onto which neurotransmitters attach

The synaptic cleft, which is about 20nm in width, is the gap between the presynaptic and postsynaptic endings.

In order for an action potential to travel across the synaptic cleft it’s carried by chemicals called neurotransmitters. They’re stored in synaptic vesicles located in the end of the axon and released by the pre-synaptic neurone from which the impulse is being transmitted.

The post-synaptic neurone is the neurone receiving the impulse. It contains chemical-gated ion channels known as neuroreceptors to which the neurotransmitters bind to specific sites.

When an action potential reaches a synapse it causes the voltage-gated calcium channels at the end of the pre-synaptic neurone to open. Calcium ions are then able to flow into the cell.

The calcium ions make the synaptic vesicle fuse with the membrane of the cell. This causes them to release their contents, neurotransmitters, by exocytosis. The neurotransmitters then travel across the synaptic cleft by diffusion and bind to the neuroreceptors of the post-synaptic membrane.

The voltage-gated sodium channels open allowing sodium ions to flow into the cell. Depolarisation occurs and, if the threshold is reached, an action potential is initiated.

Within the synaptic cleft neurotransmitter is constantly being broken down by different enzymes. The pre-synaptic neurone can then absorb these broken down products by endocytosis. They’re recycled to produce more neurotransmitter using ATP created by the mitochondria. This process prevents the synapse being constantly activated.