What is a nerve synapse and how does it work?

What is a nerve synapse?

A synapse refers to the junction between two neurons. When an impulse (action potential) reaches the end of a neuron (the pre-synaptic neuron), the synapse allows this signal to be transmitted to the next neuron (the post-synaptic neuron), and then on to the target (effector) of the impulse, for example a muscle. An action potential cannot jump between neurons, and so synapses have to make use of molecules called neurotransmitters in order to transmit the impulse instead.

How does it work?

A synapse consists of a pre-synaptic knob (this is just the end of the pre-synaptic neuron, ie. the one carrying the action potential), the post-synaptic neuron (the one the action potential is to be transmitted to), and the synaptic cleft (a narrow space between the two). When an action potential arrives at the pre-synaptic knob, it stimulates voltage-dependant calcium channels in the cell membrane here to open, causing calcium to rush in. This causes vesicles which contain neurotransmitters to move towards the cell membrane and fuse with it, releasing neurotransmitters into the synaptic cleft by exocytosis. Receptors on the post-synaptic neuron respond to these neurotransmitters and generate a new action potential by opening sodium channels, and so the signal continues. Finally, in order to prevent over-activation of the post-synaptic neuron, the neurotransmitters floating in the synaptic cleft are reabsorbed back into the pre-synaptic neuron, where they can be used again.