The gap or space between two neurons is referred to as the synaptic cleft. As an action potential travels down a presynaptic axon, it allows sodium ions to rush into the cell leading to depolarization. This wave of depolarization moves in a one-way direction away from the cell body towards the synapse. When the action potential reaches the presynaptic terminal, it allows an influx of calcium into the preganglionic cell which in turn leads to the neurotransmitter vesicles fusing with the presynaptic membrane so that the neurotransmitter is released into the synaptic cleft. The transmitter then migrates across the cleft and binds to a postsynaptic receptor.