A Synaptic cleft is the gap between the presynaptic membrane and the postsynaptic membrane of neurones, it is used to transfer a signal triggered by Action Potentials through the body to initiate a response from a stimulus. This gap is approximately 50 nm in size.  The process begins when an Action Potential is initiated that exceeds threshold potential and depolarises the presynaptic membrane causing the Voltage-gated Calcium channels to open. This in turn causes an influx of calcium ions and causes the Synaptic Vesicles to fuse with the Presynaptic membrane by Exocytosis allowing the neurotransmitter within the vesicles diffuses across the Synaptic Cleft and bind to the receptors on the post-synaptic membrane. This causes the voltage gated ion channels to open, which depolarise the neurone. Drugs and hormones also act on receptors/ vesicles e.c.t in the cleft, which allows regulation of the signal. If there was no synaptic cleft, there would be no transport medium for the transfer of neurotransmitters from one neurone to the next.