Synapse nerve junctions. Coloured scanning electron micrograph (SEM) of nerve cells showing the synapses (junctions, bulges) between them. At a synapse an electrical signal is transmitted from one cell to the next in only one direction. When an electrical signal reaches a synapse it triggers the release of neurotransmitter chemicals from vesicles (yellow) in the presynaptic cell. The neurotransmitters cross a microscopic gap called the synaptic cleft and bind to receptors on the postsynaptic cell, causing it to propagate an electrical impulse.