Synapse Coloured transmission electron micrograph TEM of a synapse a junction between two nerve cells in the brain At a synapse an electrical signal is transmitted from one cell to the next in only one direction The nerve cells are green with the presynaptic cell at left and the postsynaptic cell at right Mitochondria supplying the cells with energy are blue When an electrical signal reaches a synapse it triggers the release of neurotransmitter chemicals from vesicles red at the end of the presynaptic cell The neurotransmitters cross a microscopic gap and bind to receptors on the post synaptic cell Magnification x when printed centimetres across