Chemical synaptic transmission involves the release of specific molecules, neurotransmitters, which diffuse through the intercellular space and interact with specific receptors located on an adjacent neuron. In the electrical transmission mediated by gap junctions, the plasma membranes of adjacent neurons are separated by a gap of about two nanometers (two-billionths of a meter), but contain small channels (the gap junctions) that connect the cytoplasm of the adjoining neurons, permitting the diffusion of small molecules and the flow of electric current.