Most voltage drops caused by the opening of a handful of ion channels in a synapse will have little or no effect. But if lots of neurotransmitter arrives, then lots of ion channels will flip open. The ensuing rush of positive ions into the cell causes its membrane voltage to dip below a critical threshold of about −0.04 volts. When this happens, another set of nerve ion channels come into play. These are voltage-gated ion channels, which means they are sensitive not to neurotransmitters but to the voltage difference across the membrane.
