Action potentials are used in neurons to conduct signals along the axon and occur in electrically excitable cells like neurons and cardiac muscle cells. The action potentials travel in a wave along the membrane causing the voltage sensitive channels to open to allow influx of Na+ thereby causing the conduction of the signal along the axon. The resting membrane potential of cells including neurons is -70mv. An action potential is generated by a change in the membrane potential from -70mv to +40mv when voltage gated ion channels open altering membrane permeability to Na+ and K+.
The action potential travels down the axon and reaches the pre-synaptic terminal depolarizing the membrane in the pre-synaptic terminal. The depolarization causes the voltage-gated Ca2+ channels to open allowing the influx of Ca2+ that signals the release of neurotransmitter into the synaptic cleft.