an end plate potential is produced at the terminal cisternae when voltage-sensitive receptors open calcium-release channels. on the motor end plate when ligand-gated channels open and sodium ions diffuse into the muscle fiber. in the synaptic knob when voltage-gated calcium channels open and calcium ions enter the synaptic knob. along the transverse tubules when the opening of sodium channels and potassium channels produce alternating phases of depolarization and repolarization.