The effects of depolarization of motor nerve terminals upon the release of transmitter by nerve impulses.

1. Depolarizing currents were applied to motor nerve terminals in the rat phrenic nerve-diaphragm muscle preparation in vitro.2. During the passage of depolarizing currents the amplitude of the presynaptic nerve action potentials and of end-plate potentials (e.p.p.s) was reduced in proportion to the current strength.3. The reduction in e.p.p. amplitudes was shown to be due to a reduction in the number of quanta released.4. An excess of Mg ions or the previous application of a hyperpolarizing current could prevent the reduction of e.p.p. amplitudes and quantal contents by depolarizing current.5. Depolarizing current application prevented later hyperpolarizing currents affecting e.p.p. amplitudes.