Transmitter release by mammalian motor nerve terminals in response to focal polarization

1. A method is described by which mammalian motor nerve terminals may be uniformly polarized by focally applied current, and the extra-cellular potential in the synaptic cleft, corresponding to any current, estimated.2. The relationship between log m.e.p.p. frequency and local extra-cellular field is flat for hyperpolarization and ascends linearly with depolarization. With depolarization, m.e.p.p. frequency is multiplied about tenfold for every - 18 mV. This characteristic becomes steeper the closer the polarizing electrode to the nerve terminal with a limiting value of ten-fold per - 15 mV.3. There exists a population of small m.e.p.p.s which are generated at the same end-plate as normal m.e.p.p.s.4. Following a prolonged depolarizing pulse there is an increase of m.e.p.p. frequency which continues for periods of up to several minutes.5. With hyperpolarizing pulses m.e.p.p. frequency may increase in a characteristic ;bursty' manner. Similar bursts of m.e.p.p.s also occur spontaneously, but far less frequently, without polarization.6. During a depolarizing pulse, m.e.p.p. frequency becomes maximal or near maximal within 2 sec. There is little subsequent alteration of m.e.p.p. frequency. Numbers of m.e.p.p.s occurring during depolarizing pulses follow the Poisson distribution.7. Following a depolarizing pulse, numbers of m.e.p.p.s released by a subsequent pulse may be either increased or diminished.8. Comparison of the response of m.e.p.p. frequency to raised [K] and to extrinsic presynaptic polarization leads to the conclusion that the presynaptic transmembrane potential change corresponding to any focal current pulse is about two thirds of the local extracellular potential field. Hence the slope of the linear portion of the presynaptic transfer function is about tenfold per 10 mV presynaptic depolarization.