Electrotonic properties of motor nerve terminals

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Abstract
The electric membrane properties of frog motor nerve terminals were examined with depolarizing or hyperpolarizing current pulses of 2-8 ms duration to the preterminal, depolarization induced transmitter release was affected by electrotonic spread of potential. Na channels were blocked by tetrodotoxin. Under this condition a hyperpolarizing current pulse produced inhibition of release, followed by potentiation of release. Inhibition lasted > 100 ms with a time constant of 50-150 ms. When K channels were blocked by 3,4-diaminopyridine or tetraethylammonium a depolarizing current pulse potentiated transmitter release for a period up to 50 ms. Inward currents in the nerve terminal are carried mainly by Na+ and Ca2+ and outward currents by K+ while leak conductances are negligible. A low leak conductance and therefore a high specific membrane resistance facilitates the spread of electrotonic potentials and thereby explains the longlasting effects on transmitter release of brief current pulses to the preterminal.

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