The effect of voltage on the time course of end‐plate currents

Abstract

1. End‐plate currents have been studied in glycerol‐treated frog sartorius nerve—muscle preparations with the voltage clamp technique. 2. End‐plate currents follow a simple exponential time course over most of their declining phase. 3. The rate constant α that characterizes this exponential decay depends upon membrane potential V according to the relationship α (V) = B e AV, with A = 0·00795 ± 0·00043 ( S.E.) mV−1 and B = 1·67 ± 0·04 ( S.E.) msec−1. 4. Voltage sensitivity decreases (that is, A in the above equation becomes smaller) as the recording and current‐passing electrodes are moved away from the end‐plate region. 5. The voltage sensitivity of α is decreased by decreasing the gain of the voltage clamp amplifier. 6. Changing the end‐plate current amplitude by curare treatment, by increased calcium ion concentration, and by facilitation and depression has essentially no effect on end‐plate current time course. 7. When membrane potential is changed step‐wise during the decaying phase of the end‐plate conductance change, currents begin to decline with a rate constant α appropriate to the new membrane potential in less than 0·2 msec. 8. Treatment with prostigmine methylsulphate in concentrations up to 50 μg/ml. slows end‐plate current decay but has little effect on voltage sensitivity. That is, B in the above equation is decreased by prostigmine treatment, but A is relatively unaffected.