Changes in fraction of current penetrating an axon as a function of duration of stimulating pulse

Single nerve fibers, from the frog sciatic were mounted on an isolation bridge. Threshold current amplitude vs. pulse duration relationships were determined using (a) transmembrane stimulation and (b) external electrodes. The shape of the strength-duration relationships obtained by means of these two modes of stimulation from the same fiber, were found to be significantly different. Chronaxies varied by up to 50 %. The difference between the two strength-duration relationships was maximal for stimulating pulses of 0·1-0·4 msec. These results are discussed in view of the changes in the fraction of current penetrating the cells at different pulse durations. A computational method providing a correction for the frequency dependency of the fraction of current penetrating C.N.S. and other cells, when stimulated by means of external electrodes is described.