Characterization of the relationship between motor end-plate jitter and the safety factor

A mathematical model developed for a personal computer was used to simulate the jitter phenomenon in neuromuscular transmission in order to characterize the relationship between jitter and the safety factor. Four models of normal and abnormal neuromuscular transmission were investigated. In a human experiment, the mean consecutive difference (MCD) values for stimulated single-fiber electromyography at firing rates of 1, 2, 5, and 10 Hz were measured in 137 motor end-plates of myasthenic patients. Results of the computer simulations show that the relationship between jitter and the safety factor is exponential like. Variations in jitter are most prominent in end-plates with low safety factors. This relationship agrees with results of human end-plate studies. Changes in the MCD values caused by presynaptic depression or facilitation are linearly correlated to the initial jitter, whereas the logarithmic values of MCD are not. It is very important to keep in mind this nonlinear relationship when relating single-fiber jitter to the safety factor.