Existence of a strength-duration curve for spinal cord motor evoked potentials in cats

A new technique has been developed to estimate the chronaxie of fibers carrying action potentials that are responsible for short latency motor evoked potentials (MEPs). In a 6-cat study, electrical stimuli were applied to the exposed motor cortex, and spinal cord potentials (at the vertebral level of T9/10 and L2/3) were recorded with needle electrodes using signal averaging. From a plot of MEP amplitude versus stimulus current amplitude for stimuli of 70, 100, 200 and 500 microseconds duration, it was possible (by extrapolation and using the linear relationship between charge and duration) to estimate chronaxie for the first 2 prominent peaks in MEP recordings. Mean chronaxie values ranging from 190 to 337 microseconds were obtained. This study describes acquisition of strength-duration curves for short latency MEP peaks, but does not address the origin of these peaks.