Characteristics of M spikes in cat motoneurons and their significance for the measurement of small composite la EPSPs

Some characteristics of intracellularly recorded M spikes in cat motoneurons have been determined in order to evaluate techniques for subtracting this spike from recordings of homonymous monosynaptic composite Ia EPSPs. M spikes were recorded in 44 hyperpolarized motoneurons in three deeply anesthetized cats with sectioned dorsal roots. The decay of the M spike, consisting of several exponential components, was longer than has been supposed. This time course depends on the passive electrical properties of the cell, as shown by comparisons of (1) the M spike's longest time constant with that of the motoneuron membrane and (2) the M spike's time course with that predicted by a simulation. An average of normalized, individual M spikes was scaled to the M spike in small, homonymous, monosynaptic M + Ia EPSP records and subtracted to obtain an estimate of EPSP amplitude ("subtraction method"). This method was compared to previous methods requiring extrapolation of the M spike ("extrapolation method") or recording EPSP waveforms above and below the M spike threshold ("addition method"). Estimates of EPSP amplitude by the addition and subtraction methods gave the same results, while estimates by the extrapolation method were 0.3-0.6 mV larger on average. No significant differences were found between EPSP estimates by the subtraction method and measured EPSP maxima in 12 cells in which maximum EPSPs were obtained at stimulus strengths below the M spike threshold, supporting the absolute accuracy of the subtraction and addition methods. It is concluded that the M spike makes a significant contribution to the apparent amplitude of small composite Ia EPSPs. When an M spike is present in an EPSP record, accurate estimates of EPSP amplitude can be obtained using either the subtraction method or the addition method. However, the subtraction method has the advantage of requiring fewer recordings in each experimental trial.