Relationship between the time course of the afterhyperpolarization and discharge variability in cat spinal motoneurones

1. We elicited repetitive discharges in cat spinal motoneurones by injectin g noisy current waveforms through a microelectrode to study the relationshi p between the time course of the motoneurone's afterhyperpolarization (AHP) and the variability in its spike discharge. Interspike interval histograms were used to estimate the interval death rate, which is a measure of the i nstantaneous probability of spike occurrence as a function of the time sinc e the: preceding spike. It had been previously proposed that tart: death ra te can be used to estimate the AHP trajectory We tested the accuracy of thi s estimate by comparing the AHP trajectory predicted from discharge statist ics to the measured AHP trajectory of the motoneurone. 2. The discharge statistics of noise-driven cat motoneurones shared a numbe r of features with those previously reported for voluntarily activated huma n motoneurones. At lon discharge rates, the interspike interval histograms were often positively skewed with an exponential tail. Title standard devia tion of the interspike intervals increased with the mean interval, and the plots of standard deviation versus the mean interspike interval generally s howed an upward I,end, the onset of which was related to the motoneurone's AHP duration. 3. The AHP trajectories predicted from the interval death rates were genera lly smaller in amplitude (i.e. less hyperpolarized) than the measured AHP t rajectories. This discrepancy may result from the fact that spike threshold varies during the interspike interval, so that the distance to threshold a t a given time depends upon both the membrane trajectory and the spike thre shold trajectory Nonetheless, since the interval death rate is likely to re flect the instantaneous distance to threshold during the interspike interva l, it provides a functionally relevant measure of fluctuations in motoneuro ne excitability during repetitive discharge.