Delayed and prolonged effects of a near threshold EPSP on the firing time of human alpha-motoneurones.

In order to investigate the effects of near-threshold excitatory inputs on the precise timing of the action potentials during the tonic discharge of human motoneurones, the activity of single motor units was recorded in the extensor carpi radialis muscles while tendon taps (indentation, 0.1 mm; duration, 1 ms) were being delivered irregularly at a mean rate of 0.8 s(-1). New methods of analysis, such as the phase response function, were used to study the relative changes in the interspike interval (ISI1) during which the stimulus was being delivered and in the three subsequent intervals (ISI2, ISI3, ISI4) as a percentage of the pre-stimulus interspike interval (ISI0). The consistency of the effects of the actual stimulus as regards the spontaneous variability was assessed by comparing the data with those obtained with virtual stimulation. When the stimulus occurred at the end of ISI1, and triggered a spike, ISI1 and ISI3 were generally shortened, whereas ISI2 was lengthened, probably due to the negative correlation induced by the summation of the after-hyperpolarisations (AHPs). When the stimulus occurred in the middle of ISI1 without triggering a spike, ISI1, ISI2 and more rarely ISI3 were shortened. Lastly, when the stimulus occurred during the AHP scoop in ISI1, ISI2 was shortened although ISI1 remained unchanged. ISI4 was not consistently affected in any of these cases. The present results show that the tendon tap-induced inputs (probably from muscle spindle primary endings) mediated delayed and prolonged shortening effects of the ISIs on most of the alpha-motoneurones tested (n = 16). These effects undetected in classic peri-stimulus histogram analysis may involve long-lasting conductance changes although the contribution of polysynaptic pathways cannot be excluded. The changes in ISI were quite moderate (< 15% of ISI) but highly consistent. Their functional involvement in the synchronisation or desynchronisation processes and/or the mechanisms of optimisation of muscle contraction still remains to be explored.