Short‐term synchronization of intercostal motoneurone activity.

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Abstract
The joint occurrence of unitary excitatory post-synaptic potentials (EPSP) evoked in motoneurons by branches of common stem pre-synaptic fibers may cause short-term synchronization of their discharge during the rising phases of the unitary EPSP. This hypothesis was tested using the pre- and post-stimulus time (PPST) histogram to detect synchronized firing among groups of [cat] intercostal motoneurons discharging in response to their natural synaptic drives. Motor nerve action potentials were recorded monophasically from nerve filaments of the external intercostal muscles of anesthetized, paralyzed cats maintained on artificial ventilation. Computer methods were used to measure peak spike amplitude, spike interval and filament identification for simultaneous recordings from 4 filaments. The spike amplitude histograms were derived for each filament and groups of spikes were selected for analysis. With spikes of 1 group designated as stimuli (occurring at zero time) and those of a 2nd as response, the PPST histogram was computed with different time bin widths. With bin widths of 100 and 10 ms the central respiratory periodicity was apparent in the PPST histogram. With 1.0 ms bins the PPST histogram showed a narrow central peak extending to .+-. 3.0 ms at its base. This short-term synchronization supported the hypothesis of joint firing due to common presynaptic connectivity. Detection of short-term synchronization was critically dependent on a sufficient quantity of data, but, provided a simple criterion of adequate counts per bin in the PPST histogram was met, short-term synchronization could be detected between intercostal motoneurons of the same and adjacent segments.