Variations in the time course of the synchronization of intercostal motoneurones in the cat

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Abstract
Synchronization of intercostal motoneurons was studied by the construction of cross-correlation histograms which related the firing times of paired groups of efferent inspiratory or expiratory discharges recorded from filaments of the external or internal nerves of anesthetized or decerebrate cats. The principal feature of the histograms was always a central peak, but the time course of the central peak showed considerable variation. Three forms of synchronization were defined on the basis of the time course of the central peak: short-term synchronization (Sears et Stagg), where the peak was narrow, extending over about .+-.3 ms, but sometimes with weak shoulders to about .+-.5 ms; broad-peak synchronization where the peak was wider than this (often .+-.20 ms or more), but where there were no strong periodicities; high-frequency oscillation (HFO) synchronization, which was named from the related phenomena in medullary and phrenic recordings (Cohen), where there were periodic peaks on either side of the central peak with a frequency in the range 60-120 Hz. Combinations of these forms of synchronization were seen in some histograms. Short-term synchronization, due mainly to the branching of presynaptic axons, is generated mainly by those axons which transmit the respiratory drive, that drive providing most of the excitation of the motoneurons in moderately deep anesthesia; HFO synchronization arises from the periodic synchronization of the discharges in these same presynaptic axons; and broad-peak synchronization is generated by the activity of other presynaptic neurons whose discharges are also synchronized, but aperiodically, these neurons most likely include spinal cord interneurons which are active in light anesthesia or when released by spinal cord lesions.