The spatial distribution of synchronization of intercostal motoneurones in the cat

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Abstract
The 3 different types of synchronization of intercostal motoneurons which were described in the preceding paper (Kirkwood et al.) were studied for motoneurons of the same segment or for different segmental separations (up to 5 segments) and for motoneurons on opposite sides of the same segment. The strength of synchronization declined with segmental separation for all 3 categories, although the rate of decline was more variable for broad-peak synchronization than for the 2 other types. Short-term synchronization was undetectable for separations > 3 or 4 segments, but clear peaks were still visible in the cross correlation histograms for high-frequency oscillation (HFO) or broad-peak synchronization at a segmental separation of 5. Synchronization between motoneurons on opposite sides of the cord was generally weak although less so for broad-peak or HFO components. The decline in strength of short-term synchronization with segmental separation could not be explained by temporal dispersion of impulses in presynaptic axons. A time shift was observed in the position of the cross-correlation histogram peak which was dependent on segmental separation and equivalent to a mean descending conduction velocity of 28 m/s in the assumed common input. This figure is similar to the mean conduction velocity of bulbospinal respiratory neurons derived from published values. The short-term synchronization in these preparations is generated by the bulbospinal respiratory neurons and the majority of their axons do not branch to make strong synaptic connections to motoneurons over more than 3-4 segments. Interpretations of the different distributions of the other types of synchronization are discussed.