EVIDENCE FROM MOTONEURON SYNCHRONIZATION FOR DISYNAPTIC PATHWAYS IN THE CONTROL OF INSPIRATORY MOTONEURONS IN THE CAT

1. Motoneurone synchronization was measured by cross-correlation betwe en paired inspiratory discharges in external and internal intercostal nerves or their intramuscular branches (T3 to T8) or in the phrenic ne rve (C5 root or both C5 and C6 roots independently) in anaesthetized, paralysed cats. 2. All cross-correlation histograms showed central pea ks, for which the durations at half-amplitude (half-widths) from inter nal nerve pairs in adjacent segments were all less than for external n erve pairs in adjacent segments or within a segment (means, 1.6 ms vs. 3.4 ms for adjacent segments). Values for external-internal pairs cov ered the ranges for both these two. Lowest values came from two phreni c pairs (1.2 and 1.4 ms). 3. The peaks from ipsisegmental external-int ernal pairs were usually asymmetric and the maximum of the peak was of ten displaced to a lag of about -1 ms (external nerve providing the re ference spikes), whereas peaks from external-external pairs were alway s symmetrical and centred on zero. Phrenic-internal peaks gave maxima with lags about 1 ms less than for phrenic-external peaks from the sam e segments. 4. Two explanations were considered possible for the diffe rences in duration and timing: an extra synapse on the pathway to the external nerve motoneurones, or a correlation kernel for a monosynapti c connection to the external nerve motoneurones that had a slower time course than that for the internal or phrenic nerve motoneurones. Comp uter simulations, assuming the extra synapse, gave a good fit to the o bserved time courses of the correlation peaks for all categories of ne rve pairs using single values of parameters (e.g. EPSP rise time) cons istent with those in the literature. This could not be achieved with t he different correlation kernel model. The timing of high-frequency os cillation (HFO), which was sometimes present in the correlations, was also better predicted with the extra synapse model. 5. It is concluded that most of the synchronization between external nerve motoneurones is derived from disynaptic common inputs and that any motoneurone sync hronization peak with a half-width greater than about 2.2 ms should be assumed to be likely to contain di- or oligosynaptically derived comp onents.