The effects of single afferent impulses on the probability of firing of external intercostal motoneurones in the cat

1. Recordings of afferent discharges from external intercostal muscle spindles were made from in‐continuity dorsal roots of anaesthetized, paralysed cats and the afferents were characterized as described in the preceding paper (Kirkwood & Sears, 1982). 2. The strengths and time courses of the raised probabilities of firing of external intercostal motoneurones evoked by the synaptic actions of impulses in the afferents were measured by constructing cross‐correlation histograms from the discharges of the individual afferents and the discharges of the motoneurones, which were simultaneously recorded from the cut central ends of intercostal nerve filaments. Other dorsal roots, apart from the rootlet containing the afferent fibre, were cut to prevent afferent synchronization from affecting the results. 3. Cross‐correlation histograms involving single efferent discharges showed narrow peaks (mean half‐width 0·99 msec) at monosynaptic latencies. 4. This mean half‐width is slightly longer than the mean rise‐time of the e.p.s.p.s described in the preceding paper, but much shorter than the mean half‐width of those e.p.s.p.s. These measurements, together with the shapes of individual histogram peaks thus confirm the prediction of Kirkwood & Sears (1978) that the shape of such a peak should be approximated by the time differential of the underlying e.p.s.p. plus the e.p.s.p. time course itself. Amplitudes of the peaks were also consistent with the predictions of Kirkwood & Sears (1978). 5. The cross‐correlation histograms involving multi‐unit efferent discharges were entirely consistent with those involving single efferent discharges and were used to demonstrate the patterns of projections of individual afferents to different groups of intercostal motoneurones. 6. Individual afferents excited α motoneurones of all spike sizes in the intercostal filament recordings but excitation of γ motoneurones was not detected. 7. Individual afferents excited motoneurones of the same segment and both adjacent segments, but no projection beyond these was detected. 8. The amplitude of the multi‐unit cross‐correlation peak was taken as a measure of the over‐all strength of the connexions to a given segment. When this amplitude was measured for motoneurones of the same segment as the afferent, for sixteen out of nineteen afferents there was a highly significant positive correlation with the conduction velocity of the afferent. The remaining three afferents gave a strength of connexion which appeared to be much stronger than would be expected from the rest of the population of afferents. 9. The strength of connexion to adjacent segments was generally lower than to the same segment.