Sympathetic Discharges in Intercostal and Abdominal Nerves

There are hardly any published data on the characteristics of muscle nerve sympathetic discharges occurring in parallel with the somatic motoneurone discharges in the same nerves. Here, we take advantage of the naturally occurring respiratory activity in recordings of efferent discharges from branches of the intercostal and abdominal nerves in anesthetized cats to make this comparison. The occurrence of efferent spikes with amplitudes below that for alpha motoneurones were analyzed for cardiac modulation, using cross-correlation between the times of the R-wave of the ECG and the efferent spikes. The modulation was observed in nearly all recordings, and for all categories of nerves. It was strongest for the smallest amplitude spikes or spike-like waveforms, which were deduced to comprise postsynaptic sympathetic discharges. New observations were: (1) that the cardiac modulation of these discharges was modest compared to most previous reports for muscle nerves; (2) that the amplitudes of the sympathetic discharges compared to those of the somatic spikes were strongly positively correlated to nerve diameter, such that, for the larger nerves, their amplitudes overlapped considerably with those of gamma motoneurone spikes. This could be explained by random summation of high rates of unit sympathetic spikes. We suggest that under some experimental circumstances this overlap could lead to considerable ambiguity in the identity of the discharges in efferent neurograms.