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In deeply anesthetized cats, determinations of motor-axonal conduction velocity (CV) were made using extracellular potentials recorded from single, functionally isolated motor axons innervating the muscle tibialis posterior. Axons were activated by suprathreshold electrical stimulation at the ventral-root level. Action potentials were recorded with 3 bipolar electrodes located on the muscle nerve at the level of the popliteal fossa. The most proximal and distal of the bipolar muscle-nerve electrodes were 16.4-22.0 mm apart. Estimates were made of CV from ventral root to muscle nerve (conventional CV) and between the proximal and the distal pairs of muscle-nerve electrodes (muscle-nerve CV). An evaluation was based on comparison of these CVs, estimates of uncertainties in time and distance measurements and simulations of the effects of recording conditions on CV estimates. The analysis indicated that the uncertainty in the conventional CV measurement of mammalian motor axons is at least +/- 2%. However, variability may be as great as 20% between muscle-nerve CV measurements from different experiments, probably due to such factors as regional variation in CV and differences in recording configuration.

A test for synchronization among the spike trains of muscle afferents or motor units is described which utilizes averages of neurograms and rectified neurograms. Synchronization is quantified by the increase of a synchronization index Is above a theoretical value for asynchrony. The dependence of the Is on signal amplitude and certain experimental conditions and a method of estimating confidence limits for the test are presented.

This report describes the use of a synchronization index (Is; Hamm et al., 1985a) and its sensitivity to various forms and degrees of synchrony between spike trains. The dependence of the Is on signal-to-noise ratio, the number of synchronized spike trains and their degree of synchrony is shown in analog and digital simulations. These simulations and a comparison with peristimulus time histograms under conditions of induced synchrony reveal that the Is is a sensitive measure of synchronization in a population of spike trains.