Motor activity during walking in the cockroach Periplaneta americana. II. Tethered walking

The patterns of electrical activity in selected muscles of the rear legs were studied during locomotion in tethered cockroaches. Stepping movements did not appear significantly different in a tethered insect compared to one which was freely moving. During walking, the muscle potentials due to 'fast' flexor and 'slow' extensor motoneurons were always grouped into alternating bursts separated by periods of silence, but those due to 'slow' flexor motoneurons were not usually grouped in this way. Muscle potentials due to 'slow' flexor activity were usually modulated in frequency such that the highest frequencies occurred when in the free walking insect one would have expected a burst to occur. 'Slow' extensor and 'fast' flexor bursts were studied in detail. Minor fluctuations of the frequency of impulses within each burst were not related to any other parameter of activity which was measured. For 'slow' extensor bursts, the duration of the bursts, the number of impulses per burst and the mean interval between impulses in a burst each increased significantly as the duration of the associated step increased. As in free walking animals, the rate at which the latter two changed as a function of step duration was often significantly higher for steps shorter than about 300 msec. than for longer ones. However, the absolute value of these two parameters in tethered preparations was about half that at corresponding step durations in free walking preparations. For 'fast' flexor bursts, weak positive correlations between step duration and burst duration, number of impulses per burst and mean interval per burst could be demonstrated only in a few preparations. The relationships were not significantly different from those seen in free walking preparations. The results suggest that reflex feedback loops can strongly affect some parameters of motor output, such as discharge rate within a burst, but affect others, such as burst duration, only very slightly if at all. A model of the mechanism by which steps may be generated is modified accordingly.