Responses of interposed and dentate neurons to perturbations of the locomotor cycle.

This study examined the relationship of antidromically identified neurons in the dentate and interposed nuclei to perturbed and unperturbed locomotion in the pre-collicular, mid-mamillary, decerebrate cat. During treadmill locomotion two methods were used to perturb the step cycle. In the first, the treadmill was braked in different phases of the step cycle, the "treadmill" perturbation. In the second, the motion of the ipsilateral forelimb was interrupted by a rod placed transiently in the limb's path, the "single limb" perturbation. Most interposed cells were modulated during locomotion, their discharge being highly correlated with the EMG of the ipsilateral biceps or triceps. When the locomotion was perturbed, the modulation ceased for the duration of the perturbation. A few interposed cells displayed activity patterns unrelated to the EMG but were responsive to perturbations of a single limb. These responses may be explained by the putative activation of peripheral afferents produced by the perturbation. Most dentate cells were not modulated during unperturbed locomotion but did respond to features of the treadmill perturbation. Usually the response was coupled to the resumption of treadmill motion. A minority of dentate neurons was modulated slightly during unperturbed locomotion. Their modulation was less dramatic than that of interposed cells and was only weakly related to limb movement or EMG activity. Like the interposed neurons, these dentate cells responded to the treadmill perturbation with a cessation of modulation. All dentate cells were unresponsive to single limb perturbations. In a preparation lacking cerebral cortical input, the findings show that neurons of the interposed and dentate nuclei are modulated differently during perturbed and unperturbed treadmill walking in the decerebrate cat. The activity of interposed neurons is related to specific features of EMG activity recorded from muscles in the ipsilateral forelimb. Although some dentate cells were weakly modulated during unperturbed locomotion, the majority of these neurons responded most dramatically to the occurrence of a perturbation which completely stopped the walking behavior.