Activity of different classes of neurons of the motor cortex during locomotion.

This study examines the activity of different classes of neurons of the motor cortex in the rabbit during two locomotion tasks: a simple (on a flat surface) and a complex (overstepping a series of barriers) locomotion. Four classes of efferent neurons were studied: corticocortical (CC) neurons with ipsilateral projection (CCIs), those with contralateral projection (CCCs), descending corticofugal neurons of layer V (CF5s), and those of layer VI (CF6s). In addition, one class of inhibitory interneurons (SINs) was investigated. CF5 neurons and SINs were the only groups that were strongly active during locomotion. In most of these neurons a clear-cut modulation of discharge in the locomotion rhythm was observed. During simple locomotion, CF5s and SINs were preferentially active in opposite phases of the step cycle, suggesting that SINs contribute to formation of the step-related pattern of CF5s. Transition from simple to complex locomotion was associated with changes of the discharge pattern of the majority of CF5 neurons and SINs. In contrast to CF5 neurons, other classes of efferent neurons (CCI, CCC, CF6) were much less active during both simple and complex locomotion. That suggests that CC interactions, both within a hemisphere (mediated by CCIs) and between hemispheres (mediated by CCCs), as well as corticothalamic interactions via CF6 neurons are not essential for motor coordination during either simple or complex locomotion tasks.