Interlimb Coordination During Locomotion

SYNOPSIS. Studies of interlimb control during cat locomotion are directed at four different levels of organization. Interlimb stepping patterns are described from studies of the timing of electromyographic activity of muscles of different limbs. Patterns of coordination are based on the frequency of occurrence of the phasing of step cycles of the different limbs. Selective spinal cord lesions are used to perturb those patterns of coordination and have implicated two ascending spinal systems in interlimb control: long ascending propriospinal neurons (LAPNs) and neurons of the ventral spinocerebellar tract (VSCT). The results of neuroanatomical tract tracing experiments indicate that two different populations of LAPNs exist which might provide direct connection between cervical and lumbosacral locomotor centers but that neurons of the VSCT do not make such connections. These results imply that the role of the VSCT in interlimb control is by way of the cerebellum. Unit recordings made from axons of the VSCT during treadmill locomotion are consistent with the VSCT carrying information about the timing of both hindlimb step cycles.