Dynamic mechanism of adaptive interlimb coordination in cat locomotion

Central pattern generators (CPGs) in the spinal cord control animal locomotion. They generate rhythmically alternating flexor and extensor activities producing coordinated limb movements. Afferent feedback allows animals to adapt locomotion to the environment. Responses of neural activities to afferent feedback stimulation have been previously examined during fictive locomotion in immobilized cats. However, locomotion of behaving animals is governed by dynamic interactions between central neuronal circuits (CPGs), afferent feedback, the musculoskeletal system, and the environment. To study these interactions, we developed a neuromusculoskeletal model that included two interconnected half-center CPG models, each controlling one limb of a musculoskeletal model of cat hindlimbs. The model was used to investigate and analyze the role of afferent feedback in adaptive locomotion, particularly when during walking a foot stepped into a hole. Using a dynamic system approach, we have conducted a nullcline analysis of locomotion in these conditions and revealed how the durations of flexor and extensor phases change to produce adaptive interlimb coordination that stabilizes locomotion.