Ten studies are designed to continue our investigations of stereotypic limb movements controlled by lumbosacral centers: walking, scratching, and paw shaking. Our continued interest in these cyclic behaviors stems from the general hypothesis that basic elements of the motor program for each are provided by networks of spinal interneurons that are modulated by motion- related feedback and input from supraspinal centers. We quantify motor patterns (via EMG) in normal animals, and these baseline data are compared with similar data from deafferented and spinalized preparations. Also, by assessing limb dynamics, we determine joint torque components due to muscle and those due to gravity and intersegmental (inertial and centripetal) forces; these data are critical to understanding the role of muscle in limb movements. Five studies in Project I are devoted to understanding the unique demands placed on the CNS for the control backward (BWD) walking. We propose that forward (FWD) and BWD walking in the cat are controlled by a basic flexor- extensor synergy provided by common spinal network and that specific differences in the details of the motor pattern (relative timing and burst amplitude) as well as the gating of the stumbling-corrective reactions rely on a combination of supraspinal input (which sets unique postures for each form of walking) and motion-related feedback (which is direction dependent). Moreover, we hypothesize that FWD and BWD walking place functionally different demands on the fore- and hindlimbs and that different joints are responsible for producing and absorbing mechanical power during stance. Two studies in Project II are designed to investigate the role of motion- related feedback during the contact phase of the hindlimb scratch cycle. We predict that extensor muscle activity during contact is modulated by cutaneous and proprioceptive feedback and that in the absence feedback, extensor activity is terminated. We predict, therefore, that the effects of motion-relation feedback during the paw-contact phase are to prolong extensor activity during the contact phase and delay the onset of flexor activity, particularly at the ankle joint. Three studies in Project III are planned to assess paw-shake cycle is programed centrally, while the sequential timing of anterior muscles is regulated by motion-related feedback and will be altered in deafferented preparations. We predict that forelimb responses, being investigated for the first time, operate under different control strategies than those for hindlimb responses; thus, muscle synergy and kinematics data for this behavior will lead to new insights about the regulation of rapid-cyclic motions. Knowledge about the control of stereotypic limb movements is fundamental to our understanding of how the central nervous system regulates purposeful and adaptive motions of the limbs. Our unique approach of combining the study of limb dynamics with neuromuscular patterns yields important insights about the requirements for neuromotor control in normal and diseased states.
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