[Neuronal control of posture and locomotion in decerebrated and spinalized animals]

We have found that the brainstem-spinal cord circuitry of decerebrated cats actively maintain the equilibrium during standing, walking and imposed mechanical perturbations similar to that observed in intact animals. The corrective hindlimb motor responses during standing included redistribution of the extensor activity ipsilateral and contralateral to perturbation. The postural corrections in walking cats were due to considerable modification of EMG pattern in the limbs as well as changing of the swing-stance phases of the step cycle and ground reaction forces depending of perturbation side. Thus the basic mechanisms for balance control of decerebrated animals in these two forms of motor behavior are different. Balance-related adjustments relied entirely on the integration of somatosensory information arising from the moving hindquarters because of the suppression of vestibular, visual, and head-neck-trunk sensory input. We propose that the somatosensory input from the hindquarters in concert with the lumbosacral spinal circuitry can control the dynamics of the hindquarters sufficient to sustain balance. We found that, after isolation from the brainstem or forebrain, lumbosacral circuits receiving tonic epidural electrical stimulation can effectively control equilibrium during standing and stepping. Detailed analyses of the relationships among muscle activity, trunk kinematics, and limb kinetics indicate that spinal motor systems utilize a combination of feedback and feedforward strategies to maintain dynamic equilibrium during walking. The unexpected ability of spinal circuitries to exert efficient postural control in the presence of epidural electrical stimulation in decerebrated and spinal cats have significant implications for the potential of humans with a severe spinal cord injury to regain a significant level of functional standing and walking capacities.