The long-term goal of the proposed research is to understand the role of the spinal cord in normal and abnormal motor coordination. The spinal cord contains neuronal circuits that receive sensory information from muscles and distribute this information back to the motoneurons of many muscles including the muscle or origin. The functions of this proprioceptive feedback and of the corresponding neural circuitry are not well understood, but they may include the modification of centrally generated motor patterns and the mediation of responses to mechanical disturbances. Prior research supported by this grant showed that distal hindlimb muscles having important postural and antigravity functions are linked by powerful inhibitory feedback from muscle receptors. It was proposed that this feedback promotes interjoint coordination during responses of the limb to mechanical perturbations and constitutes a postural control system in the spinal cord. This hypothesis resulted from studies of the distribution of sensory feedback from muscle spindles and Golgi tendon organs and the 3-dimensional actions of muscles in reduced animal preparations. In the next grant cycle, it is proposed to test these hypotheses in the context of global limb posture and states that represent a wider behavioral repertoire. The use of intramuscular electrical stimulation to selectively activate Golgi tendon organs in a given muscle will be refined and then the reflex effects, in terms of activation patterns of other muscles and end-point forces, will be evaluated using electromyographic and force-plate analysis. The investigations will focus on the three dimensional mechanical actions of the complex muscles crossing the hip and the neural pathways linking these muscles with those that cross the knee and ankle. In addition, the relative roles of excitatory and inhibitory feedback from Golgi tendon organs will be evaluated by comparing the distributions of these pathways during postural maintenance and locomotion. These experiments will test two major hypotheses concerning the role of Golgi tendon organs in motor coordination. In addition, they will provide a basis for understanding the deficits in motor coordination that result from damage to proprioceptive feedback.