The mammalian spinal cord contains pattern generating circuits for locomotion. In addition, there are sensory pathways in the spinal cord that regulate muscular stiffness, inter-joint coordination, and the pattern-generating networks. Evidence exists also that the same sensory information is used to regulate posture and stability in three dimensions during both standing and movement, but the identity of the underlying neural pathways is unknown. The purpose of the proposed project is to build on the work from previous grant cycles and test the hypothesis that the distributed network of spinal pathways from muscle spindle receptors and Golgi tendon organs can mediate appropriate postural responses in three dimensions in a way that resembles the strategy for maintaining balance and stability in normal animals. Alternatively, a neural network in the brainstem might be required for this regulation while the spinal pathways provide more local regulation of the mechanical properties of joints and limbs. In the proposed project, the decerebrate cat preparation will be used to investigate the roles of sensory receptors, neural pathways in the spinal cord, and mechanics of the musculoskeletal system in postural regulation. First, intra-axonal recordings from single muscle spindle receptors and Golgi tendon organs from a variety of muscles will be used to determine the manner in which these receptors respond to translations of the support surface in three dimensions. Second, ground reaction forces and electromyographic responses will be recorded during platform translations to determine whether the strategy used by intact animals can be observed in the decerebrate preparation, and the manner in which the strategy that is observed changes with limb orientation. Third, the organization of intermuscular, sensory pathways will be investigated during treadmill locomotion to understand how spinal mechanisms of balance and stability are altered during locomotion as compared to quiet standing. Demonstration of an intrinsic ability of the spinal cord to regulate posture in three dimensions would greatly facilitate the design of treatments for patients with spinal injury and other motor disorders, since standing and balance are basic to voluntary movement.