NICHOLS, T. Richard; Emory University, Atlanta, GA: Differing Roles of Length and Force Feedback in the Regulation of Motor Coordination

The study of musculoskeletal biomechanics and muscle physiology is critical to understanding the manner in which neural mechanisms mediate coordinated movement. Substantial progress has been made in elucidating the roles of sensory feedback from muscle spindle receptors and Golgi tendon organs in motor coordination by investigating the interactions of neural and mechanical circuits in the feline motor system. At the level of the spinal cord, excitatory length feedback from the primary endings of muscle spindles is distributed primarily to the muscles containing the receptors and to muscles having synergistic mechanical actions and regulates the mechanical properties of these target muscles. This feedback is distributed to muscles that are undergoing similar mechanical changes to the muscle of origin. Force feedback at the spinal level is subdivided into at least two types. Force feedback can be excitatory, in which case it is distributed primarily autogenically as is the case for length feedback, or inhibitory, in which case it is distributed to muscles that cross different joints and different axes of rotation than the muscle containing the receptors. Excitatory force feedback is also context dependent, appearing during stepping or locomotion but not steady force generation. The response properties of tendon organs and the distribution of force feedback suggest that this feedback adjusts the magnitude of stiffness of the main muscle and of the endpoint of the limb, and also promotes interjoint coordination rather than compensating for nonlinearities in the individual muscles.