Orderly recruitment among motoneurons supplying different muscles

Virtually all movements involve the recruitment of motor units from multipl e muscles. Given the functional diversity of motor units (motoneurons and t he muscle fibers they supply), the effective production of specific movemen ts undoubtedly depends upon some principle(s) to organize the ensemble of a ctive motor units. The principle acting to organize the recruitment of moto r units within muscles is the size principle, whereby the first motor units to be recruited have the smallest values for axonal conduction velocity an d contractile force, and are the slowest to contract and fatigue. Here we c onsider the possibility that the size principle applies in the recruitment of motor units across muscles, i.e., that regardless of their muscles of or igin, active motor units are recruited in rank order, for example, from low to high conduction velocity. The benefits of orderly recruitment across mu scles could be similar to the acknowledged advantages of orderly recruitmen t within muscles. One benefit is that the neural process involved in organi zing active motor units would be simplified. In a muscle-based scheme, the size principle would organize only those motor units within individual musc les, leaving the nervous system with the additional task of coordinating th e relative activities of motor units from different muscles. By contrast, i n an ensemble-based scheme, orderly recruitment of all motor units accordin g to the size principle would automatically coordinate motor units both wit hin and across motor nuclei. Another potential benefit is the provision for movements with smooth trajectory, the result of interleaving the divergent torque contributions made by motor units from muscles that differ in their orientations about joints. Otherwise, if order were restricted within musc les, the torque trajectory of a joint would change unevenly as participatin g muscles begin contracting at different times and grade activity at differ ent rates. These considerations support speculation that motor units recrui ted from co-contracting muscles are collectively recruited according to the size principle. (C) Elsevier, Paris.