AHN, Anna N; Harvey Mudd College: Translating a Neural Signal into a Mechanical Output

The mechanical behavior of muscle during locomotion is often predicted from its anatomy, stimulation pattern, basic contractile properties, and joint kinematics. Cockroaches and frogs provide model systems to examine these assumptions. In the cockroach leg, two muscles operating at the same joint are innervated by a single motor neuron. Direct measurements under in vivo running conditions reveal that the same neural signal and joint kinematics can result in different mechanical behaviors from two anatomically similar muscles with the same contraction kinetics, force-length properties, and force-velocity properties. In the frog semimembranosus muscle(SM), in vivo segment length changes along the muscle’s length were examined during hopping to link data addressing in vitro sarcomere behavior with in vivo muscle behavior. As expected, the proximal and mid segments of the SM shortened similarly during hopping. During the same hop, however, the strain of the distal segment was lower and more variable, often lengthening before shortening. These differences in strain amplitude and pattern imply that adjacent segments along a parallel-fibered muscle can operate on different regions of their force-length and force-velocity relationships. Moreover, these in vivo segment strain patterns differ from the patterns of sarcomere heterogeneity seen in vitro in single fibers. A single neural input to two muscles within group and to adjacent segments along a muscle can result in variable mechanical outputs during locomotion.