MUSCLE SPINDLE FEEDBACK AND LIMB COORDINATION DURING LOCOMOTION
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Understanding how the nervous system controls locomotion is of great importance to improve treatments of mobility disorders. The numerous muscles and joints in a limb participating in a given movement, have to feed sensory information back to the central nervous system, so that, individuals have the capacity to perform functional movements and cope with diverse environmental variables. The integration of proprioceptive feedback derived from muscle spindles and Golgi tendon organs into the central nervous system is still a fascinating topic requiring a deeper understanding. We have analyzed locomotor patterns in different behavioral conditions in mice, in which proprioceptive feedback was genetically removed or attenuated by adeno-associated virus technology. Our results showed that proprioceptive feedback from muscle spindles and Golgi tendon organs are essential for the coordination of flexor and extensor muscles necessary for functional terrestrial locomotion or swimming. Wide-spectrum muscle spindle feedback deficiency changed muscular activation patterns, generated incapability to precisely target the limbs after a stumble event during walking, and harmed the regulation of muscle activity-strength or speed-dependent EMG amplitude. Our results show that the ablation of proprioceptive feedback from an individual group of muscles, neither diminished functional locomotion nor altered muscle activation pattern. Nonetheless, the feedback from muscle spindles of the ankle extensor muscles, the triceps surae, is the main source of speed-dependent modulation of activity-strength, in contrast to the muscle spindle feedback from the knee joint which has no influence on speed-dependent amplitude modulation. We conclude that proprioceptive feedback is essential for controlling variable aspects of locomotion. The group Ia/II and Ib afferents carry the information used by the spinal cord to control muscle activity strength and regulate the onset and offset-timing of muscle activity patterns during locomotion.