Muscle spindle discharge in normal and obstructed movements

1. The discharge activity of muscle spindle endings located in tail and hind limb muscles was recorded during voluntary movements in the cat. 2. During active shortening of the receptor-bearing muscles, both primary and secondary endings tended to fall silent. This was more pronounced, the higher the rate of muscle shortening. We suggest that in unobstructed movements in which muscle velocities exceed 0.2 resting lengths per second (lr/sec), the firing patterns of spindle afferents are dominated by their responses to the length variations. At velocities lower than 0.2 lr/sec, fusimotor action may predominate. 3. When active muscle shortening was unexpectedly halted, both primary and secondary endings resumed firing, but the increases in discharge rate were not as abrupt as might have been expected had there been strong co-activation of fusimotor and skeletomotor neurones. Rather, for the types of movements studied, fusimotor action appears to have been quite modest.