Testing a gamma-activated multiple spike-generator hypothesis for the Ia afferent

For several years we have studied the input-output relations of muscle spindles and tendon organs using random length perturbations as stimuli. While studying the effect of gamma stimulation on spindle afferent responses, we made the chance observation that although static gamma fibers altered the response of a spindle Ia afferent to random length stimuli, in response to specific length sequences with concurrent gamma stimulation, the receptor produced an unexpectedly large number of spikes in exactly the same temporal relation to the repeated “random” sequence. We report here on the data obtained in one such experiment in which a gamma fiber we had classified as “static” modified the Ia afferent response to muscle stretch.