The discharge frequencies of primary muscle spindle endings during simultaneous stimulation of two fusimotor filaments

SummaryThe primary muscle spindle endings of the cat's gastrocnemius muscle were investigated with and without stimulation of two different γ-filaments separately and then simultaneously while ramp stretches of the same muscle were performed to register discharge patterns of the primary ending. The stretch length was varied from 4–12 mm, the stretch rate from 20 up to 300 mm/sec. For each discharge pattern the spontaneous activity, the dynamic and the static response were evaluated. The effect of a γ-stimulation on these responses is calculated as the difference between the discharge frequency with and that without a γ-stimulation. This difference represents the effect of a fusimotor stimulation.During simultaneous γ-stimulation the result was equal to the sum of the separate γ-stimulations at 68% of the responses for the spindles investigated. In those cases where no such summation took place the experimental values were generally lower than the values for a summation. Apart from this, a lack of summation was especially evident at high stimulation frequencies. The results are discussed together with the statement of other investigators that a summation never occurs during simultaneous stimulation of two γ-filaments. Finally, a concept is outlined which could explain the occurrence as well as the absence of a summation.