Sinusoidal responses of primary and secondary endings in deefferented spindles of anesthetized cats were studied over the low frequency range 0.001 to 0.1 Hz. Stretch amplitudes were chosen conservatively small (25 to 100 μm peak to peak) so as to lie within the linear region. At 0.1 Hz, average sensitivity was 350 pps/mm for primary endings and 80 pps/mm for secondary endings. Sensitivity fell to low values at lower frequencies, but even at 0.001 Hz, corresponding to 17 min/cycle, sensitivity remained elevated above static values determined with large stretches. Phase lead varied from 5 to 50° and, in the case of primary endings, tended to be greater at lower frequencies. Except for the different scaling factors, the only apparent difference between the frequency responses of primary and secondary endings was a tendency for primary endings to show a greater phase lead over the range 0.001 to 0.01 Hz. Dynamic responsiveness was assessed theoretically from frequency response data by calculating responses to ramps at various velocities. Over most of the velocity range, dynamic responses were not proportional to velocity. The greater dynamic responsiveness of primary endings during large (6 mm) ramp stretches might be related to frequency response below 0.01 Hz. Certain aspects of dynamic responsiveness to large ramps (6 mm) were accounted for by assuming all phases of responses were attenuated by 25 dB in the case of primary endings and 20 dB in the case of secondary endings. The nonlinearity responsible for attenuation appears to occur at an early stage in the sensory process. Comparison of individual responses to slow ramps with predictions based on linear theory indicated the presence of abrupt departures from linearity for primary and secondary endings.
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