Static and dynamic fusimotor action on the response of IA fibres to low frequency sinusoidal stretching of widely ranging amplitude

Abstract

Single fusimotor fibers were stimulated repetitively to test their action on the responsiveness of muscle spindle primary endings in the cat soleus to sinusoidal stretching of both large and small amplitude. Frequencies of 0.06-4 Hz were used at amplitudes from 10 .mu.m-3 mm. The response was assessed by fitting a sinusoid to the cycle histogram of the afferent firing throughout the course of the cycle; this linear approximation measured the fundamental of the response and ignored any harmonics. The sine was allowed to project to negative values and any empty bins in the histogram were ignored when fitting. With small amplitudes of stretching the histograms were reasonably sinusoidal, but with large amplitudes they showed appreciable distortion of the wave form for the passive ending and during dynamic fusimotor stimulation. Non-linearity of response manifested itself also, with increase amplitude of stretching, by an increase in the phase advance of the response, by increasing rms [root mean square] deviation of the histogram points from the fitted sine and (for dynamic stimulation) by an increase in the mean value of the fitted sine. With increasing amplitude the response modulation ceased to increase proportionately with the stimulus, so that the sensitivity of the ending to a large stretch (defined as afferent modulation/stretch amplitude) was appreciably less than for a small stretch. This effect was most pronounced for the passive ending. Whatever the amplitude of movement the modulation during static stimulation was less than that for the passive or during dynamic stimulation. For small amplitudes the response during dynamic stimulation was less than that of the passive, but for large amplitudes the response during dynamic stimulation was always the greater. At some intermediate cross-over amplitude the 2 responses were the same size, though still differing slightly in other respects. The value of the cross-over amplitude was usually about 200 .mu.m at 1 Hz and increased on lowering the frequency. Thus dynamic fusimotor action did not uniformly produce either an increase or a decrease in the sensitivity of the ending in relation to the passive. Bode plots, for each amplitude, of sensitivity and phase against frequency suggested that under all conditions the ending was relatively insensitive to frequency in the range studied, for the slope of the log-log sensitivity lines was only 0.15-0.2 (3.5-6 db/decade). The mechanism which made a non-linearity was not particularly frequency sensitive. Static fusimotor stimulation did not change the frequency sensitivity of the ending. Dynamic fusimotor stimulation very slightly increased the frequency sensitivity of the ending for large amplitudes. The major effect of fusimotor action, whether static or dynamic, was to regulate the sensitivity of the primary ending to stretching for all amplitudes of movement (i.e., gain) rather than to control the relative values of its sensitivity to length and to velocity (i.e., crudely, the damping in a feed-back loop).