The responses of primary spindle afferents to fusimotor stimulation at constant and abruptly changing rates.

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Abstract
Single fusimotor fibers to de-efferented soleus of the cat were stimulated to investigate the size and time course of the responses elicited in single primary spindle afferents. The muscle was kept at constant length close to the physiological maximum. Constant and alternating rates of fusimotor stimulation were used: repetitive stimulation at constant rate (maintained stimulation); and modulated stimulation with the rate of activation alternating between 2 constant levels at repeat frequencies between 0.09 and 2 Hz (rectangular stimulation). The responses were averaged and displayed as post-stimulus time (pst) histograms or as cycle histograms. During static fusimotor stimulation the pst histograms could be clearly modulated over a range of rates of stimulation. Histogram modulation was not a prerequisite of static action since with different fibers the degree of modulation could range from deeply modulated to completely non-modulated. Dynamic fusimotor stimulation was almost always accompanied by non-modulated pst histograms. Primary spindle afferents responded to rectangular stimulation of either kind of fusimotor fiber with an approximately rectangular modulation of the rate of discharge. At the repeat frequencies studied, the size of the responses was appreciably larger with static than with dynamic activation. It was assessed as fusimotor rate-sensitivity during alternating stimulation by the response/stimulus ratio which is defined as change in firing/change in alternating rate of stimulation, in impulses/stimuli. The mean values of rate-sensitivity were 1.35 impulses/stimuli (statics) and 0.29 (dynamics), with a static/dynamic ratio of 4.7. The afferents fusimotor rate-sensitivity during steady stimulation (change in firing/change in maintained rate of stimulation) was also determined. The mean values were 0.78 (static) and 0.37 (dynamics), with a static/dynamic ratio of 2.1. The time course of the responses to rectangular stimulation was of the same order of magnitude for static and dynamic fibers. It was assessed by fitting a single exponential to the rising and falling phase of cycle histograms. The mean values of the time constants for static fibers were 58 ms (rising phase) and 59 ms (falling phase), and for dynamic fibers 34 ms (rising phase) and 49 ms (falling phase). The differences were statistically non-significant.

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