Effects upon spindle discharges of electrical stimulation of static fusimotor fibers with concomitant application of muscle vibration.

Discharges of single afferent fibers from the primary endings of the soleus muscle spindles were recorded from thin dorsal root filaments in cats anesthetized with urethane and chloralose. The distal cut end of the ventral root was split into fine filaments to obtain functionally single fusimotor fibers. The fusimotor fibers obtained in this study were of the static type. The soleus muscle was sinusoidally stretched at 70 Hz with various amplitude concurrently with 100 Hz electric stimulation of fusimotor fiber. The spindle afferent discharges were analysed by compiling inter-spike interval histograms and cross-correlograms between the afferent spikes and the stimulus pulses applied to the fusimotor fiber. The same analysis was also made between the afferent spikes and peak extensions of muscle yielded by vibratory stimulation. One-third of the fusimotor fibers were capable of driving the spindle afferents. The driving of fusimotor stimulation was replaced by driving by muscle dibration of more than 10 mum amplitude applied concurrently with fusimotor stimulation. The remaining two-thirds of the fusimotor fibers could not drive the spindle afferents. In this case, the driving by muscle vibration was obtained when vibration of more than 5 mum amplitude was applied concurrently with fusimotor stimulation. It was suggested that fusimotor fibers which produced driving of the spindle afferents would terminate on nuclear chain fibers and those not producing driving on nuclear bag fibers, or the latter would terminate relatively distant from the primary ending as compared with the former.