Frequencygrams of rabbit spindle primary endings elicited by stimulation of fusimotor fibres.

1. Frequencygrams of rabbit spindle primary endings elicited by stimulation of single fusimotor axons were obtained by the method described by Bessou, Laporte & Pagès (1968a) which gives indirect information on the contraction of intrafusal muscle fibres.2. Repetitive stimulation of the majority of static axons (twenty out of twenty-five axons studied) gave frequencygrams showing discrete large peaks corresponding to each stimulus, even for rates of stimulation as high as 300-500/sec.3. Large responses to single stimuli were elicited by thirteen of these axons. The responses consisted of a fast-rising phase lasting 5-10 msec and declining phase lasting 30-50 msec.4. Repetitive stimulation of some static axons (five out of twenty-five) gave frequencygrams displaying a smooth contour with no sign of individual responses to single stimuli and with a slow rising and declining phase.5. Both kinds of ;static' frequencygrams may be obtained from the same spindle.6. Repetitive stimulation of dynamic axons gave frequencygrams of small amplitude with a slow rising and declining phase and no sign of individual responses, even at low rates of stimulation.7. Although rabbit spindles lack nuclear chain muscle fibres (Barker & Hunt, 1964), rabbit's frequencygrams were essentially similar to those of cat spindles. The functional implications of this similarity are discussed.