Frequencygrams of spindle primary endings elicited by stimulation of static and dynamic fusimotor fibres.

1. Frequencygrams of cat spindle primary endings, obtained by the method described by Bessou, Laporte & Pagès (1968), were elicited by the stimulation of single static and dynamic fusimotor fibres.2. Stimulation of static fibres by single stimuli may elicit: (i) no response, (ii) responses of small amplitude, (iii) responses of large amplitude with a pause after the rising phase.3. Stimulation of static fibres by repetitive stimuli gives frequencygrams resembling records of tetanic contraction of skeletal muscle. At a frequency of stimulation of 180-200/sec periodic oscillations are perceptible on the plateau of the frequencygram.4. Stimulation of dynamic fibres by single stimuli may elicit: (i) no response, (ii) responses smaller and longer than the responses given by static fibres.5. When the muscle is stretched the amplitude of the responses elicited by dynamic fibres decreases whereas the amplitude of the responses given by static fibres increases.6. Frequencygrams obtained by repetitive stimulation of dynamic fibres show periodic oscillations only at low rates of stimulation. At rates of stimulation higher than the minimal rate giving frequencygrams with a smooth contour, the amplitude of the frequencygram increases.