Action of static and dynamic fusimotor fibres on secondary endings of cat's spindles

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Abstract
The effects exerted on secondary endings by repetitive stimulation of single static or single dynamic fusimotor fibers were studied in 9 tenuissimus spindles in cats. The discharge of each secondary ending was recorded simultaneously with the discharge of the primary ending belonging to the same spindle. All the 19 static fusimotor fibers studied activated secondary endings. Of 8 dynamic fusimotor fibers, 7 had no action on secondary endings. One dynamic fiber activated an atypical secondary ending which displayed some phasic sensitivity. No difference in conduction velocity was found between static and dynamic fibers. The implications of these observations for the mechanism of action of static and dynamic fusimotor fibers are discussed.