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

1. The effects exerted on secondary endings by repetitive stimulation of single static or single dynamic fusimotor fibres were studied in nine tenuissimus spindles in cats.2. The discharge of each secondary ending was recorded simultaneously with the discharge of the primary ending belonging to the same spindle.3. All the nineteen static fusimotor fibres studied activated secondary endings.4. Of eight dynamic fusimotor fibres, seven had no action on secondary endings. One dynamic fibre activated an atypical secondary ending which displayed some phasic sensitivity.5. No difference in conduction velocity was found between static and dynamic fibres.6. The implications of these observations for the mechanism of action of static and dynamic fusimotor fibres are discussed.