Spindle secondary ending responses elicited by stimulation of static fusimotor axons.

1. Indirect evidence on the time course of intrafusal events associated with stimulation of a static fusimotor axon was obtained by analysis of the responses from muscle spindle secondary endings of the cat.2. In many instances, a single impulse in a single static fusimotor axon to a muscle spindle evoked a transient increase in discharge of the secondary ending the duration of which ranged from 30 to 60 msec with a rising phase of 6-20 msec. Increasing the length of the muscle increased the slope of the rising phase and augmented the magnitude of the response.3. Different static fusimotor axons produced different effects upon secondary endings. A single impulse in some fusimotor axons did not produce an effect while repetitive stimulation produced a smooth increase in discharge frequency. When efficacious, a single fusimotor impulse evoked approximately a similar duration of increased discharge from a secondary ending and a primary ending of the same muscle spindle. The relative magnitude of effects of a given fusimotor axon on primary and secondary endings of a spindle varied.4. During partial curarization the changes in the discharges of a secondary ending evoked by stimulation of a single static fusimotor axon occurred in a step-like fashion.5. These results are interpreted as supporting the idea that static fusimotor axons induce twitch-like contractions of intrafusal elements (nuclear chain) through the intermediary of propagated action potentials.