Fusimotor actions on sensitivity of spindle secondary endings to slow muscle stretch in cat peroneus tertius

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Abstract
The effects of stimulating single .gamma.-axons on the responses of secondary endings to very slow ramp stretches have been examined in cat peroneus tertius muscle; 180 examples of the action of a .gamma.-axon on the response of a secondary ending were observed. Of the 3-6 .gamma.-axons acting on a secondary ending, only 1 or 2 elicited a significant steepening in the slope (i.e., increase in discharge frequency produced by 1 mm lengthening of the muscle) of the ending''s response to slow muscle stretch. The average change in slope observed in such instances was a two-fold increase for fusimotor stimulation at 50/s. These .gamma.-axons did not exert similar effects on all the endings they activated. In about half of the instances in which .gamma. stimulation steepened the slopes of secondary endings responses to slow muscle stretch, this effect increased with the rate of stimulation up to 100/s. In the other instances maximal effects were observed for relatively low rates of stimulation (30-70/s); for higher rates there was no further increase in the slope or even a relative decrease. The actions of fusimotor axons on the position sensitivity of secondary endings measured under static conditions were similar to their actions on lengthening sensitivity measured during slow movement. The action of a .gamma.-axon on the sensitivity of a secondary ending may depend on a specific pattern of distribution of the axon to intrafusal muscle fibers.