MODULATION OF SENSORY DISCHARGES BY EFFERENT SPINDLE EXCITATION

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Abstract
Afferent frequency changes during motor spindle activation were studied in the M. extensor longus dig. IV of toads. Single or repeated activation of large motor fibers (10-40 m/second) produces an initial high frequency sensory acceleration followed, by a slower discharge rate. Activation of small motor axons (2-8 m/second) produces gradually increasing sensory acceleration after a facilitation period of 1-2 seconds. Sensory frequency changes produced by independent stimulation of single large or small axons are not significantly modified by activation of additional large or small motor fibers. The sensory burst produced by activation of large motor fibers is followed by a pause ("off effect"). Such an effect does not appear after stimulation of small nerve fibers or when large and small motor fibers are activated simultaneously. In summary, sensory discharge patterns obtained during motor stimulation of the spindle seem to reflect intrafusal mechanical events such as twitch-like activity and slow, graded muscle contractions.