Impulse rates and sensitivity to stretch of soleus muscle spindle afferent fibers during locomotion in premammillary cats

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Abstract
Impulses from soleus muscle afferents were recorded in premammillary cats that were walking on a treadmill. In normal walking, the effects of .gamma.-motoneurons on impulse rates of muscle spindle afferents are confounded by the effects of the large length changes that occur. To isolate the effects of .gamma.-motoneurons the leg was fixed in place for recording and denervated except for soleus muscle. Because .gamma.-motoneurons produce marked effects on the stretch sensitivity of muscle afferents, soleus muscle was oscillated about a preset length so the stretch sensitivity of its afferents could be determined. The impulse [imp] rate of secondary muscle spindle afferents in soleus muscle was generally increased at all phases of the step cycle. The mean rate approximately doubled during walking (82 imp/s), compared with nonwalking (rest) periods (44 imp/s). The sensitivity to sinusoidal length changes was generally reduced throughout the step cycle (mean reduction = 33%). With this pattern of activating .gamma.-motoneurons, the secondary muscle spindle afferents will provide a good feedback signal of the large length changes that normally occur in the muscle during locomotion. The changes in sensitivity of primary muscle spindle afferents will complement central changes so the gain of the stretch reflex from extensors is high during extension (when required to help support the weight of the body) and low during flexion (when a high gain would be counterproductive).