Effects of slow muscle stretch on the responses of primary and secondary endings to small amplitude periodic stretches in de-efferented soleus muscle spindles.

The responses of primary and secondary endings of de-efferented soleus spindles to small amplitude periodic stretches superimposed on slow ramp stretches have been studied. For primary endings, the responses become progressively larger especially during the last period of the ramp whereas for secondary endings, after an initial period of moderate growth, the amplitude of the responses display a relative reduction precisely when the responses of the primary endings are the largest. These differences can be interpreted in terms of progressive increase in stiffness of the striated polar portions of the intrafusal muscle fibers.