Nonselective motor innervation of intrafusal fibers in muscle spindles of the rat

Summary Distributions of motor axons to different types of intrafusal fiber were reconstructed from serial 1-m thick transverse sections of six poles of muscle spindle in the rat soleus. Motor axons innervated (dynamic) bag₁ fibers, or (static) bag₂ fibers in conjunction with chain fibers. However, approximately forty percent of axons that supplied the spindles synapsed on both bag₁ and bag₂, or bag₁ and chain fibers. The significance of this co-innervation of dynamic and static intrafusal fibers is discussed relative to the general organization and function of mammalian spindles.