A further observation of muscle spindles in the extensor digitorum longus muscle of the aged rat.

We observed three novel muscle spindles in the extensor digitorum longus muscle of the aged (20 months) rat. Two muscle spindles of the three contained thin muscle fibers lacking sensory innervation between the layers of the spindle capsule and within the periaxial space, respectively. The other one contained sensory-innervated thin muscle fibers with an indistinct equatorial nucleation between the layers of the spindle capsule. These findings suggest that the occurrence of thin muscle fibers may be intimately related to the degeneration and regeneration of extrafusal muscle fibers during aging and that these newly formed thin muscle fibers may often fail to receive sensory innervation.