Formation of muscle spindles in the absence of motor innervation.

Whether muscle spindles can form in muscles innervated only by afferents was investigated by removing the lumbosacral segment of the spinal cord immediately after crushing the nerve to the medial gastrocnemius (MG) muscle in newborn rats, and administering nerve growth factor for 10 days afterwards. The nerve-crushed MG muscles reinnervated by afferents in the absence of motor innervation were examined at postnatal (P) days 7, 9 and 30 for the presence of spindles by light and electron microscope. Reinnervated MG muscles contained spindle-like encapsulations of 1-4 fibers at 7, 9 and 30 days after the nerve crush. The number of spindles exceeded that of normal MG muscles, suggestive of de novo formation of spindles. All nerve-muscle contacts in the spindles had features of sensory endings, and intrafusal fibers expressed the spindle-specific slow-tonic myosin heavy chain (MHC) isoform at P30. No motor endplates were visible on any muscle fibers and extrafusal fibers were atrophied, as would be predicted in the absence of motor innervation. Thus, efferents are not essential for the formation and differentiation of muscle spindles in reinnervated muscles of neonatal rats.