The fine structure of denervated and reinnervated muscle spindles: morphometric study of intrafusal muscle fibers.

The fine structure of normal, denervated, and reinnervated muscle spindles in lower lumbrical muscles of rats was studied morphometrically at time intervals ranging from 3-14 months. In control spindles, the mean transverse area of mitochondria was estimated to be more than twice as large in nuclear chain than in typical nuclear bag fibers. Following denervation, there was a severe decrease of the mean number and transverse area of mitochondria, and a moderate, but statistically significant decrease of the mean transverse area of intrafusal muscle fibers (IMFs) despite an increase of the number of IMFs. At 12-14 months of reinnervation, changes of the transverse areas of IMFs were statistically insignificant, but the mean values for the mitochondria were incompletely restored. At 4 x 3 months, after fourfold repeated crush injuries to the nerve, most of the values estimated (transverse area of mitochondria; number, shape, and transverse area of IMFs and nuclei) tended to approach those in denervated rather than in reinnervated IMFs. The differences of the reactions of intra- and extrafusal muscle fibers following complete motor and sensory denervation appeared to be in accordance with their normal dimensional dissimilarities.