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

Abstract

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.