Long-term effects of partial denervation on sprouting and muscle fiber area in rat plantaris.

The long-term effects of partial denervation on the muscle fiber cross-sectional area, degree of axonal sprouting, and end-plate morphology were examined 1, 3, 6, 9, and 12 months postsurgery in rat plantaris muscle. After 1 month of partial denervation, mean cross-sectional areas of type I and type II myofibers were significantly lower than that of sham-operated controls; fiber atrophy and hypertrophy was observed. After 3 months, we found no statistical difference in the mean cross-sectional fiber area between the two experimental groups. After 6 months, the fiber areas were now significantly larger than controls, possibly the result of compensatory work hypertrophy, due to the overuse of remaining hyperexpanded motor units. Preterminal, intranodal, and intraterminal sprouting were found to significantly increase from 1 to 6 months following partial denervation. While sprouting was enhanced, the number of terminal branch points per end plate did not change. Following 9 months of partial denervation and overuse, the mean fiber areas significantly decreased as compared to controls. The former muscles were found to contain angulated fibers, group atrophy, and increased levels of axonal sprouting. The number of terminal branches per end plate was now significantly increased over control values, possibly a compensatory response to reduced synthesis of neurotrophic factor(s) and/or transmitter-related components. At 12 months, fiber areas, axonal sprouting, and the number of terminal branches per end plate have all decreased. Degenerating end plates, denervated myofibers, angulated fibers, and group atrophy were observed. It would appear that aging-like changes are occurring earlier in chronically stressed, partially denervated muscles.