Morphological features of collateral innervation and supernumerary innervation in the skeletal muscles of presenile rats.

Using silver impregnation either with or without cholinesterase staining, this study was designed to investigate the morphological patterns of remodeling in both the arborization of axon terminals and in the subneural apparatus of the motor endplate in adult rats (3, 6, and 12 months old). The coincidental growth of the nerve terminal and muscle fiber was observed to continue, and the number of muscle fibers remained unchanged up to 12 months of age. In 12-month-old muscles, as compared with those of the younger subjects, the frequencies of 1) terminals with signs of degeneration, growth, or both, 2) denuded postsynaptic cholinesterase sites not associated with the overlying axon, 3) collateral innervation, and 4) supernumerary innervation, were all seen to have increased. The functional terminal innervation ratio was increased to 1.06. The characteristic features consisted of enhanced collateral formation by means of ultraterminal sprouting, and of multiple axons proceeding to an already innervated endplate. An imbalance between growth and degeneration in the motor endplates during the presenile stages is thus a likely stimulus for the particular compensatory changes to increase the size of the motor unit.