AbstractFollwing partial deneration motor units can increase (by selfreinnervation) as much as four to five times their normal size. To investigate the still unknown quantitative reinnervation capacity of a motor nerve in the case of foreign‐reinnervation, in adult male rats the denervated sternomstoid muscle was either self‐reinervated by its original nerve or foreign‐reinnervation by the omohyoid nerve, which had to reinnevate the three times the amount of muscle fibers and six times the amount of muscle mass. After survival times of 7, 8, 9, or 10 months, nerves and muscles were investigated histochemically and immunohistochemically. The omohyoid nerve could fully reinnervate the sternomastoid muscle, but at 7 and 8 months this muscle still revealed nearly the same proportions of IIA and IIB fibers as were seen in the self‐reinnervated sternomastoid at all stages. However, in the following 2 months a shift of the fiber pattern toward that of the normal omohyoid was observed, as evidenced by a strong increase in type IIB fibers (from 24% to 62%), at the expense of type IIA fibers. These findings are in contrast to those after foreign (cross) reinnervation of leg muscles where the fiber transfor mation (according to the foreign motor input) occurs in parallel with the reinnervation process during the first 2‐3 months. The delayed fiber transformation observed could be the consequence of the highly enlarged peripheral field of the omohyoid motoneuron pool or could merely reflect a general difference between limb and neck muscles. Whatever the case may be, in the present experiments the afferent input, which is not fully restored until 9 months, could play a crucial role in contrast to limb muscles, where the successful motor reinnervation takes place in the absence of sensory
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