Variations of collateral branching in the motor nerve fibers in the skeletal muscle of adult rats.

Collateral branching (CB) applies to a collateral branch resulting in the innervation of two or more muscle fibers by one motor axon. Using a silver impregnation with or without cholinesterase (ChE) staining, the morphological pattern of innervation was investigated in the motor endplates of the adult rat soleus. Two types of variations of CB, type A and B, were noted. In type A (elaboration of CB), marked ramification of a single motor axon results in double, discrete endings (main and smaller accessory endings) on one muscle fiber and collateral formation on the other. The accessory endings were apposed by some small, clearly-defined separated units of the postsynaptic ChE site. In type B (double endings per endplate), small endings lying close to the main ending were either due to ramification of the motor axon innervating the neighboring muscle fiber (type B 1) or due to direct contact of non-branched, non-original motor axons (type B 2). The incidence of CB and its variations was small and the number of subterminal axons innervating more than one muscle fiber (functional terminal innervation ratio) was far below the level of partial denervation. In response to the local events of muscle, either type A or type B branching could serve as a useful tool to remodel the motor endplate for, e.g., its renewal, replacement and/or enlargement.