Branching pattern of the motor nerve endings in a skeletal muscle of the adult rat.

We have performed a morphometric analysis of the branching pattern of the rat sternomastoid motor nerve terminals in normal and regenerated singly-innervated synaptic areas, in junctions with an accessory ending and also in dually-innervated synaptic areas. We tried to seek measurable differences related to the terminal complexity and size in these different situations. The number of sole-plate nuclei is always significantly correlated with the nerve terminal parameters and unrelated to the muscle cell diameter. A significant correlation between the terminal complexity and the postsynaptic size is found only in the smallest endings whereas correlation is lost with the increase in nerve terminal complexity. In normal singly-innervated areas, the mean length of the distal free-end segments of the terminal arborisation becomes reduced with the increase in complexity of the terminals, whereas the mean length of the segments between branching points and the mean angle of branch emergence is, on average, constant. Whereas the accessory endings show the same branching pattern as the endings during the initial days of regeneration, the parent ending can be considered as a miniature version of the endings in normal singly-innervated junctions. In dually-innervated areas, the smallest endings show a similar branching pattern to the accessory endings and the endings during the initial process of reinnervation, whereas the larger endings show a significant reduction in the length of the distally placed segments compared to the normal endings in singly-innervated areas. Results indicate that the terminal branching pattern is not haphazard but, on the contrary, general rules and tendencies can be measured.