Motor unit and muscle fiber type grouping after peripheral nerve injury in the rat

• Motor unit muscle fibers supplied by one motoneuron are distributed in a mosaic.
• There are more adjacent muscle fibers in reinnervated motor units.
• Reinnervated motor units obey the same size principle as normal.
• Regenerating as well as normal nerves branch in a size-dependent manner.
• Regenerating slow nerves show preference for ‘slow’ motor nerve pathways.

Muscle unit (MU) fibers innervated by one motoneuron and corresponding muscle fiber types are normally distributed in a mosaic. We asked whether, 4–8 months after common peroneal nerve transection and random surgical alignment of nerve stumps in rat tibialis anterior muscles 1) reinnervated MU muscle and muscle fiber type clumping is invariant and 2) slow and fast motoneurons regenerate their nerve fibers within original endoneurial pathways. MU contractile forces were recorded in vivo, the MUs classified into types according to their contractile speed and fatigability, and one MU subjected to alternate exhaustive stimulation-recovery cycles to deplete glycogen for histochemical MU fiber recognition and enumeration, and muscle fiber typing. MU muscle fibers occupied defined territories whose size increased with MU force and muscle fiber numbers in normal and reinnervated muscles. The reinnervated MU muscle fiber territories were significantly smaller, the fibers clumped within 1–3 groups in 90% of the MUs, and each fiber lying adjacent to another significantly more frequently. Most reinnervated slow muscle fibers were normally located in the deep muscle compartment but substantial numbers were located abnormally in the superficial compartment. Our findings that well reinnervated muscle fibers clump in small muscles contrast with our earlier findings of clumping in large muscles only when reinnervated MU numbers were significantly reduced. We conclude that fiber type clumping is predictive of muscle reinnervation in small but not large muscles. In the latter muscles, clumping is more indicative of sprouting after partial nerve injuries than of muscle reinnervation after complete nerve injuries.