Motor unit and muscle fiber type grouping after peripheral nerve injury in the rat.
Muscle unit (MU) fibers innervated by one motoneuron and corresponding muscle fiber types are normally distributed in a mosaic. We asked whether, 4-8months 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.
Department of Surgery, Division of Plastic Surgery, The Hospital for Sick Children, University of Toronto, ON M5G 1X8, Canada. Electronic address: tessat.gordon@gmail.com.
Tessa
T
Gordon
Division of Neuroscience, Faculty of Medicine and Dentistry, University of Alberta, Edmonton T6G2S2 T6G, Canada.
Joanne E Totosy
JET
de Zepetnek
D000818:Animals; D004195:Disease Models, Animal; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor; D006003:Glycogen / Q000172:deficiency; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000473:pathology*; D018482:Muscle, Skeletal / Q000473:pathology / Q000503:physiopathology; D009416:Nerve Regeneration / Q000502:physiology; D010523:Peripheral Nervous System Diseases / Q000473:pathology* / Q000503:physiopathology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D006003:Glycogen
Fiber type grouping; Muscle fiber type distribution; Nerve branching; Nerve regeneration; Skeletal muscle reinnervation; The size principle