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Motor Unit Muscle Fiber Clumping in Large but not Small Reinnervated Muscles as an Indicator of Reduced Innervation after Peripheral Nerve Injuries and the Return of Fiber Type Compartmentalization
Tessa Gordon, PhD; DSc; Joanne Totosy de Zepetnek, PhD
University of Alberta, Edmonton, AB, Canada
Introduction: Motor unit (MU) muscle fibers innervated by one motoneuron, and muscle fiber types are normally distributed in a mosaic pattern in muscle cross-sections. A muscle nerve normally branches to supply muscle fibers in separate muscle compartments with slow and fast nerve fibers innervating deep and superficial compartments, respectively. We showed that this distribution is restored after reinnervation of large hindlimb muscles with MU and fiber type clumping apparent only when the nerve supply was reduced. Here we asked whether 1) MU muscle fiber and muscle fiber types 'clump' (defined as a significant increase in numbers of the fibers that are adjacent to one another) in a smaller reinnervated rat tibialis anterior (TA) muscle, and 2) regenerating slow and fast nerves reinnervate their original muscle compartments as they do in larger muscles.
Materials and Methods: TA muscle and MU contractile forces were recorded in normal muscles and in reinnervated muscles, 4-6 months after common peroneal nerve transection and surgical repair with random alignment of the nerve stumps. Glycogen-depletion for visualization and enumeration of MU muscle fibers was accomplished by exhaustive electrical stimulation of the nerve supply. Muscle fiber types identified with histochemical staining, were counted.
Results: First, the MU muscle fibers occupied defined territories in serial muscle cross-sections. The territories were significantly smaller in reinnervated TA muscles despite all motoneurons regenerating their nerves and reinnervating the muscles. The reinnervated MU muscle fibers and fiber types were clumped within 1 to 3 groups with significantly more MU muscle fibers lying adjacent to one another in parallel with a corresponding increase in muscle fiber type adjacencies. Second, reinnervated slow muscle fibers were localized to the deep muscle compartment as they are normally despite the failure of regenerating nerve fibers to reinnervate their former muscle fibers. Numbers of reinnervated slow muscle fibers were significantly increased and a significant proportion were located abnormally in the superficial compartment.
Conclusions: We conclude that 1) reinnervated rat MU muscle fibers clump within smaller muscle territory areas and 2) most but not all reinnervated slow muscle fibers locate normally to the deep muscle compartment. The significance of the findings is that fiber type clumping in small muscles that have relatively few MU muscle fibers in small territories, is not a reliable indicator of the extent of nerve regeneration whereas, in large muscles, 'clumping' may serve to indicate reduced numbers of intact MUs after complete or partial nerve injuries.
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