Self-reinnervated cat medial gastrocnemius muscles. II. Analysis of the mechanisms and significance of fiber type-grouping in reinnervated muscles. Rafuse, Victor F., and Tessa Gordon. Department of Pharmacology, Division of Neuroscience, 513 Heritage Medical Research Centre, University of Alberta, Edmonton, Alberta, Canada, T6G 2S2.
APStracts 2:0267N, 1995.
SUMMARY AND CONCLUSIONS
1. The technique of glycogen depletion was used to determine whether 1) regenerating motor axons reestablish the normal regionalization of motor units (MUs) in the cat medial gastrocnemius (MG) muscle, 2) the extent of clumping between MU fibers and/or type-grouping of muscle fibers progressively increases with a decrease in reinnervated MU numbers and 3) the pattern of innervation can explain why MUs fail to increase significantly in size when the cut nerve is sutured directly to the muscle even when few axons make functional connections. 2. Distributions of MU fibers were analyzed in 5 normal and 14 reinnervated cat MG muscles 4.5 - 16 mo after sectioning its nerve and suturing the proximal end to the distal nerve sheaths (N-N suture) or directly to the muscle fascia (N-M suture). Muscle unit distributions were quantified according to location, territory size, density, and extent of clumping between fibers from the same MU. 3. Normal MU fibers were regionalized within 5 regions along the muscle's longitudinal and transverse axes. Reinnervated MUs were located within similar regions indicating that regenerating axons follow the major proximal nerve branches to restore normal compartmentalization. 4. Muscle unit fibers were diffusely scattered within discrete MU territories in normal muscles. Territory size tended to increase with MU size while density of muscle unit fibers within the territory decreased. 5. Territories increased with MU size after N-N suture but were smaller and showed little size variation after N-M suture. The extent of muscle unit fiber clumping was inversely related to the number of reinnervated MUs. On average the extent of clumping was substantially higher in muscles reinnervated after N-M suture. These results indicate that distal nerve sheaths facilitate proximal axon branching which establishes MU territory size. Once the territory is established motor axons branch distally to increase MU size which in turn compensates for reduced MU numbers. 6. Muscles reinnervated by <80% of the MUs exhibited fiber type-grouping of Type I fibers and on average the extent of clumping was substantailly higher in muscles reinnervated after N-M suture fibers. With less innervation, type-grouping increased inversely with the number of reinnervated MUs. However, for a similar number of MUs, Type I fiber type-grouping was substantially higher in muscle reinnervated after N-M suture. Type-grouping therefore, reflects muscle unit fiber clumping under conditions where MU size increased (N-N suture) or MU territory size decreased (N-M suture). 

Received 12 May 1995; accepted in final form 9 August 1995.
APS Manuscript Number J264-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.