Self-reinnervated cat medial gastrocnemius muscles. I. Comparisons of the capacity for regenerating nerves to form enlarged motor units after extensive peripheral nerve injuries. 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 2:0266N, 1995.
SUMMARY AND CONCLUSIONS
1. The aims of this study were to determine whether 1) regenerating motor axons have the capacity to form enlarged motor units (MUs) in muscles reinnervated by few motoneurons and 2) the type of nerve injury, repair and/or growth environment affects this capacity. 2. MU innervation ratio (IR) was estimated by measuring isometric unit tetanic force in reinnervated cat medial gastrocnemius (MG) muscles 3-16 mo after denervation by either 1) crushing its nerve, 2) transecting the nerve and suturing the proximal end to either the distal stump (N-N suture), or 3) directly to the muscle fascia (N-M suture). In addition, the number of regenerating axons was experimentally reduced by cutting one of two contributing ventral roots. 3. Muscles were reinnervated by 2-88% of their normal complement of MUs. Mean unit tetanic force increased as the number of reinnervated MUs decreased in reinnervated muscles after nerve crush or N-N suture, but not after N-M suture even when few axons made functional connections. When MU number was <20% of normal, mean unit force was significantly higher in reinnervated muscles following nerve crush as compared to muscle reinnervated after N-N suture. 4. The cross-sectional areas (CSAs) of all muscle fiber types were similar to normal in reinnervated muscles after nerve crush, but the CSAs of Type IIa and IIb fibers were significantly smaller in muscles reinnervated after complete nerve transections (ie. N-N or N-M sutures). 5. When MU force was normalized to mean muscle fiber CSA, cut motor axons displayed the same capacity to form enlarged MUs as crushed motor axons. The force of the MUs increased by as much as 5-8 times that of normal, provided the axons grew along the distal nerve stump (N-N suture). 6. Tetanic force increased in the normal order S 
Received 12 May 1995; accepted in final form 9 August 1995.
APS Manuscript Number J290-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.