ELSEVIER Brain Research 676 (1995) 113-123 BRAIN RESEARCH Research report The hyper-reinnervation of rat skeletal muscle Todd A. Kuiken a,*, Dudley S. Childress a, W. Zev Rymer a,b,c a Department of Biomedical Engineering, Northwestern University, Evanston, IL, USA b Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL, USA c Veterans Administration, Lakeside, Chicago, IL, USA Accepted 27 December 1994 Abstract This study examines muscle recovery and related changes in the motor unit population of 'hyper-reinnervated' rat skeletal muscle. Medial gastrocnemius (MG) muscles were hyper-reinnervated by either cutting the MG nerve and implanting it on the MG muscle together with additional hind limb nerves, or by crushing the MG nerve and excising the medial portion (50-70%) of the MG muscle. Our findings were that muscles hyper-reinnervated with multiple nerves recovered muscle mass and strength more fully than did the self-reinnervated muscles, more motor units were formed (up to three times the normal number were found), and the mean motor unit size was significantly smaller. A relatively small percentage of muscle fibers became polyneuronally innervated. In contrast, the number of motor units that were formed in the muscle reduction experiments were not significantly larger than was expected considering the mass of the muscles. We conclude that hyper-reinnervation improves muscle recovery, it may be a useful technique for improving function in denervated muscle, and may serve to provide added sources of EMG control signals in some amputees. Keywords: Reinnervation; Hyperinnervation; Hyper-reinnervation; Motor unit; Rat; Muscle 1. Introduction When a motor axon is cut or severely crushed the segment distal to the injury degenerates. Over time, the recovering proximal segment of the axon then sends out projections that regenerate into the dener- vated muscle, and try to establish functional connec- tions with muscle fibers by some kind of competitive reinnervation process. At present, we do not fully understand the factors determining the size of the resulting motor units, and more specifically, the rela- tive impact of such variables as the total number of available reinnervating neurons, and other regional anatomical constraints influencing the regeneration of motor axons. Our study used the experimental inter- vention of grafting an excessive number of motoneu- rons onto denervated muscle, which increased the com- petition between regenerating neurons and stressed * Corresponding author. Rehabilitation Institute of Chicago, Med- ical Education, 345 E. Superior St., Chicago, IL 60611, USA. Fax: (1) (312) 908-6998. 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0006-8993(95)00102-6 the process of motor unit formation. This model, called 'hyper-reinnervation', provides an interesting test of the competitive reinnervation process. Hyper-reinnervation also has the potential for im- proving the recovery of denervated muscle. Muscle often fails to regain its normal mass and strength after nerve injury, especially if the nerve to the muscle has been transected [4]. This is due to a failure of some muscle fibers to become reinnervated. We hypoth- esized that grafting an increased number of motoneu- rons on a denervated muscle would increase the likeli- hood that any given muscle fiber would be found by a motoneuron and reinnervated. This would improve the overall recovery of the muscle, and potentially provide useful therapeutic outcomes. In theory, there are three ways in which an excess of motoneurons could influence motor unit formation in the hyper-reinnervation model. The size of the motor units formed may be smaller; only a fraction of the motoneurons may reinnervate the muscle; and/or the motor units formed could overlap with each other, so that individual muscle fibers may be multiply inner- vated from different axons (i.e. polyneuronal innerva-