Brain Advance Access originally published online on August 10, 2006
Brain 2006 129(9):2471-2483; doi:10.1093/brain/awl184
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Comparison of the fastest regenerating motor and sensory myelinated axons in the same peripheral nerve
1 Division of Neurophysiology, Institute of Medical Physiology Panum Institute, University of Copenhagen, Denmark 2 Department of Plastic Surgery, The Neuroscience Center Rigshospitalet, Copenhagen, Denmark 3 Department of Plastic Surgery, Clinical Neurophysiology, The Neuroscience Center Rigshospitalet, Copenhagen, Denmark
Correspondence to: Christian Krarup, MD, DMSc, FRCP, Department of Clinical Neurophysiology, The Neuroscience Center, NF3063, Rigshospitalet, 9 Blegdamsvej, 2100 Copenhagen, Denmark E-mail: ckrarup{at}rh.dk
Functional outcome after peripheral nerve regeneration is often poor, particularly involving nerve injuries far from their targets. Comparison of sensory and motor axon regeneration before target reinnervation is not possible in the clinical setting, and previous experimental studies addressing the question of differences in growth rates of different nerve fibre populations led to conflicting results. We developed an animal model to compare growth and maturation of the fastest growing sensory and motor fibres within the same mixed nerve after Wallerian degeneration. Regeneration of cat tibial nerve after crush (n = 13) and section (n = 7) was monitored for up to 140 days, using implanted cuff electrodes placed around the sciatic and tibial nerves and wire electrodes at plantar muscles. To distinguish between sensory and motor fibres, recordings were carried out from L6–S2 spinal roots using cuff electrodes. The timing of laminectomy was based on the presence of regenerating fibres along the nerve within the tibial cuff. Stimulation of unlesioned tibial nerves (n = 6) evoked the largest motor response in S1 ventral root and the largest sensory response in L7 dorsal root. Growth rates were compared by mapping the regenerating nerve fibres within the tibial nerve cuff to all ventral or dorsal roots and, regardless of the lesion type, the fastest growth was similar in sensory and motor fibres. Maturation was assessed as recovery of the maximum motor and sensory conduction velocities (CVs) within the tibial nerve cuff. Throughout the observation period the CV was 
14% faster in regenerated sensory fibres than in motor fibres in accordance with the difference observed in control nerves. Recovery of amplitude was only partial after section, whereas the root distribution pattern was restored. Our data suggest that the fastest growth and maturation rates that can be achieved during regeneration are similar for motor and sensory myelinated fibres.
Key Words: nerve; regeneration; laminectomy; cuff electrodes; cat
Abbreviations: CMAP, compound muscle action potential; CNAP, compound nerve action potentials; CRP, compound root action potential; CV, conduction velocity
Received March 28, 2006. Revised May 30, 2006. Accepted June 15, 2006.