• Mika Karasawa, Kumiko Yokouchi, Akira Kakegawa, Kyutaro Kawagishi, Tetsuji Moriizumi, and Nanae Fukushima.
• Department of Anatomy, Shinshu University School of Medicine, Matsumoto, Japan.
• J. Neurosurg. 2015 Jun 1; 122 (6): 143814431438-43.

ObjectThe purpose of this study was to determine the minimum amount of nerve fibers required to maintain normal motor function after nerve injury in rats.MethodsThe authors first confirmed that a common peroneal nerve injury caused more aggravating effects on lower limb motor function than tibial nerve injury, as assessed by the static sciatic index (SSI). Thereafter, rats were subjected to varying degrees of crush injury to the common peroneal nerve. At 48 hours after the injury, motor function was assessed using the SSI and slope-walking ability (with slope angles of 30° and 45°). The tibialis anterior muscle, a main muscle innervated by the common peroneal nerve, was removed. Muscle sections were co-labeled with neuronal class III β-tubulin polyclonal antibody to identify the presence of axons and Alexa Fluor 488-conjugated α-bungarotoxin to identify the presence of motor endplates.ResultsThe evaluation of neuromuscular innervation showed a correlation between SSI scores and ratios of residual axons (rs = 0.68, p < 0.01), and there was a statistically significant difference between slope-walking ability and ratios of residual axons (p < 0.01). Moreover, the ratios of residual axons in the nerve-crushed rats with normal motor function (SSI above -20) ranged from 36.5% to 88.7%, and those ratios in the success group with slope-walking angles of 30° and 45° ranged from 14.7% to 88.7% and from 39.8% to 88.7%, respectively.ConclusionsIn this study of rodents, less than half of the motor axons were sufficient to maintain normal motor function of the lower limb.

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