Changes in motor unit properties in SOD1 (G93A) rats.

INTRODUCTION: In amyotrophic lateral sclerosis (ALS), progressive death of motor neurons results in denervation and reinnervation of muscles. It is not clear how ALS affects the properties of motor units (MUs). METHODS: Properties of single MUs in the medial gastrocnemius (MG) muscle of rats bearing the human mutated superoxide dismutase gene type 1 (SOD1) were determined at 3 stages: asymptomatic (ALS I); early symptomatic (ALS II); and terminal (ALS III). RESULTS: In ALS II, higher proportions of FF (fast fatigable) and S (slow) MUs were observed, whereas in ALS III higher percentages of S and lower percentages of FF MUs were noted compared with controls. S motor neurons reinnervated fast muscle fibers, and those MUs gained some properties of fast MUs, including lower fatigue resistance, greater force generation, and higher action potential amplitudes. CONCLUSION: Changes in MU properties of SOD1 rats have progressive and multidirectional character and speed depending on the MU type.