Reorganization of motor-unit properties in reinnervated muscles of the cat.

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Abstract
Single motor units from the triceps surae muscle group were studied in cats using newly developed chronic recording methods as well as more conventional acute methods. Motor-unit properties in each muscle were recorded before and at different intervals after nerve section and surgical repair so that the pattern of reinnervation could be compared to control values for that muscle. In normally innervated muscles, the tension and contractile speed of single motor units are directly correlated with the amplitude of the extracellularly recorded axonal potential. These normal relationships were absent when the muscle 1st became reinnervated after sectioning and suturing the medial gastrocnemius (MG) or lateral gastrocnemius and soleus (LGS) nerves to their distal nerve stump or directly to the muscle. As the reinnervated nerves and muscles recovered from denervation, the normal relationships again emerged. The relationship between the contractile speed and the tension of motor units returned to control levels when the muscle had recovered about 50% of the final tension eventually attained. The relationship between the tension of the motor unit and the amplitude of the extracellularly recorded axonal potential recovered more slowly, but eventually approached control levels. Motor units were further classified according to their speed of contraction and susceptibility to fatigue into three groups: fast, fatigable (FF); fast, fatigue resistant (FR) and slow (S). The percentage of each type of unit in reinnervated muscles was similar to that in control muscles. The relative amplitude of the extracellular motor axon potential and the twitch tension produced was ordered, with FF > FR > S in reinnervated and control motor units. Nerve and muscle properties become well matched in single motor units several months after reinnervation. Since nerve axons do not reinnervate their original muscle fibers, this matching involves a respecification of properties in single muscle fibers. Possible mechanisms underlying these changes are discussed.