Contractile properties of cat motor units enlarged by motoneurone sprouting.

The fast-twitch flexor digitorum longus (FDL) muscle was partially denervated by a unilateral section of the L7 ventral root. After approximately 100 days the isometric, force-velocity and histochemical properties of single motor units from the partially denervated muscle were determined. The diameter of the nerve axons supplying the muscle were also examined. The extrapolated maximum speed of sarcomere shortening (Vmax) of the enlarged motor units was significantly less than that of the control whole muscle Vmax. It is concluded that the motoneurone is capable of expanding its territory and therefore its tension generating capability without any apparent change in axonal diameter. In addition, the intrinsic properties of the muscle fibres are altered.