Is resistance of a muscle to fatigue controlled by its motoneurones?

Abstract

The original experiment of Buller et al.¹ and the many subsequent confirmatory reports clearly show that the time-to-peak tension and many other speed-related parameters of slow and fast muscle fibres are dictated by the motoneurone². It has been concluded that the motoneurone exerts this control of the physiological and associated biochemical properties by the frequency at which it excites the muscle fibre³. However, no studies have been reported on the fatigue properties and the associated biochemical characteristics after cross-reinnervation. Based on the ‘size principle’ of motoneurones⁴, it would be reasonable to assume that a muscle fibre reinnervated by a small motoneurone would be active often and that this would be manifested biochemically as an elevated oxidative capacity⁵. Also, it has been shown repeatedly that the mitochondrial content of a muscle fibre can be modified by daily endurance type exercise⁶. Thus, it would seem that the motoneurone at least indirectly also controls the mitochondrial content of a muscle fibre by controlling the degree of activity. We have now tested this hypothesis using self- and cross-reinnervated muscles in cats. We found that fast- and slow-twitch muscles retained their characteristic fatigue resistance properties regardless of whether the nerve to which they had become connected had originally innervated a fatigue-resistant or relatively fatiguable muscle.