Muscle Plasticity During Sprouting and Reinnervation

Abstract

SYNOPSIS. When peripheral nerves are cut, the axotomized nerves and denervated muscles undergo atrophic changes which are reversed only when functional connections are remade in the periphery. The restored interaction completely reverses the effects of axotomy and denervation and leads to rematching of the size of the motoneuron, muscle unit force, speed and histochemical properties, according to the size principle. Differences in unit force and fatigue characteristics between motor unit types are not fully restored in reinnervated muscles but do not obscure size relationships between the motoneurons and their muscle units. Although intact motoneurons will supply increased numbers of muscle fibers after partial nerve injuries, regenerating axons appear to be limited in their ability to enlarge their muscle units. Increased motor unit force in reinnervated slow motor units is accounted for primarily by an increase in fiber diameter; fast motor units do not increase their mean force output. As a result of the rematching of muscle unit properties with the size of the motoneurons that reinnervate them, motor unit properties are appropriate for fine control of movement after complete or partial nerve injuries. However, regenerating axons do not reinnervate their original muscle fibers and unless the fibers are injured close to the muscles, they often fail to reinnervate their original muscles. The mismatching of motor pools with inappropriate target muscles is probably the main factor responsible for poor recovery of motor function after complete nerve injuries.