Chapter 17 Limited Plasticity of Adult Motor Units Conserves Recruitment Order and Rate Coding

Abstract

Publisher Summary Motoneuron and muscle unit properties are remarkably well matched for function. Normally, motoneurons and their muscle fibers, the motor units, are recruited according to size, with the smallest and slowest motor units recruited before the larger and faster motor units. The duration of the after-hyperpolarization of the motoneurons' action potential limits the rate of firing and is well matched to the contraction time of the muscle units. The slow motor units that are the most excitable and most readily recruited are also the most fatigue resistant (FR). They contract with little or no fatigue. Progressively, larger and faster motor units are recruited into activity as muscle force is increased by voluntary effort. The low force fast motor units are FR in contrast to the large forceful fast motor units that are classified as fast fatigable (FF) units in accordance with their high susceptibility to fatigue. The basis for the matching of motoneuron and their muscle unit properties is not fully understood.