Chapter 4 - Motor Units and Muscle Receptors

Abstract

The motor unit is composed of the motoneuron, its axon branching within a muscle, and the innervated muscle fibers. There are three types of motor units: slow (S), fast resistant to fatigue (FR), and fast fatigable (FF). Differences between them concern morphological and electrophysiological properties of motoneurons, a number, morphology, biochemical characteristics, and metabolism of muscle fibers forming each unit, contractile times, forces and fatigue resistance. Recruitment of motor units into a contraction depends on the excitability of motoneurons and correlates with the properties of muscle fibers. The number of active units, their position and mutual relations within a muscle as well as frequencies and patterns of discharges generated by motoneurons determine the muscle force during subsequent phases of movement. The properties of motoneurons and motor units are subject to plastic modifications during development and aging, as a result of various forms of motor activity or disease. The motor unit activity is closely connected to the muscle receptors, primarily muscle spindles, and tendon organs which send information about the spatial position of the body and the limbs to the central nervous system, thus contributing to the control of precise movements.