<P> The number of muscle fibers within each unit can vary within a particular muscle and even more from muscle to muscle; the muscles that act on the largest body masses have motor units that contain more muscle fibers, whereas smaller muscles contain fewer muscle fibers in each motor unit . For instance, thigh muscles can have a thousand fibers in each unit, while extraocular muscles might have ten . Muscles which possess more motor units (and thus have greater individual motor neuron innervation) are able to control force output more finely . </P> <P> Motor units are organized slightly differently in invertebrates; each muscle has few motor units (typically less than 10), and each muscle fiber is innervated by multiple neurons, including excitatory and inhibitory neurons . Thus, while in vertebrates the force of contraction of muscles is regulated by how many motor units are activated, in invertebrates it is controlled by regulating the balance between excitatory and inhibitory signals . </P> <P> The central nervous system is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units . Henneman's size principle indicates that motor units are recruited from smallest to largest based on the size of the load . For smaller loads requiring less force, slow twitch, low - force, fatigue - resistant muscle fibers are activated prior to the recruitment of the fast twitch, high - force, less fatigue - resistant muscle fibers . Larger motor units are typically composed of faster muscle fibers that generate higher forces . </P> <P> The central nervous system has two distinct ways of controlling the force produced by a muscle through motor unit recruitment: spatial recruitment and temporal recruitment . Spatial recruitment is the activation of more motor units to produce a greater force . Larger motor units contract along with small motor units until all muscle fibers in a single muscle are activated, thus producing the maximum muscle force . Temporal motor unit recruitment, or rate coding, deals with the frequency of activation of muscle fiber contractions . Consecutive stimulation on the motor unit fibres from the alpha motor neuron causes the muscle to twitch more frequently until the twitches "fuse" temporally . This produces a greater force than singular contractions by decreasing the interval between stimulations to produce a larger force with the same number of motor units . </P>

Which of the following best describes the three critical components of a single motor unit