18 Overview of Motor Systems
The firing rates of our motor neurons, and therefore the states of contraction of our muscles, are determined by multiple influences. Simple reflex arcs like the stretch reflex and more complex motor programs like the basic pattern generator for walking are built into the spinal cord and brainstem. These reflex arcs and motor programs, as well as the motor neurons themselves, are in turn influenced by various descending pathways. Finally, activity in the descending pathways is modulated by other cortical areas, the basal ganglia (see Chapter 19), and the cerebellum (see Chapter 20).
Each Lower Motor Neuron Innervates a Group of Muscle Fibers, Forming a Motor Unit
Each lower motor neuron innervates a fraction of the muscle fibers in one muscle. The combination of a lower motor neuron and all the muscle fibers it innervates is a motor unit (Fig. 18-1). Although there is a range of motor unit sizes in every muscle, their average size varies in a predictable way—those involving finely controlled muscles (e.g., extraocular muscles) contain very few muscle fibers, and those involving less finely controlled muscles may have hundreds of muscle fibers.
Lower Motor Neurons Are Arranged Systematically
The cell bodies of lower motor neurons are arranged systematically in the anterior horn (just as things like body parts and retinal areas are represented systematically in pathways and cortical areas). At any given spinal level, motor neurons for more proximal muscles are located medial to those for more distal muscles, and motor neurons for flexors are located dorsal to those for extensors (Fig. 18-2).
There Are Three Kinds of Muscle Fibers and Three Kinds of Motor Units
We use most muscles for multiple purposes that require different forces of contraction, from relatively weak contractions used in holding positions for long periods of time (e.g., standing) to powerful contractions that cannot be sustained for very long (e.g., sprinting, jumping). Corresponding to this, there are three different kinds of muscle fibers: slow fibers (S) that produce little force but do not fatigue much, fast fibers (FF) that produce a lot of force but fatigue quickly, and intermediate fibers (FR) with intermediate properties. All the muscle fibers innervated by a given motor neuron are of the same type, so there are also three kinds of motor units (Table 18-1).
