Most mature extrafusal skeletal muscle fibers in mammals are innervated by only a single α motor neuron. Since there are more muscle fibers by far than motor neurons, individual motor axons branch within muscles to synapse on many different fibers that are typically distributed over a relatively wide area within the muscle, presumably to ensure that the contractile force of the motor unit is spread evenly (Figure 16.4). In addition, this arrangement reduces the chance that damage to one or a few α motor neurons will significantly alter a muscle's action. Because an action potential generated by a motor neuron normally brings to threshold all of the muscle fibers it contacts, a single α motor neuron and its associated muscle fibers together constitute the smallest unit of force that can be activated to produce movement. Sherrington was the first to recognize this fundamental relationship between an α motor neuron and the muscle fibers it innervates, for which he coined the term motor unit.

Figure 16.4

The motor unit. (A) Diagram showing a lower motor neuron in the spinal cord and the course of its axon to the muscle. (B) Each motor neuron synapses with multiple muscle fibers. The motor neuron and the fibers it contacts defines the motor unit. Cross (more...)

Both motor units and the α motor neurons themselves vary in size. Small α motor neurons innervate relatively few muscle fibers and form motor units that generate small forces, whereas large motor neurons innervate larger, more powerful motor units. Motor units also differ in the types of muscle fibers that they innervate. In most skeletal muscles, the small motor units innervate small “red” muscle fibers that contract slowly and generate relatively small forces; but, because of their rich myoglobin content, plentiful mitochondria, and rich capillary beds, such small red fibers are resistant to fatigue. These small units are called slow (S) motor units and are especially important for activities that require sustained muscular contraction, such as the maintenance of an upright posture. Larger α motor neurons innervate larger, pale muscle fibers that generate more force; however, these fibers have sparse mitochondria and are therefore easily fatigued. These units are called fast fatigable (FF) motor units and are especially important for brief exertions that require large forces, such as running or jumping. A third class of motor units has properties that lie between those of the other two. These fast fatigue-resistant (FR) motor units are of intermediate size and are not quite as fast as FF units. As the name implies, they are substantially more resistant to fatigue, and generate about twice the force of a slow motor unit (Figure 16.5).

Figure 16.5

Comparison of the force and fatigability of the three different types of motor units. In each case, the response reflects stimulation of a single motor neuron. (A) Change in muscle tension in response to a single motor neuron action potential. (B) Tension (more...)

These distinctions among different types of motor units indicate how the nervous system produces movements appropriate for different circumstances. In most muscles, small, slow motor units have lower thresholds for activation than the larger units and are tonically active during motor acts that require sustained effort (standing, for instance). The threshold for the large, fast motor units is reached only when rapid movements requiring great force are made, such as jumping. The functional distinctions between the various classes of motor units also explain some structural differences among muscle groups. For example, a motor unit in the soleus (a muscle important for posture that comprises mostly small, slow units) has an average innervation ratio of 180 muscle fibers for each motor neuron. In contrast, the gastrocnemius, a muscle that comprises both small and larger units, has an innervation ratio of 1000–2000 muscle fibers per motor neuron, and can generate forces needed for sudden changes in body position. More subtle variations are present in athletes on different training regimens. Thus, muscle biopsies show that sprinters have a larger proportion of powerful but rapidly fatiguing pale fibers in their legs than do marathoners. Other differences are related to the highly specialized functions of particular muscles. For instance, the eyes require rapid, precise movements but little strength; in consequence, extraocular muscle motor units are extremely small (with an innervation ratio of only 3!) and have a very high proportion of muscle fibers capable of contracting with maximal velocity.