Muscles Change Their Strength of Contraction by Varying the Number of Active Motor Units or the Rate of Motor Unit Activation
Even though each muscle fiber is innervated by only one neuron, each motor neuron’s axon branches as it reaches the muscle and innervates several muscle fibers. A motor unit is defined as one alpha (α) motor neuron and all the extrαfusαl (force-generating, striated) muscle fibers that it innervates (Figure 6-6, A).
All the muscle fibers of each motor unit are of the same functional type (e.g., fast or slow twitch), and an action potential on the motor neuron causes all the muscle fibers to contract simultaneously. In motor units a relationship exists among the functional type of muscle fiber innervated, the number of muscle fibers innervated, and motor neuron size. Small motor units tend to be made up of a motor neuron with a small cell body and a narrow, slower-conducting axon that innervates a small number of slow-twitch fibers. Large motor units have a motor neuron with a large cell body and a faster-conducting, wide axon innervating a large number of fast-twitch fibers. Activation of a small motor unit produces a smaller, slower, less fatiguable increment of contractile force in the muscle compared with a larger motor unit. The neuronal cell bodies of all the motor units from a single muscle form a cluster within the central nervous system (CNS) called the motor neuron pool of that muscle (Figure 6-6, B). Within the motor neuron pool for a given muscle, there is a range of motor unit sizes. Muscles with a larger proportion of smaller motor units tend to be amenable to finer control of contractile force.Although an action potential on a motor neuron produces a simultaneous, brief twitch in all the muscle fibers of the motor unit, the pattern of excitation of the units originating from within the CNS produces the smooth, graded contraction of which most muscles are capable. The CNS can instruct a muscle to contract with greater force primarily by increasing the number of motor units that contract at any one time; this is called recruitment or spatial summation.
The force of contraction can also be increased by increasing the frequency of activation of a motor unit, in which a subsequent twitch begins before relaxation of the previous twitch; this is called temporal summation. The recruitment of motor units to increase contractile force occurs in an orderly manner, according to motor unit size, with the smaller units activated first. This results in force being increased gradually in small, more precise amounts when the muscle force required is low. As the force required increases, faster and larger increases in contractile force are progressively added by orderly activation of the larger motor units. This produces an overall smoothness of contraction, keeping the movement as precise as possible until larger, grosser increments are needed, usually when significant tension has already been generated in the muscle.In some skeletal muscles the CNS can command some percentage of motor units to be active for extensive periods (various motor units take turns)» thus continually shortening the distance between the origin and insertion tendon. When contraction of a whole muscle belly occurs without relaxation, the muscle is said to be in tetany. Tetanization of cardiac muscle would be fatal, because heart muscle must relax to allow cardiac filling before it contracts to pump out the blood. Chapter 19 discusses how cardiac muscle prevents tetany.