CHAPTER 3 Structure and Function of Muscles Muscle at Rest Myosin


57 Z line


Actin Isometric Contraction Myosin


Actin


Figure 3.7 The wrist extensors are performing an isometric contraction holding the wrist in slight extension while the fingers move across the keyboard. The actin and myosin filaments bind but do not change their position in relation to each other.


THE MOTOR UNIT


Sarcomeres develop tension that results in a muscle con- traction when they are activated by an efferent motor neuron. These motor neurons, whose cell bodies are located in the anterior (ventral) horn of the spinal cord, receive impulses from the CNS, spinal interneurons, or peripheral afferent sensory neurons. This input travels along the motor neuron axons to the muscle, where it divides into smaller and smaller branches before termi- nating at the motor end plate near the sarcolemma of a single muscle fi ber. The motor neuron and all of the fi bers it innervates make up a motor unit (Fig. 3.8). When the unit is stimulated to contract, all the fi bers of the unit contract maximally or not at all. When an entire muscle contracts, many motor units are fi ring repeatedly but at various times.


The magnitude of the muscle contraction depends on the total number of motor units that are activated and the frequency of fi ring of each of the units. The number, size, and function of motor units vary within muscles and between muscles. Some motor units have small cell bodies with axons of small diameters. Because of this design, it takes longer for nerve impulses to travel down the axon to reach the muscle fi ber. Conversely, axons with large diameters transmit the impulses at faster speeds. Motor units also vary in the number of muscle fi bers they innervate. Some motor units may innervate only two to three muscle fi bers; others may correspond to a few thou- sand muscle fi bers. Generally, muscles used during fi ne motor activities of the hands or during eye movements, where adjustments in muscle tension are small, have small motor units with smaller cells and axons and fewer numbers of fi bers. Muscles that generate larger gross movements tend to have larger cell bodies and axons and a larger number of muscle fi bers per motor unit. The gastrocnemius muscle, which assists with propelling the body forward during walking, can have 2,000 muscle


TABLE 3.2 Motor Units Large Motor Units


Larger number of muscle fi bers; can have several thousand fi bers


Larger cell bodies and diameter of axons


Innervate primarily muscle fi bers related to strength (type II muscle fi bers)


Recruited during forceful contractions


Small Motor Units


Smaller number of muscle fi bers; can have as few as three muscle fi bers


Smaller cell bodies and diameter of axons


Innervate primarily muscle fi bers related to endurance (type I muscle fi bers)


Recruited fi rst with most activities


fi bers per motor unit, whereas muscles that control eye movement can have fewer than 10 fi bers per motor unit. Rather than one type, most muscles in the body have a mix of small and large motor unit types. Table 3.2 summarizes the characteristics of large and small motor units.


Recruitment of Motor Units


Recruitment of motor units in a muscle seems to follow a rule of recruiting only what is needed to perform a given task. The number and types of motor units recruited depend on the nature of the task and the type of muscle contraction needed to perform the task. Generally, iso- metric contractions recruit smaller motor units before larger units. Less muscle tension needed for a task results in recruitment of smaller units that expend less energy than larger motor units. Depending on the baseline


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