Motor Unit—All the Muscle Fibers Innervated by a Single Nerve Fiber. Each motoneuron that leaves the spinal cord innervates multiple muscle fibers, with the number of fibers innervated depending on the type of muscle. All the muscle fibers innervated by a single nerve fiber are called a motor unit (Figure 1). In general, small muscles that react rapidly and whose control must be exact have more nerve fibers for fewer muscle fibers (for instance, as few as two or three muscle fibers per motor unit in some of the laryngeal muscles). Conversely, large muscles that do not require fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit. An average figure for all the muscles of the body is questionable, but a good guess would be about 80 to 100 muscle fibers to a motor unit.

Fig1. A motor unit consists of a motor neuron and the group  of skeletal muscle fibers it innervates. A single motor axon may  branch to innervate several muscle fibers that function together as a  group. Although each muscle fiber is innervated by a single motor  neuron, an entire muscle may receive input from hundreds of different motor neurons. 

The muscle fibers in each motor unit are not all bunched together in the muscle but overlap other motor units in microbundles of 3 to 15 fibers. This inter digitation allows the separate motor units to contract in support of one another rather than entirely as individual segments.

Muscle Contractions of Different Force—Force Summation. Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of con traction, which is called frequency summation and can lead to tetanization.

Multiple Fiber Summation. When the central nervous system sends a weak signal to contract a muscle, the smaller motor units of the muscle may be stimulated in preference to the larger motor units. Then, as the strength of the signal increases, larger and larger motor units begin to be excited as well, with the largest motor units often having as much as 50 times the contractile force of the smallest units. This phenomenon, called the size principle, is important because it allows the gradations of muscle force during weak contraction to occur in small steps, whereas the steps become progressively greater when large amounts of force are required. This size principle occurs because the smaller motor units are driven by small motor nerve fibers, and the small motoneurons in the spinal cord are more excitable than the larger ones, so naturally they are excited first.

Another important feature of multiple fiber summation is that the different motor units are driven asynchronously by the spinal cord; as a result, contraction alternates among motor units one after the other, thus providing smooth contraction even at low frequencies of nerve signals.

Frequency Summation and Tetanization. Figure 2 shows the principles of frequency summation and tetanization. Individual twitch contractions occurring one after another at low frequency of stimulation are displayed to the left. Then, as the frequency increases, there comes a point when each new contraction occurs before the preceding one is over. As a result, the second contraction is added partially to the first, and thus the total strength of contraction rises progressively with increasing frequency. When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuous, as shown in the figure. This process is called tetanization. At a slightly higher frequency, the strength of contraction reaches its maximum, and thus any additional increase in frequency beyond that point has no further effect in increasing contractile force. Tetany occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials.

Fig2. Frequency summation and tetanization. 

Maximum Strength of Contraction. The maximum strength of tetanic contraction of a muscle operating at a normal muscle length averages between 3 and 4 kilograms per square centimeter of muscle, or 50 pounds per square inch. Because a quadriceps muscle can have up to 16 square inches of muscle belly, as much as 800 pounds of tension may be applied to the patellar tendon. Thus, one can readily understand how it is possible for muscles to pull their tendons out of their insertions in bone.

Changes in Muscle Strength at the Onset of Contraction—The Staircase Effect (Treppe). When a muscle begins to contract after a long period of rest, its initial strength of contraction may be as little as one half its strength 10 to 50 muscle twitches later. That is, the strength of contraction increases to a plateau, a phenomenon called the staircase effect, or treppe.

Although all the possible causes of the staircase effect are not known, it is believed to be caused primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the sarcoplasm to recapture the ions immediately.

Skeletal Muscle Tone. Even when muscles are at rest, a certain amount of tautness usually remains, which is called muscle tone. Because normal skeletal muscle fibers do not contract without an action potential to stimulate the fibers, skeletal muscle tone results entirely from a low rate of nerve impulses coming from the spinal cord. These nerve impulses, in turn, are controlled partly by signals transmitted from the brain to the appropriate spinal cord anterior motoneurons and partly by signals that originate in muscle spindles located in the muscle itself. Both of these signals are discussed in relation to muscle spindle and spinal cord function in Chapter 55.

Muscle Fatigue. Prolonged and strong contraction of a muscle leads to the well-known state of muscle fatigue. Studies in athletes have shown that muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen. Therefore, fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. However, experiments have also shown that transmission of the nerve signal through the neuromuscular junction, which is discussed in Chapter 7, can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle contraction. Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue within 1 or 2 minutes because of the loss of nutrient supply, especially the loss of oxygen.

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مواضيع ذات صلة

Secretion of Acetylcholine by The Nerve Terminals

Energetics of Muscle Contraction

Interaction of One Myosin Filament, Two Actin Filaments, and Calcium Ions to Cause Contraction

Molecular characteristics of the contractile filaments

General Mechanism of Muscle Contraction

Physiological Anatomy of Skeletal Muscle

المفاصل Joints

العضلة القلبية (Cardiac muscle)

العضلة الناعمة متعددة الوحدات (multi-unit smooth muscle)

العضلة الناعمة المفردة (single-unit smooth muscle)

العضلة الناعمة (smooth muscle)

آلية التقلص والانبساط العضلي