174 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. The neuromuscular system can produce slow, gentle movements as well as fast, forceful movements. This is accomplished by regulating the number of motor units activated, as well as the number and frequency of action potentials. Only a small number of action potentials are needed for slow, gentle movements, while fast or forceful movements require a large number of action potentials, released rapidly. Maximum Tension and Return to Relaxation When it receives an action potential, a motor unit always develops maximum tension. This physiological principle is known as the all-or-none law. However, because each whole muscle includes multiple motor units, simultaneous activation of many motor units is required for the muscle to develop maximum tension. The diagram in Figure 6.8 displays the relationship between the number and frequency of action potentials and the development of tension in the muscle. With high-frequency stimulation, the muscle develops a sustained, maximal level of tension called tetanus. Almost all skeletal motor units develop tension in a twitch-like fashion, generating maximum tension very briefly and then immediately relaxing. After the action potential has traveled the length of the muscle fiber, chemical processes return the fiber to its resting state. Sodium ions diffuse back out of the cell into the interstitial fluid, and calcium ions return to storage sites within the cell. The actin filaments slide back to their original positions as the cross bridges release them, and the muscle fiber returns to a state of relaxation. SELF CHECK 1. What structures make up a motor unit? 2. Describe the neuromuscular junction. 3. What is an action potential? 4. Why is a small motor unit more suitable than a large one for fine motor skills? Types of Skeletal Fibers Have you ever noticed that some athletes are espe- cially good at events or tasks that require endur- ance, whereas others excel at activities that require explosive strength or speed? The reason is definitely related to the ways in which these individuals train, but that is only a small part of the explanation. Also This change in electrical charge is known as depolarization. Depolarization triggers the opening of additional channels in the fiber membrane that allow only entry of additional Na+. The flood of positive ions into the fiber generates an electrical charge called an action potential. Sarcomere Contraction Glucose stored in the form of glycogen within the muscle cell provides the energy for creating an action potential. Phosphocreatine within the cell enables the transfer of energy to protein filaments known as actin and myosin. Actin and myosin are contractile proteins that reside in functional units called sarcomeres inside the muscle fiber. The release of calcium ions (Ca++) triggers the actin filaments to slide over the myosin filaments, resulting in a contraction of the sarcomere (Figure 6.7). Notice in Figure 6.7 that the myosin filaments are encircled by small protrusions called heads. When the sarcomere is activated by an action potential, these heads attach to receptor sites on the actin filaments, forming cross bridges. The cross bridges contract, pulling the actin filaments toward the center of the sarcomere. During the process of sarcomere contraction, the cross bridges attach, pull, and release multiple times. The Ca++ ions released with the arrival of the action potential enable the myosin heads to attach to the actin filaments. Myosin Actin Heads Relaxed Contracted © Body Scientific International Figure 6.7 The sarcomere is the contractile unit of muscle. When the muscle is stimulated, the actin filaments slide together, producing contraction of the sarcomere.