Sr

- is the internal tubular structure that is the site of Ca2+ storage and release for excitation-contraction coupling.

- has terminal cisternae that make intimate contact with the T tubules in a triad arrangement.

- membrane contains Ca2+-ATPase (Ca2+ pump), which transports Ca2+ from intracellular fluid into the SR interior, keeping intracellular [Ca2+] low.

- contains Ca2+ bound loosely to calsequestrin.

- contains a Ca2+ release channel called the ryanodine receptor.

B. Steps in excitation-contraction coupling in skeletal muscle (Figures 1-9 and 1-10)

1. Action potentials in the muscle cell membrane initiate depolarization of the T tubules.

2. Depolarization of the T tubules opens Ca2+ release channels in the nearby SR, causing release of Ca2+ from the SR into the intracellular fluid.

3. Intracellular [Ca2+] increases.

Figure 1-9. The cross-bridge cycle in skeletal muscle. (A) Rest. (B) Ca2+ binds to troponin C. (C) Tropomyosin is moved out of the way to expose myosin binding sites, and cross-bridges form. (D) Cross-bridges break. ADP = adenosine diphosphate; ATP = adenosine triphosphate; P, = inorganic phosphate. (Reprinted with permission from Bullock J, Boyle J III, Wang MB: Physiology, 2nd ed. Baltimore, Williams & Wilkins, 1991, p 30.)

Figure 1-9. The cross-bridge cycle in skeletal muscle. (A) Rest. (B) Ca2+ binds to troponin C. (C) Tropomyosin is moved out of the way to expose myosin binding sites, and cross-bridges form. (D) Cross-bridges break. ADP = adenosine diphosphate; ATP = adenosine triphosphate; P, = inorganic phosphate. (Reprinted with permission from Bullock J, Boyle J III, Wang MB: Physiology, 2nd ed. Baltimore, Williams & Wilkins, 1991, p 30.)

✓Action potential

/ Intracellular [Ca2+]

✓Twitch tension

Time

Figure 1-10. Relationship of the action potential, the increase in intracellular [Ca2+], and muscle contraction in skeletal muscle.

4. Ca2+ binds to troponin C on the thin filaments, causing a conformational change in troponin that leads to the following events in the cross-bridge cycle.

a. Tropomyosin is moved out of the way.

b. Actin and myosin bind, the heads of the cross-bridges pivot, the thin and thick filaments slide over each other, and ATP is hydrolyzed.

c. Subsequently, the cross-bridges break and a new molecule of ATP binds to the myosin head to begin a new cycle.

d. Cross-bridge cycling continues as long as Ca2+ is bound to troponin C.

5. Relaxation occurs when Ca2+ uptake into the SR by Ca2+-ATPase lowers the intracellular [Ca2+]. ATP is consumed in the process of Ca2+ uptake (and during the cross-bridge cycle).

6. Mechanism of tetanus. A single action potential causes the release of a standard amount of Ca2+ from the SR and produces a single twitch (see Figure 1-10). However, if the muscle is stimulated repeatedly, then more Ca2+ is released from the SR and there is a greater increase in intracellular [Ca2+], greater Ca2+ binding to troponin C, and greater cross-bridge formation. As a result, more tension is generated by the muscle (tetanus).

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