Figure 1114

Time relations between a skeletal-muscle fiber action potential and the resulting shortening and relaxation of the muscle fiber.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Muscle CHAPTER ELEVEN

Muscle CHAPTER ELEVEN

Sarcoplasmic Reticulum The sarcoplasmic reticulum in muscle is homologous to the endoplasmic reticulum found in most cells and forms a series of sleevelike structures around each myofibril (Figure 11-15), one segment surrounding the A band and another the

I band. At the end of each segment there are two enlarged regions, known as lateral sacs that are connected to each other by a series of smaller tubular elements. The lateral sacs store the calcium that is released following membrane excitation.

Sarcoplasmic Reticulum Lateral SacsMuscle Fiber Anatomy

FIGURE 11-15

(a) Diagrammatic representation of the sarcoplasmic reticulum, the transverse tubules, and the myofibrils. (b) Anatomical structure of transverse tubules and sarcoplasmic reticulum in a single skeletal-muscle fiber. %

FIGURE 11-15

(a) Diagrammatic representation of the sarcoplasmic reticulum, the transverse tubules, and the myofibrils. (b) Anatomical structure of transverse tubules and sarcoplasmic reticulum in a single skeletal-muscle fiber. %

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

11. Muscle

PART TWO Biological Control Systems

© The McGraw-Hill Companies, 2001

A separate tubular structure, the transverse tubule (T tubule), crosses the muscle fiber at the level of each A-I junction, passing between adjacent lateral sacs and eventually joining the plasma membrane. The lumen of the T tubule is continuous with the extracellular fluid surrounding the muscle fiber. The membrane of the T tubule, like the plasma membrane, is able to propagate action potentials. Once initiated in the plasma membrane, an action potential is rapidly conducted over the surface of the fiber and into its interior by way of the T tubules. The action potential in a T tubule adjacent to the lateral sacs activates voltage-gated proteins in the T-tubule membrane that are physically or chemically linked to calcium-release channels in the membrane of the lateral sacs. Depolarization of the T tubule by an action potential thus leads to the opening of the calcium channels in the lateral sacs, allowing calcium to diffuse from the calcium-rich lumen of the lateral sacs into the cytosol. The rise in cy-tosolic calcium concentration is normally enough to turn on all the cross bridges in the fiber.

A contraction continues until calcium is removed from troponin, and this is achieved by lowering the calcium concentration in the cytosol back to its pre-release level. The membranes of the sarcoplasmic reticulum contain primary active-transport proteins, Ca-ATPases, that pump calcium ions from the cytosol back into the lumen of the reticulum. As we just saw, calcium is released from the reticulum upon arrival of an action potential in the T tubule, but the pumping of the released calcium back into the reticulum requires a much longer time. Therefore, the cytosolic calcium concentration remains elevated, and the contraction continues for some time after a single action potential.

Contraction Relaxation

® Muscle action

Contraction Relaxation

® Muscle action

Muscle Contraction Relaxation

I Ca2+ binding to troponin removes blocking action of tropomyosin

I Ca2+ removal from troponin restores tropomyosin blocking action

Thin filament

I Ca2+ binding to troponin removes blocking action of tropomyosin

I Ca2+ removal from troponin restores tropomyosin blocking action

Troponin Tropomyosin

Thick filament

Staying Relaxed

Staying Relaxed

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