Porth's Essentials of Pathophysiology, 4e

25

Cell Structure and Function

C h a p t e r 1

represents the cell membrane. Embedded throughout the sarcoplasm are the contractile elements actin and myosin, which are arranged in parallel bundles (i.e., myofibrils ). Each myofibril consists of regularly repeat- ing units called sarcomeres along its length. Sarcomeres, which are the structural and functional units of striated muscle, extend from one Z line to another Z line (Fig. 1-17B). The central portion of the sarcomere contains the dark band (A band) consisting mainly of myosin filaments, with some overlap with actin filaments. Straddling the Z line, the lighter I band contains only actin filaments; therefore, it takes two sar- comeres to complete an I band. An H zone is found in the middle of the A band and represents the region where only myosin filaments are found. In the center of the H zone is a thin, dark band, the M band or M line, pro- duced by linkages between the myosin filaments. Z lines consist of short elements that interconnect and provide the thin actin filaments from two adjoining sarcomeres with an anchoring point. Muscle contraction involves the actin filaments sliding inward among the myosin filaments (Fig. 1-17C). In the relaxed state, the ends of the actin filaments extend from two successive Z lines, barely overlapping one another. In the contracted state, these actin filaments have been pulled inward among the myosin filaments so their ends overlap one another. The sarcoplasmic reticulum , which is comparable to the smooth ER, is composed of longitudinal tubules that run parallel to the muscle fiber and surround each myo- fibril (Fig. 1-17D). This network ends in enlarged, saclike regions called the lateral sacs or terminal cisternae . These sacs store calcium that is released during muscle contrac- tion. A second system of tubules consists of the transverse

or T tubules , which are extensions of the cell membrane and run perpendicular to the muscle fiber. The hollow por- tion or lumen of the T tubule is continuous with the extra- cellular fluid compartment. Action potentials, which are rapidly conducted over the surface of the muscle fiber, are in turn propagated by the T tubules into the sarcoplasmic reticulum. As the action potential moves through the lat- eral sacs, the sacs release calcium, initiating muscle con- traction. The membrane of the sarcoplasmic reticulum also has an active transport mechanism for pumping calcium ions back into the reticulum. This prevents interactions between calcium ions and the actin and myosin myofila- ments after cessation of a muscle contraction. Skeletal Muscle Contraction. Muscle contraction involves the sliding of the thick myosin and thin actin filaments over each other to produce shortening of the muscle fiber, while the actual length of the individual thick and thin filaments remains unchanged. The thick myosin filaments consist of a thin tail, which provides the structural backbone for the filament, and a globu- lar head that forms cross-bridges with the thin actin filaments (Fig. 1-18A). Myosin molecules are bundled together side by side in the thick filaments such that one half have their heads toward one end of the filament and their tails toward the other end, and the other half are arranged in the opposite manner. Each globular myosin head contains a site able to bind to a complementary site on the actin molecule. Besides the binding site for actin, each myosin head has a separate active site that catalyzes the breakdown of ATP to provide the energy needed to activate the myosin head so it can form a cross-bridge with actin. After contraction, myosin also

ATP binding site

2

Cross- bridge

Actin binding site

Myosin

Head

Tail

Cross-bridge attachment

3

1

A

P

i

ADP

Actin

Troponin complex

Power stroke

Cocking of myosin head

TnC

Tropomyosin Actin

TnT

TnI

ATP

4

B

C

Cross-bridge detachment

FIGURE 1-18. Molecular structure of the thicker myosin filament (A) and the thinner actin filament (B) of striated muscle.The thin filament is a double-stranded helix of actin molecules with tropomyosin and troponin molecules lying along the grooves of the actin strands. (C) Sequence of events involved in sliding of adjacent actin and myosin filaments: (1) cocking of the myosin head, which occurs as adenosine triphosphate (ATP) is split to adenosine diphosphate (ADP); (2) cross- bridge attachment; (3) power stroke during which the myosin head bends as it moves the actin forward; and (4) cross-bridge detachment, which occurs as a new ATP attaches to the myosin head.

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