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interdigitate with the myosin filaments. The Z disc, which itself is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber. Therefore, the entire muscle fiber has light and dark bands, as do the individual myofibrils.These bands give skeletal and cardiac muscle their striated appearance.

The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z discs is called a sarcomere. When the muscle fiber is contracted, as shown at the bottom of Figure 6-4, the length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. We will see later that, at this length, the muscle is capable of generating its greatest force of contraction.

What Keeps the Myosin and Actin Filaments in Place? Titin Filamentous Molecules. The side-by-side relationship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called titin. Each titin molecule has a molecular weight of about 3 million, which makes it one of the largest protein molecules in the body. Also, because it is filamentous, it is very springy. These springy titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sar-comere will work. There is reason to believe that the

Figure 6-2

Electron micrograph of muscle myofibrils showing the detailed organization of actin and myosin filaments. Note the mitochondria lying between the myofibrils. (From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

titin molecule itself acts as template for initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments.

Sarcoplasm. The many myofibrils of each muscle fiber are suspended side by side in the muscle fiber. The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mitochondria that lie parallel to the myofibrils. These supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria.

Sarcoplasmic Reticulum. Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive reticulum (Figure 6-3), called the sarcoplas-mic reticulum. This reticulum has a special organization that is extremely important in controlling muscle contraction, as discussed in Chapter 7.The very rapidly contracting types of muscle fibers have especially extensive sarcoplasmic reticula.

General Mechanism of Muscle Contraction

The initiation and execution of muscle contraction occur in the following sequential steps.

1. An action potential travels along a motor nerve to its endings on muscle fibers.

2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine.

3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple "acetylcholine-gated" channels through protein molecules floating in the membrane.

Figure 6-3

Sarcoplasmic reticulum in the extracellular spaces between the myofibrils, showing a longitudinal system paralleling the myofibrils. Also shown in cross section are T tubules (arrows) that lead to the exterior of the fiber membrane and are important for conducting the electrical signal into the center of the muscle fiber. (From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This initiates an action potential at the membrane.

5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes.

6. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.

7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process.

8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump, and they remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease.

We now describe the molecular machinery of the muscle contractile process.

Molecular Mechanism of Muscle Contraction

Sliding Filament Mechanism of Muscle Contraction. Figure 6-4 demonstrates the basic mechanism of muscle contraction. It shows the relaxed state of a sarcomere (top) and the contracted state (bottom). In the relaxed state, the ends of the actin filaments extending from two successive Z discs barely begin to overlap one another. Conversely, in the contracted state, these actin filaments have been pulled inward among the myosin

Figure 6-4

Relaxed and contracted states of a myofibril showing (top) sliding of the actin filaments (pink) into the spaces between the myosin filaments (red), and (bottom) pulling of the Z membranes toward each other.

Figure 6-4

Relaxed and contracted states of a myofibril showing (top) sliding of the actin filaments (pink) into the spaces between the myosin filaments (red), and (bottom) pulling of the Z membranes toward each other.

filaments, so that their ends overlap one another to their maximum extent. Also, the Z discs have been pulled by the actin filaments up to the ends of the myosin filaments. Thus, muscle contraction occurs by a sliding filament mechanism.

But what causes the actin filaments to slide inward among the myosin filaments? This is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive, but when an action potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils. The calcium ions in turn activate the forces between the myosin and actin filaments, and contraction begins. But energy is needed for the contractile process to proceed. This energy comes from high-energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy. In the next few sections, we describe what is known about the details of these molecular processes of contraction.

Molecular Characteristics of the Contractile Filaments

Myosin Filament. The myosin filament is composed of multiple myosin molecules, each having a molecular weight of about 480,000. Figure 6-5A shows an individual molecule; Figure 6-5B shows the organization of many molecules to form a myosin filament, as well as interaction of this filament on one side with the ends of two actin filaments.

The myosin molecule (see Figure 6-5A) is composed of six polypeptide chains—two heavy chains, each with a molecular weight of about 200,000, and four light chains with molecular weights of about 20,000 each.

Cross-bridges Hinges Bo

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