CHAPTER 2 THE MUSCLE AND ITS CONTRACTION

1. CHAPTER 2 THE MUSCLE AND ITS CONTRACTION 主讲：黄文英

2. Section1 Skeletal Muscle Architecture Section4 The Activation of The Muscle Section3 The Classification of The Muscle Files Section 2 The contraction Mechanism of the Skeletal Muscle CHAPTER 2 THE MUSCLE AND ITS CONTRACTION

3. Section 1 Skeletal Muscle Architecture The muscles are bundles of contractile muscle fibers that join into a tendon at each end. Each bundle of fascicle consists of thousands of muscle fibers, individually wrapped in a thin layer of connective tissue, the endomysium. A layer of connective tissue called the permysium surrounds each fascicle. These fascicles, usually about the thickness of a match or a toothpick, are easy to distinguish in fresh meat. The muscle itself is wrapped in a sheath of connective tissue called the epimysium. In most muscle the epimysium forms a smooth surface, allowing the muscle to move freely in relation to neighboring muscle or other structures during contraction. In such cases the epimysium is called fascia.

4. 1 The Muscle Cell Membrane The cell membrane of the muscle fiber possesses all the characteristics of cell membrane in general, including the ability to create and maintain a membrane potential so vital to its contractile function. Structurally, it is closely apposed to the basement membrane surrounding each muscle fiber. The transverse tubules, or T-tubules, are tubular invaginations of the muscle cell membrane, running more or less perpendicularly to the long axis of the muscle fiber, hence the name . T-tubules are found at regular intervals along the length of the muscle fiber. In mammalian skeletal muscle they are found at each A-I junction .

5. 2 The Interior of the Muscle Cell （1）The Macroscopic structure of the Myofibril The interior of a skeletal muscle cell is dominated by the contractile elements, the myofibrils, running parallel to the long axis of the muscle cell. Each myofibril consists of regularly repeating units, called sarcomeres, arranges in series. The most striking feature, however, is the regularity of the arrangement of sarcomeres in neighboring myofribrils, which gives rise to the transverse striations so typical of skeletal muscle cells. The traditional view is that each sarcomere is composed of two types of protein filaments, actin filaments and myosin filaments, often collectively named myofilaments. In addition to actin molecuels, the action filaments contain two types of regulatory proteins, troponin and tropomyosin.

6. 2 The Interior of the Muscle Cell (2) Myofibrillar Fine Structure The myosin filament, 1.6 µm long, is a multimolecular aggregate of myosin molecules, each 150 nm long. the form of the myosin molecule has been compared to that of a golf club, with a shaft forming part of the stem of the myosin filament and a head sticking out toward a neighboring actin filameng. The myosin head contains the site for attachment to the actin filament and for contractile work. Several hundred myosin molecules are packed in a sheaf, with their heads points in one direction along half the filament and in the opposite direction along the other half, leaving a zone devoid of protruding heads midway along the filament’s length .

7. 2 The Interior of the Muscle Cell (3) Nucleolus We have already mentioned that one muscle cell may contain many nuclei, sometimes as many as several thousands (Roy, Monke et al. 1999). In relation to training adaptations, it is important to realize that each nucleus seems to govern a part of the muscle cell cytoplasm surrounding it, called a nuclear domain (Pavlath et al. 1989). Among other things, this implies that the number of muscle cell nuclei has to increase if the volume of the muscle cell increases by hypertrophy to keep the nucleus/cytoplasm ratio constant. This happens by recruitment and fusion of satellite cells with the hypertrophying muscle fiber.

8. Section 2 The contraction Mechanism of the Skeletal Muscle 1 Interaction of Actin and Myosin There can be no doubt any longer that contraction of a skeletal muscle fiber takes place by a sliding movement of its action andmyosin filaments relative to each other. During this sliding movement the actin filaments in each half sarcomere are pulled toward the center of the sarcomere by the myosin heads, resulting in a progressive narrowing of the Ι-bands but leaving the A-bands unchanged. Because the actin filaments are attached to the Z-disks, they are drawn together too, and the whole sarcomere shortens.

9. 2 From Nerve Signal to Muscle Contraction: Excitation-Contraction Coupling The instantaneous concentration of calcium ions in the cytosol depends both on their rate of release from the SR and on their reuptake into the SR by means of Ca2+ATPases. In the present context, the rate of uptake into the SR can be regarded as constant in a given fiber. This leaves the frequency of the impulse train more or less alone in charge of the instantaneous cytosolic calcium concentration. A low frequency gives a low calcium concentration and a low contraction force. Increasing the impulse frequency progressively increases calcium concentration and force, until a maximal force is obtained, possibly because all TnC calcium-binding sites are occupied.

10. 3 Relaxation Is As Important As Contraction in Normal Movements Contraction of a skeletal muscle fiber takes place by movement of the actin and myosin filaments in relation to each other, neither of the filaments changes length during contraction. These action potentials spread out from the motor endplate toward both ends of the muscle fiber, at the same time penetrating into the interior of the muscle cell by means of the T-tubules. The T-tubules are in close contact with the lateral sacs of the SR, forming the resulting release of calcium ions from the SR. The calciumions bind to troponin C, thereby removing the inhibition imposed by the regulatory proteins on the contractile process. relaxation of the muscle concentration in the cytosol due to actinve relocation of the calcium to the SR by calcium ATPases in the SR membrane.

11. 4 Force Transmission Force transmission from the muscle fibers to the tendon takes place both through specialized myotendinous junctions in the ends of the muscle fibers and through the endomysial connective tissue surrounding each fiber. In the latter case, force transmission takes place through shearing forces. This type of force transmission may be the only one in muscle fibers that are shorter than the fascicles in which they lie. The connective tissue of a muscle is, however, rather compliant, making contractions that appear to be isometric on the whole-muscle level at least partly concentric on the muscle fiber level.

12. Section 3The Classification of The Muscle Files 1 The Muscle Cell Membrane The cell membrane of the muscle fiber possesses all the characteristics of cell membrane in general, including the ability to create and maintain a membrane potential so vital to its contractile function. Functionally, the muscle cell membrane is a mosaic of functional domains .Examples are the neuromuscular and myotendinous junctions and the transverse tubules. The transverse tubules, or T-tubules, are tubular invaginations of the muscle cell membrane, running more or less perpendicularly to the long axis of the muscle fiber, hence the name (figure 2.3). T-tubules are found at regular intervals along the length of the muscle fiber. In mammalian skeletal muscle they are found at each A-I junction (figure 2.3). Text in here

13. Section 3The Classification of The Muscle Files 2 Strategies for Typing A muscle cell is a long, multinucleated cell resulting from the fusion of mononucleated myoblasts in fetal life. Due to its considerable length in relation to its diamenter, amuscle cell is commonly known as a muscle fiber. Each muscle fiber is surrounded by a basement membrane, which in turn borders directly on the connective tissue of the endomysium. The interior of the muscle fiber is dominated by the myofibrils the main components of which are actin and myosin myofilaments. In addition to actin molecules, the thin actin filaments contain two types of regulatory proteins, troponin and tropomyosin. Text in here

14. Section 3The Classification of The Muscle Files 3 Muscle Fiber Types: How Many and What Kind? One final word of warning seems to be appropriate regarding fiber-type designations. Unfortunately, some of the names in common use today were earlier assigned to other properties of muscle fibers. Type Ι and Type Ⅱ(or 1 and 2), for example, have been used for muscle fibers of different length (Barrett 1962), as well as for fibers with different SDH activity In the rat, one of the most common experimental animals in muscle biology, it is now customary to separate four different muscle fiber types: type Ι, type ⅡA, type ⅡB,and type ⅡX. The latter type was described independently by two groups: one in Germany (Bär and Pette 1988) and one in Ttaly Text in here

15. Section 3The Classification of The Muscle Files 4 Rat Muscle and Human Muscle The specific tension of the different muscle fiber types has been a matter of debate for a long time and still is. This may at least in part be due to differences in the experimental approach. Thus, Larsson and Moss (1993) found no difference in specific tension in chemically skinned human muscle fibers, while in freeze-dried muscle fibers the specific tension varied according to MyHC type. In whole muscle, the question of specific tension is further complicated by the presence of other tissue components in addition to the muscle fibers. For further discussion of specific tension, see Larsson and Moss (1993) and J.A.Taylor and Kandarian (1994). Text in here

16. Section 3The Classification of The Muscle Files 5 Species Differences Versus Differences Between Muscles A major question in muscle biology is to what extent the results from animal experiments are valid for human muscle as well. Apparently, some results are, while others are more uncertain. Molecular biology has shown that many on the proteins involved in the contratile process are highly conserved, which means that the proteinsin question have changed very little during the course of evolution and only those most acquainted with the field are able to tell an electron micrograph of rat muscle from one of human muscle. When it comes to other levels of organization and functional characteristics, the situation may be different. In spite of this, results from animal experiments like those reported by Burke et al. (1973) are more often than not used without reservation when describing human muscle propertied. For discussion, see Bigland-Ritchie, Fuglevand, and Thomas Text in here

17. Section 3The Classification of The Muscle Files 6 Muscle Biopsies Muscle biopsies can be obtained by an open biopsy method, where a piece of muscle is cut out under visual guidance, but more often a biopsy needle (J.Bergström 1975) or a conchotome is used (Dietrichson et al. 1987). The question is, however, how representative the biopsy sample is,. From animal muscle it is known that the deeper part of a muscle often contains a higher percentage of type Ι muscle fibers than more superficial parts. In the chicken, where color differences between fiber types are prominent, this is easy to recognize with the naked eye. Text in here

18. 7 Large Individual Variations in Fiber Types and The Determinants In a study originally designed to reveal whether the number of capillaries in the muscles increases during endurance training, half of the experimental subjects dropped out after 7 weeks of endurance training. Based on biopsy material obtained from the vastus lateralis before the training started, Ingjer and Dahl (1979) were bale to show a significantly lower percentage of type Ⅰfibers and leisure physical activity was demonstrated by Glenmark (1994). It has been shown that running and race-walding performance is significantly related to the economy of motion. Since it is quite possible that muscle fiber type influences contractile economy (Horowitz, Sidossis, and Coyle 1994), muscle fiber type’s having an effect on leisure physical activity seems not at all farfetched.

19. 8 The Genetic Factor In humans, three different types of skeletal muscle fiber types are found in locomotor muscles: one slow-twitch, called type IIA and IIX. Type IIX was formerly called IIB. Type I muscle fibers are the more economical and oxidative muscle fiber type, making them highly suitable for sustained work. Among the fast fibers, type IIA is the more oxidative. After periods of changed levels of physical activity and in old age, the prevalence of hybrid muscle fibers containing more than one isoform of MyHC increases. This tends to make the borders between the different muscle fiber types less distinct.

20. Section4The Activation of The Muscle The isometric force exerted by a muscle or, more correctly, on the actual length of its muscle fibers and their constituent sarcomeres. The obvious explanation for this is that the force developed is related to the degree of overlap between the action and myosin filaments The ability of muscle fiber to create force at high shortening velocities is closely related to its speed of cross-bridge cycling. Consequently, the slope of the curve is different for slow and fast muscle fibers, slow fibers losing force more rapidly with increased speed of shortening. intuitively, this makes slow muscle fibers unfit for contractions at high shortening velocities, and as we shall see later, the consequences for the resulting power are severe.

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