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Skeletal Muscle and its Contraction

Skeletal Muscle And Its Contraction
Muscles make up about 50% of the adult body mass. There are three types of muscle: skeletal (muscle attached to the skeleton); cardiac (muscle involved in cardiac function; Chapter 15) (both of these are morphologically striated or striped and are commonly called striated muscles); and smooth (muscle involved in many involuntary processes in the blood vessels and gut; this type is not structurally striated, hence its name; Chapter 15). A comparison of the properties of the three muscle types is shown in Appendix I.

Skeletal muscle
The skeletal muscles and the skeleton function together as the musculoskeletal system. Skeletal muscle is sometimes referred to as voluntary muscle because it is under conscious control. It uses about 25% of our oxygen consumption at rest and this can increase up to 20-fold during exercise.

Skeletal Muscle And Its Contraction

General mechanisms of skeletal muscle contraction
The functions of muscle tissue are the development of tension and shortening of the muscle. Muscle fibres have the ability to shorten a considerable amount, which is brought about by the molecules sliding over each other. Muscle activity is transferred to the skeleton by the tendons, and the tension developed by the muscles is graded and adjusted to the load.

Fine structure of skeletal  muscle  (Fig.  12a) The connective tissue surrounding the whole muscle is called the epimysium. The connective tissue that extends beyond the body of the muscle eventually blends into a tendon, which is attached to bone or cartilage. Skeletal muscle is composed of numerous parallel, elon- gated, multinucleated (up to 100) cells, referred to as muscle fibres or myofibres, which are between 10 and 100 μm in diameter and vary in length, and are grouped together to form fasciculi. Each fasciculus is surrounded by the perimysium. Each myofibre is encased by connective tissue called the endomysium. Beneath the endomysium is the sarcolemma (an excitable membrane). This is an elastic sheath with infoldings that invaginate the fibre interior, particularly at the motor end plate of the neuromuscular junction (Chapter 13). Each myofibre is made up of myofibrils 1 μm in diameter separated by cytoplasm and arranged in a parallel fashion along the long axis of the cell. Each myofibril is further subdivided into thick and thin myofilaments (thick, 10–14 nm in width and 1.6 μm in length; thin, 7 nm in width and 1 μm in length). These are responsible for the cross-striations. Thin filaments consist primarily of three proteins, actin, tropomyosin and troponin, in the ratio 7 : 1 : 1, and thick filaments consist primarily of myosin. The cytoplasm surrounding the myofilaments is called the sarcoplasm. Each myofibre is divided at regular intervals along its length into sarcomeres separated by Z-discs (in longitudinal sections, these are Z-lines). To the Z-lines are attached the thin filaments held in a hexagonal array. The I-band extends from either side of the Z-line to the beginning of the thick filament (myosin). The myosin filaments make up the A-band.
The H-zone is at the centre of the sarcomere, and the M-line is a disc of delicate filaments in the middle of the H-zone that holds the myosin filaments in position so that each one is surrounded by six actin filaments.
The thin filaments consist of two intertwining strands of actin with smaller strands of tropomyosin and troponin between the intertwining strands. Each strand of actin is made up of about 200 units of globular or G-actin. It is on these globules that there is a site for myosin to bind during contraction.
The thick filaments are made up of about 100 myosin molecules; each molecule is club shaped, with a thin tail (shaft) comprising two coiled peptide chains and a head made up of two heavy peptide chains and four light peptide chains that have a regulatory function. The ATPase activity of the  myosin  molecule  is  concentrated  in the head.
The thin tails of the myosin molecules form the bulk of the thick filaments, whereas the heads are ‘hinged’ and project outward to form cross-bridges between the thick filaments and their neighbouring thin filaments. Six thin filaments surround each thick filament.
Between the myofibrils are a large number of mitochondria and glycogen granules, as found in other cells, but muscle cells have regular invaginations which project from outside the cell and wrap around the sarcomeres, particularly where the thin and thick filaments overlap. These invaginations are called transverse or T-tubules and contain extracellular fluid. The specialized smooth endoplasmic reticulum, the sarcoplasmic reticulum, which is close to the T-tubules, is enlarged to form terminal cisternae which actively transport Ca2+ into the lumen from the sarcoplasm.
Like fingers of the hands sliding over one another, actin and myosin molecules slide past each other. The myosin heads bind to the actin chain and tilt. There is a constant process of binding, tilting, releasing and rebinding of cross-bridges, as well as rotation of the myosin filaments as they interact with the actin filaments and bind with the alternate myofibril in the hexagonal structure. This results in the con- traction of the whole muscle. The cross-bridges are formed asynchronously so that some are active, whilst others are resting.

The interaction of actin (thin filaments) and myosin (thick filaments) brings about contraction of the muscle, which is caused by the cross-bridges, a result of the interaction of troponin and Ca2+. This mechanism is called the sliding filament theory. The contraction of muscle is triggered by the release of Ca2+ from the sarcoplasmic reticulum. Ca2+ floods out of the cisternae, where it is stored by binding reversibly with a protein, calsequestrin. This raises the concentration of calcium from 0.1 μmol/L to more than 10 μmol/L1, saturating the binding sites on troponin. This results in a shift of tropomyosin, thus allowing the myosin cross-bridges to bind more strongly with actin and begin the contraction cycle (Fig. 12b). The heads tilt after attachment by hydrolysing the adenosine triphosphate (ATP) energy stores, releasing adenosine diphosphate (ADP) and inorganic phosphate (Pi), which leads to a greater binding of the cross-bridges. ADP and Pi escape from the head, freeing the head for another molecule of ATP. This releases the binding of the head and, if Ca2+ is still present, the cycle continues. Otherwise, the binding is inhibited. Contraction is maintained as long as Ca2+ is high. The duration of the contraction is dependent on the rate at which the sarcoplasmic reticulum pumps back the Ca2+ into the terminal cisternae.