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Usually, one end of the fiber attaches to tendon -- the tough tissue that holds muscle to bone -- while the other end attaches to the connective tissue in the muscle. The firm white tendons form a core for the red muscle by extending far inside its mass and emerging at the ends of the muscle to link it to bone. Muscle fibers and tendon fibers are completely different materials and do not merge. Instead, the connective tissue extending from the tendon forms a cup like receptacle for the muscle fiber endings. Other fibers tangle around the muscle fiber, binding it to the receptacle.
Skeletal muscle is striated. The striations arise from the many myofibrils, each of which is striated. The parallel arrangement of myofibrils within the muscle cell gives it its characteristic appearance. Just as a single fiber contains many myofibrils, each myofibril contains many smaller filaments arranged in a repeated pattern along the length of the fibril.
There are thick filaments composed of the protein myosin and thin filaments composed of the protein actin. The arrangement of the filaments makes the striations. The filaments, like the resulting striations, make little sense unless connected with their mechanism. Externally when a muscle contracts, its fibers appear to shorten. The electron scanning microscope shows a different occurrence. Instead of shortening, the filaments slide past each other (sliding filament theory). When the muscle receives an impulse from the central nervous system the neuro transmitters initiate a wave of electrical activity that spreads through the whole fiber. This causes the fiber's membrane to release electrically charged calcium ions that spark the mechanical process of contraction. When the calcium ions come into contact with the fiber's contractile proteins (thick myosin and thin actin filaments), they interact with two other proteins, troponin and tropomyosin.
Troponin and tropomyosin circle the thin actin filaments like delicate embroidery. The calcium chemically binds with troponin, causing it to influence the tropomyosin. The tropomyosin threads shift their hold on the actin filament, which then binds with the myosin filament. This entire chain reaction takes a few milliseconds. Emerging from the myosin filament are pairs of rounded buds called cross bridges. Crowning the cross bridges is a remarkable substance called adenosine triphosphate (ATP). ATP is an organic compound (the main source of life's energy) and is derived from food. So vital is this substance that some scientists believe the appearance of ATP on earth may have been the event that led to life itself. ATP and the myosin molecules have a great attraction to each other. In the normal muscle, almost every myosin head has ATP resting on it. The two buds of the head have different functions. One is adenosine triphosphatase (ATPase), which can split ATP and liberate energy. The other portion of the head binds to the actin filaments forming the cross bridges.
As the protein tropomyosin shifts away from the binding sites on the actin filaments, the myosin arms can link with actin. The myosin ATPase then splits ATP to release energy to fuel the muscle's contraction by throwing the cross bridges into action. The thick and thin filaments slide by each other. Without ATP to move the cross bridges, the actin and myosin filaments would remain locked together. The muscle would be unable to contract.
The term "contraction" does not always refer to the shortening of the muscle. It also refers to the tension within the muscle. There are two types of contraction. If the muscle develops tension but does not shorten, it is an isometric contraction. If the muscle shortens and retains tension, it is an isotonic contraction. The amount of resistance the muscle must overcome determines which type of contraction takes place. Whenever a muscle pulls against a resistance that it cannot overcome, an isometric contraction happens.
When a muscle can overcome a resistance it will shorten, and an isotonic contraction occurs. The movement of a muscle is concentric if it pulls and shortens to overcome resistance. The movement of a muscle is eccentric when it is pulled by another muscle and lengthens against resistance. When a single nerve impulse strikes a muscle, the response is a twitch. Each muscle twitch has three phases: 1) the latent period when the cell starts its chemical processes and the cross bridges build force; 2) the contraction when the muscle shortens; and 3) the relaxation period when the muscle returns to its resting state.
A succession of individual muscle twitches will produce a greater strength of contraction. This is the "staircase effect". When the contraction force reaches a peak, the muscle will briefly maintain it and then return to rest. The force of the contraction will drop with the depletion of ATP. The loss of tension is muscle fatigue. Eventually, even a strong stimulus will not cause a muscle response.
The number of fibers in a muscle is fixed at birth. weightlifters have the same as everyone else. However, they do have larger muscle fibers and muscles with more connective tissue. Exercise stimulates the production of greater amounts of the contractile proteins actin and myosin. Exercise makes more cross bridges available for more work. With the increase in proteins, the myofibrils thicken and the fibers expand. Muscle enlargement is called hypertrophy.
The heart also can hypertrophy. The cavity of the left ventricle enlarges in endurance athletes so that the heart can supply more blood to power prolonged activity. In athletes trained for spurts of strength, the left ventricle wall thickens enabling the athlete to cope with the sudden rise in blood pressure these sports produce.
Muscle also can atrophy. If the nerve fibers are injured, the muscle cannot contract. The amount of actin and myosin in the fibers reduce, and the fibers themselves grow smaller. Even if there is no damage to the nerve fibers, the muscle can atrophy from disuse.
By and large, the greatest cause of atrophy in muscle is
aging. Muscular strength will peak at the age of thirty. As
you get older, the myofibrils degenerate and the number and
size of muscle fibers dwindle. Connective tissue replaces
the lost fibers, making the muscle more rigid and slow to
respond. Steady exercise is a valuable measure of preventive
medicine that helps delay fiber loss and maintains strength.
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