CONCEPTUAL LIFE SCIENCE MUSCULAR SYSTEM TYPES OF MUSCLES Smoothmuscle is thetype ofmuscle found in all oftheinvoluntaryorgans except the heart. The tissue is made ofindividual muscle cells, each with its own nucleus. The cells of smooth muscle do not have striations. Striations are parallel lines that are perpendicular to the long axis ofthe muscle cell. They are found in striated and cardiac muscle but not in smooth muscle. Striated muscle is also called voluntary muscle. It is found attached to the bones oftheskeletonbytendons. Atendonisaconnectivestructurecomposedofdense connective tissue. The individual cells ofstriated muscle are very long, as long as the muscle itself. Cells this long must have many nuclei so muscle cells are multinucleate. Each individual nucleus controls its own nearby area of the muscle. Striated muscle has striations. Thesecross-bandsresultfromtheinnermolecularstructure ofthemuscle. Striated muscle is under voluntary control. These are the muscles that you can move when you want them to do something for you. Cardiac muscle is the type of muscle found only in the heart. The cells of cardiac muscle are striated. Cardiac muscle has individual cells connected to each other by their ends at which are located intercalated discs. Each individual cell of cardiac muscle has its own nucleus. The heart pumps constantly at an average rate of72 beats per minute. The heart muscle cells rest between each heartbeat. Systole is when the heart contracts while diastole is when the heart relaxes. Structure ofmuscle zz I--~.----+---A---+ Figure 15-1. Structure of muscle (relaxed) 15-115-2 z z H Figure 15-2. Structure ofmuscle (contracted) Muscle consists ofthick fibers called myosin and thin fibers called actin. The actin fibers are attached to vertical structures called Z-lines. The actin and myosin fibers overlap and are in close proximity to each other. This configuration is known as the actomyosin complex. These bands cause the striations of skeletal and cardiac muscle. ThebandindicatedbyAisthecompletelengthofeachsectionofmyosin. Thespace between consecutive myosin bands is called the I-band. The space between consecutive actin bands (where there is myosin alone) is called the H-band. Relaxed muscle is shown in Figure 15-1. Note that the Z-lines are quite far apart and the H and I bands are wide. Then examine Figure 15-2. When muscle contracts the actin and myosin bands slide over each other and the Z-lines get closer together. Also, the H and I-bands become much shorter. As the muscle units get closer together, the muscle gets shorter. This is how muscles contract. Muscles cannot elongate on their own. Another muscle must be used in order to elongate or relax a contracted muscle. Attachment ofmuscles Musclesareattachedtobones insuchawayastoprovidepairsofmusclesfor each moving part ofthe body. The forearm is an example. When the biceps muscle contracts, it closes the angle that the ann makes. This muscle is called ajlexor. In order to extend the ann and make it straight, the triceps muscle is used. This muscle is called an extensor. Soaflexorclosesthe angleofajoint whilethe extensorincreases the angle ofajoint. Whentheflexorcontractstheextensorisrelaxedandpermits itselftobe stretched. Likewise, when the extensor contracts, the flexor relaxes and is allowed to be stretched. Nonnally, these muscles do not contract simultaneously. However, during electric shock, such as when a person accidentally grabs a live wire, all muscles are contracted simultaneously. The stronger ones win. That is why a person cannot let go of a live
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