SKELETAL MUSCLE (Structure & Functions)
Humans rely on muscles for many of our physiological processes, and virtually all our dynamic interactions with the environment involve muscle tissue.
8.1 Functions of Skeletal Muscle
- Movement of the body– Contraction of skeletal muscles is responsible for the overall movements of the body, such as walking, running, and manipulating objects with the hands.
- Maintenance of posture- Skeletal muscles constantly maintain tone, which keeps us sitting or standing erect.
- Respiration– Muscles of the thorax carry out the movements necessary for respiration.
- Production of body heat– When skeletal muscles contract, heat is given off as a by-product. This released heat is critical to the maintenance of body temperature.
- Communication– Skeletal muscles are involved in all aspects of communication, including speaking, writing, typing, gesturing, and facial expressions.
- Constriction of organs and vessels– The contraction of smooth muscle within the walls of internal organs and vessels causes those structures to constrict. This constriction can help propel and mix food and water in the digestive tract, propel secretions from organs, and regulate blood flow through vessels.
- Contraction of the heart– The contraction of cardiac muscle causes the heart to beat, propelling blood to all parts of the body.
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Functions of Skeletal Muscle
8.2 Organization of Skeletal Muscle (Structure of Skeletal Muscle)
- Each skeletal muscle cell, also called a muscle fiber.
- These muscle fibers are bound together into bundles, or fascicles, and are supplied with a rich network of blood vessels and nerves.
- The fascicles are then bundled together to form the intact muscle.
- These muscle fibers are individually wrapped and then bound together by several different layers of fibrous connective tissue.
- The epimysium (“epi”-outside, and “mysium”-muscle) is a layer of dense fibrous connective tissue that surrounds the entire muscle. This layer is also often referred to as the fascia.
- Each skeletal muscle is formed from several bundled fascicles of skeletal muscle fibers, and each fascicle is surrounded by perimysium (“peri”-around).
- Each single muscle cell is wrapped individually with a fine layer of loose (areolar) connective tissue called endomysium (“endo”-inside).
- These connective tissue layers are continuous with each other and they all extend beyond the ends of the muscle fibers themselves, forming the tendons that anchor muscles to bone, moving the bones when the muscles contract.
Epimysium ─ a connective tissue sheath that surrounds and separates muscle.
Perimysium ─ a connective tissue that surrounds and holds fascicles together.
Endomysium ─ a connective tissue that surrounds each muscle fibre.
- Deep to the endomysium, each skeletal muscle cell is surrounded by a cell membrane known as the sarcolemma.
- The cytoplasm, or sarcoplasm contains a large amount of glycogen (the storage form of glucose) for energy, and myoglobin -a red pigment similar to hemoglobin that can store oxygen.
- Most of the intracellular space, however, is taken up by rod-like myofibrils-cylindrical protein structures.
- Each muscle fiber contains hundreds or even thousands of myofibrils that extend from one end of each muscle fiber to the other. These myofibrils take up about 80% of the intracellular space, and are so densely packed inside these cells that mitochondria and other organelles get sandwiched between them while the nuclei get pushed to the outside and are located on the periphery right under the sarcolemma.
- Each myofibril is comprised of several varieties of protein molecules that form the myofilaments, and each myofilament contains the contractile segments that allow contraction. These contractile segments are known as sarcomeres (“sarc-” -muscle, “mere” – part).
- The striations seen microscopically within skeletal muscle fibers are formed by the regular, proteins inside the cells.
Figure 13- Structure of Skeletal Muscle
- That there are light and dark striations in each cell. The dark areas are called A bands, which is fairly easy to remember because “a” is the second letter in “dark.” The light areas are called I bands, and are also easy to remember because “i” is the second letter in “light.” (“A” actually stands for anisotropic, and “I” stands for isotropic. Both of these terms refer to the light absorbing character of each band.)
Figure 14- Arrangement of Thick & Thin Filaments
- Notice that in the middle of each I band is a darker line called the z line or z disc. The Z lines are the divisions between the adjacent sarcomeres. Sarcomeres are connected end to end along the entire length of the myofibril.
- Also, in the middle of each A band is a lighter H zone (H for “helle”-“bright”), and each H zone has a darker M line (M for “middle”) running right down the middle of the A band.
- Each myofibril, in turn, contains several varieties of protein molecules, called myofilaments. The larger, or thick myofilaments are made of the protein, myosin, and the smaller thin myofilaments are chiefly made of the protein, actin.
- Let’s discuss each myofilament in turn. Each actin molecule is composed of two strands of fibrous actin (F actin) and a series of troponin and tropomyosin molecules.
- Each F actin is formed by two strings of globular actin (G actin) wound together in a double helical structure, much like twisting two strands of pearls with each other.
Figure 15- Structure of F actin
- Each G actin subunit has a binding site for the myosin head to attach to the actin.
- Tropomyosin is a long string-like polypeptide that parallels each F actin strand and functions to either hide or expose the “active sites” on each globular actin molecule (G actin).
- Associated with each tropomyosin molecule is a third polypeptide complex known as troponin.
- Troponin complexes contain three globular polypeptides (Troponin I, Troponin T, and Troponin C) that have distinct functions. Troponin I binds to actin, Troponin T binds to tropomyosin and helps position it on the F actin strands, and Troponin C binds calcium ions.
Figure 16- Structure of Troponin Complex
- There is one troponin complex for each tropomyosin. When calcium binds to Troponin C, it causes a conformational change in the entire complex that result in exposure of the myosin binding sites on the G actin subunits.
- The thick myofilaments are composed chiefly of the protein myosin.
- Myosin myofilaments, or thick myofilaments, resemble bundles of minute golf clubs .
- The parts of the myosin molecule that resemble golf club heads are referred to as myosin heads.
- The myosin heads have three important properties: (1) The heads can bind to attachment sites on the actin myofilaments; (2) they can bend and straighten during contraction; and (3) they can break down ATP, releasing energy.
Figure 17- Actin & Myosin Filaments