Cellular Level
Skeletal muscle is a highly organized tissue composed of bundles of muscle fibers called myofibers which contain several myofibrils. Each myofiber represents a muscle cell with its basic cellular unit, the sarcomere. Bundles of myofibers form fascicles, and bundles of fascicles form muscle tissue.
Skeletal muscle fibers are striated, multinucleated cells ranging from 10 to 100 micrometers in diameter and many centimeters long. The nuclei are located in the cell’s periphery, adjacent to the sarcolemma. The sarcolemma is a tubular sheath that encases and defines each muscle fiber, forming a barrier between extracellular and intracellular compartments. The sarcolemma is comprised of a plasma membrane and a polysaccharide coating that fuses with tendon fibers. Invaginations within the sarcolemma are termed transverse tubules (T tubules), which function as a major location for ion exchange.
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Grossly, skeletal muscle fibers are made up of endomysium, perimysium, and epimysium, covering the sarcolemma; each muscle fiber is a layer of connective tissue called the endomysium. Capillaries and nerve tissue are present within the endomysium to supply the individual muscle fibers. Multiple muscle fibers join to form fascicles encased by another connective tissue covering known as the perimysium. The perimysium may surround anywhere from 10 to 100 fascicles. Muscle fascicles are further grouped to form a muscle encased by a fibrous tissue envelope called the epimysium.
Each muscle fiber is composed of several hundred to several thousand myofibrils. Myofibrils are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. When viewed under electron microscopy, sarcomeres are arranged longitudinally and include the M line, Z disk, H band, A band, and I band.
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The Z line, or Z disk, is the terminal boundary of the sarcomere, where alpha-actinin acts as an anchor for the actin filaments. The M line is the central-most line of the sarcomere, where myosin filaments are anchored together through binding sites within the myosin filament. The H band contains the M line and is the central region of the sarcomere that contains only myosin filaments. The A band is a larger portion of the sarcomere that contains the entirety of the myosin fibers and includes regions of actin and myosin overlap. The I band covers the terminal regions of two adjacent sarcomeres and contains only actin filaments. The H and I bands shorten with muscle contraction, while the A band remains a constant length.[2]
Actin filaments are double-helical structures, known as filamentous-actin (F-actin), composed of monomeric G-actin units. G-actin exhibits polarity and creates a positive and negative end within the sarcomere, with the positive end situated toward the terminal end of the sarcomere. Tropomyosin is a helical protein that runs along the actin double helix within its groove.[3] Troponin binds tropomyosin by a troponin complex at every seven actin monomer and is composed of troponin-C (Tn-C), troponin-I (Tn-I), and troponin-T (Tn-T). Tn-T binds tropomyosin, Tn-I inhibits actin and myosin binding, and Tn-C binds calcium.[4]
Myosin proteins are composed of two regions: light meromyosin and heavy meromyosin. Light meromyosin binds other light meromyosin regions to anchor myosin at the M line. Heavy meromyosin is further subdivided into two regions; the S-1 portion, or the myosin head, binds actin and contains an ATPase portion, while the S-2 portion is the location of the power stroke.[5]
Support proteins within the sarcomere include titin, desmin, myomesin, C protein, nebulin, and plectin. Plectin tethers the Z disks of adjacent myofibrils to each other. Desmin helps maintain myofibril alignment, connects to the cytoskeleton and other structural elements within the cell, and distributes contractile force. Myomesin and C protein are myosin-binding proteins that function to tether and stabilize myosin at the M line. Titin is found at the Z disk and anchors myosin longitudinally within the sarcomere.
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Muscle fiber types can be broken down into three groups. Type I fibers, or slow oxidative fibers, are slow-twitching fibers. They are the smallest fiber type and have a low glycogen content. Type I fibers have a low rate of fatigue, slow contractile speed, and low myosin ATPase activity, making them best suited for endurance types of contraction, such as maintaining posture and marathon running.
Type IIa fibers, or fast oxidative fibers, are fast twitching fibers with a high myosin ATPase activity and an intermediate rate of fatigue. They are best suited for medium-duration and moderate-movement actions like walking and biking.
Type I and IIa fibers are called red fibers, meaning they have a high myoglobin content. They also obtain ATP primarily from oxidative phosphorylation and are composed of many mitochondria and capillaries.
Type IIb fibers, or fast glycolytic fibers, are fast-twitching fibers. They are the largest fibers in diameter due to their high density of actin and myosin proteins. Type IIb fibers contain few mitochondria and are termed white fibers due to their low myoglobin content. These fibers obtain ATP primarily from anaerobic glycolysis, have a high myosin ATPase activity, and have a fast rate of fatigue. They are best suited for short-duration, intense movements such as sprinting and weight-lifting.
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