Howver, some of these structures are specialized in muscle fibers. Within a muscle fiber, proteins are organized into structures called myofibrils that run the length of the cell and contain sarcomeres connected in series. Because myofibrils are only approximately 1.
The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized arrangement of contractile, regulatory, and structural proteins. It is the shortening of these individual sarcomeres that lead to the contraction of individual skeletal muscle fibers and ultimately the whole muscle. A sarcomere is defined as the region of a myofibril contained between two cytoskeletal structures called Z-discs also called Z-lines , and the striated appearance of skeletal muscle fibers is due to the arrangement of the thick and thin myofilaments within each sarcomere Figure The dark striated A band is composed of the thick filaments containing myosin, which span the center of the sarcomere extending toward the Z-dics.
The thick filaments are anchored at the middle of the sarcomere the M-line by a protein called myomesin. The thin filaments extend into the A band toward the M-line and overlap with regions of the thick filament. The A band is dark because of the thicker mysoin filaments as well as overlap with the actin filaments.
The H zone in the middle of the A band is a little lighter in color, because the thin filaments do not extend into this region. Because a sarcomere is defined by Z-discs, a single sarcomere contains one dark A band with half of the lighter I band on each end Figure During contraction the myofilaments themselves do not change length, but actually slide across each other so the distance between the Z-discs shortens.
The length of the A band does not change the thick myosin filament remains a constant length , but the H zone and I band regions shrink.
These regions represent areas where the filaments do not overlap, and as filament overlap increases during contraction these regions of no overlap decrease. The thin filaments are composed of two filamentous actin chains F-actin comprised of individual actin proteins Figure These thin filaments are anchored at the Z-disc and extend toward the center of the sarcomere.
Within the filament, each globular actin monomer G-actin contains a mysoin binding site and is also associated with the regulatory proteins, troponin and tropomyosin. The troponin protein complex consists of three polypeptides. Troponin and tropomyosin run along the actin filaments and control when the actin binding sites will be exposed for binding to myosin. Thick myofilaments are composed of myosin protein complexes, which are composed of six proteins: two myosin heavy chains and four light chain molecules.
Location and number of nuclei How are fibers connected? Morphology Location of muscle Answer:. Slides Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden. Study Mode. With this dye, the A-bands are stained dark and the I-bands light. Since both cardiac muscle fibers and skeletal muscle fibers are striated, how would you differentiate between them in a histological slide?
This is a section of the tongue. Begin by identifying groups of fasciculi cut in transverse section. Where are the nuclei located within a cell? Can you identify the endomysium and the perimysium? Where can capillaries be found? How does the location of capillaries in muscle differ from peripheral nerves? As you zoom in on the image, note the abundance of capillaries in between the cardiac muscle cells.
Why are so many capillaries necessary? Note how the cardiac muscle cells are striated, like skeletal muscle cells. However, unlike skeletal muscle, note how the end of each cells splits into branches.
In addition, the single nuclei of cardiac muscle cells are located centrally. Identify the intercalated discs. What are the functions of the intercalated discs?
This is a transverse section of the gastrointestinal tract. Locate the two layers of smooth muscle and zoom in on them.
In which direction do the fibers of each layer run? How would you describe the morphology of each smooth muscle cell? A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions.
The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle.
Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations Figure. Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle?
The three types of muscle cells are skeletal, cardiac, and smooth. Their morphologies match their specific functions in the body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated appearing as long, unbranched cylinders.
Cardiac muscle is involuntary and found only in the heart. The striated appearance of skeletal muscle tissue is a result of repeating bands of the proteins actin and myosin that are present along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded. Each I band has a dense line running vertically through the middle called a Z disc or Z line.
The Z discs mark the border of units called sarcomeres , which are the functional units of skeletal muscle. One sarcomere is the space between two consecutive Z discs and contains one entire A band and two halves of an I band, one on either side of the A band.
A myofibril is composed of many sarcomeres running along its length, and as the sarcomeres individually contract, the myofibrils and muscle cells shorten Figure 6. Myofibrils are composed of smaller structures called myofilaments.
There are two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. Thick filaments occur only in the A band of a myofibril. Thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band. The region at which thick and thin filaments overlap has a dense appearance, as there is little space between the filaments.
Thin filaments do not extend all the way into the A bands, leaving a central region of the A band that only contains thick filaments. This central region of the A band looks slightly lighter than the rest of the A band and is called the H zone.
The middle of the H zone has a vertical line called the M line, at which accessory proteins hold together thick filaments. Both the Z disc and the M line hold myofilaments in place to maintain the structural arrangement and layering of the myofibril.
Myofibrils are connected to each other by intermediate, or desmin, filaments that attach to the Z disc. Thick and thin filaments are themselves composed of proteins. Thick filaments are composed of the protein myosin. The tail of a myosin molecule connects with other myosin molecules to form the central region of a thick filament near the M line, whereas the heads align on either side of the thick filament where the thin filaments overlap.
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