It has one nucleus per cell, is branched, and is distinguished by the presence of intercalated disks. Each skeletal muscle fiber is a skeletal muscle cell.
The plasma membrane of a skeletal muscle fiber is called the sarcolemma. The sarcolemma is the site of action potential conduction, which triggers muscle contraction. Within each muscle fiber are myofibrils —long cylindrical structures that lie parallel to the muscle fiber.
Myofibrils run the entire length of the muscle fiber, and because they are only approximately 1. They attach to the sarcolemma at their ends, so that as myofibrils shorten, the entire muscle cell contracts Figure 2. Figure 2. A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm called the sarcoplasm. A muscle fiber is composed of many fibrils, packaged into orderly units. 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. Figure 3. A sarcomere is the region from one Z line to the next Z line.
Many sarcomeres are present in a myofibril, resulting in the striation pattern characteristic of skeletal muscle. 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 3. 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. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and blood vessels.
Each cell is spindle shaped with a single nucleus and no visible striations Figure 4. 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. Each cell is striated with a single nucleus and they attach to one another to form long fibers. Cells are attached to one another at intercalated disks. The cells are interconnected physically and electrochemically to act as a syncytium.
Cardiac muscle cells contract autonomously and involuntarily. Smooth muscle is involuntary. Each cell is a spindle-shaped fiber and contains a single nucleus.
No striations are evident because the actin and myosin filaments do not align in the cytoplasm. You are watching cells in a dish spontaneously contract. They are all contracting at different rates, some fast, some slow. After a while, several cells link up and they begin contracting in synchrony. Discuss what is going on and what type of cells you are looking at. The cells in the dish are cardiomyocytes, cardiac muscle cells.
With the further extraction of actin, only the Z disks remained. Strangely, the sarcomere length did not change, and these "ghost" myofibrils still exhibited elastic behavior. The breakthrough came in the s with the discovery of the gigantic protein titin. Titin anchors the A band in the middle of the sarcomere and may determine thick-filament length and thus A-band length.
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