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. 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. The primary component of thin filaments is the actin protein.
Myofibrils run parallel to the myocyte and typically run for its entire length, attaching to the sarcolemma at either end. Each myofibril is surrounded by the sarcoplasmic reticulum, which is closely associated with the transverse tubules.
Sarcomere : The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments. Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins.
These proteins are organized into regions termed sarcomeres, the functional contractile region of the myocyte. Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction.
The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance. Myofibrils are composed of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments. Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments. Thin filaments are composed of actin, tropomyosin, and troponin. The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle.
Privacy Policy. Skip to main content. Muscular System. Search for:. Smooth Muscle. Skeletal Muscle Fibers Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
Learning Objectives Outline the structure of a skeletal muscle fiber.
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