Skeletal Muscle Fiber Structure
Skeletal muscle is a form of striated muscle tissue that is controlled voluntarily. It is one of three major muscle types, along with cardiac and smooth muscle. As their name suggests, most skeletal muscles are attached to bones by bundles of collagen fibers known as tendons. The term muscle refers to multiple bundles of muscle fibers held together by connective tissue.
Skeletal muscle tissue is made up of individual muscle cells called myocytes, sometimes colloquially called "muscle fibers". They are formed from the fusion of developmental myoblasts (a type of embryonic progenitor cell that gives rise to a muscle cell) in a process known as myogenesis. Myocytes are long, cylindrical, multinucleated cells that run parallel to each other within a muscle ((Figure 1).
These cells are incredibly large, with diameters of up to 100 µm and lengths of up to 30 cm . The plasma membrane of a skeletal muscle fiber, the sarcolemma, is the site of action potential conduction, which triggers muscle contraction.
Within each muscle fiber or myoctye are myofibrils: long cylindrical structures that lie parallel to the muscle fiber. Myofibrils run the entire length of the muscle fiber. Because they are only approximately 1.2 µm in diameter, hundreds to thousands can be found inside one muscle fiber. They attach to the sarcolemma at their ends, so that as myofibrils shorten, the entire muscle cell contracts.
Myofibrils are composed of long proteins such as actin, myosin, and titin, while other proteins hold them together. These proteins are organized into thin filaments and thick filaments, which repeat along the length of the myofibril in sections called sarcomeres. Muscles contract by sliding the thin (actin) and thick (myosin) filaments along each other. Dark "A" bands and light "I" bands repeat along myofibrils. 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 (Figure 2). One sarcomere, the space between two consecutive Z discs, 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. As the sarcomeres individually contract, the myofibrils and muscle cells shorten.
Myofilaments (Thick and Thin Filaments)
Myofibrils are composed of smaller structures called myofilaments. There are two main types of filaments: thick filaments and thin filaments. Thick filaments occur only in the A band of a myofibril and 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 (Figure 3.
Thin filaments are composed of actin, tropomyosin, and troponin. Thin flilaments, attached to a protein in the Z disc called alpha-actinin, occur across the entire length of the I band and part way into the A band. Thin filaments do not extend all the way into the A bands, leaving a central region of the A band that contains only thick filaments, 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.