When a muscle is in a relaxed state there is some overlap between the myosin and the actin filaments.įorce is generated in the sarcomere through interaction between the actin and myosin filaments ( Fig. Each (thick) myosin filament is surrounded by a hexagonal array of (thin) actin filaments. From the Z-disk, (thin) actin myofilaments project towards the middle of each sarcomere and in the central region of each sarcomere they interdigitate with (thick) myosin filaments. The sarcomere is defined as the part of the myofibril enclosed between the so-called Z-disks or Z-lines ( Fig. Sarcomeres are arranged in series and in parallel and are responsible for force generation and shortening of the muscle fiber. The myofilaments are arranged in sarcomeres, which are the contractile units of the muscle fibers. The myofibrils are made up of filaments, called myofilaments. d Two myofibrils connected with intermediate filaments and costameres responsible for the perpendicular transmission of force to the extracellular matrix. c Single sarcomere, showing Z-disks and myosin and actin filaments. b Muscle fascicle with muscle fibers and myofibrils. a Relationship between muscle fibers and connective tissue in a pennate skeletal muscle: 1, capillary within the endomysium 2, muscle fiber 3, endomysium 4, perimysium 5, epimysium 6, fascia of the muscle. The relationship between the muscle cell diameter and muscle fiber length is called the aspect ratio a muscle cell with a diameter of 50 μm and a length of 15 cm is 3000 times longer than it is thick that is, it has an aspect ratio of 3000. The mean diameter of a muscle cell is approximately 50 μm, especially in untrained individuals. It is generally assumed that the length of a muscle fiber can be about 60% of the total muscle length. In humans, the length of the muscle fibers may vary between a few millimeters and many centimeters. The satellite cells are not innervated but are influenced by the microenvironment and chemical signal substances produced in the muscle fibers. Given that fully differentiated muscle fibers are postmitotic, the only cells that can divide to serve in different repair and regeneration processes are the satellite cells. These cells are the muscle fiber stem cells, also called myogenic progenitor cells. Satellite cells are located between the basal membrane and the cell membrane of the muscle fiber. Muscle fibers, or muscle cells, are cable-like structures composed of tightly packed subunits, myofibrils, that fill up most of the volume of the fibers ( Fig. The connective tissue of the muscle also contains blood vessels and nerves ( Fig. The epimysium is continuous with the fascia, which allows the muscle to slide with respect to surrounding tissues. The whole muscle is enclosed by the epimysium. Muscle fiber bundles have a diameter of approximately 1 to 3 mm. The individual fibers together with their endomysium are arranged in bundles (fascicles) and each bundle is surrounded by another layer of connective tissue, the perimysium. A thin layer of connective tissue, the endomysium, surrounds each individual muscle fiber. The muscle fibers are arranged in parallel and normally extend from one tendon to another ( Fig. The muscle fibers are the active elements, while connective tissues are the passive elements and serve as the connection between the muscle fibers and the skeleton. Each muscle is composed of muscle fibers and connective tissue. It accounts for approximately 40% of total body weight and is organized into hundreds of separate entities, or body muscles, each of which has been assigned a specific task to enable the great variety of movements that are essential to normal life. Skeletal muscle is the largest organ in the body.
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