Smooth muscle is found in the walls of hollow organs throughout the body. Smooth muscle contractions are involuntary movements triggered by impulses that travel through the autonomic nervous system to the smooth muscle tissue. The arrangement of cells within smooth muscle tissue allows for contraction and relaxation with great elasticity.
The smooth muscle in the walls of organs like the urinary bladder and the uterus allow those organs to expand and relax as needed. The smooth muscle of the alimentary canal the digestive tract facilitates the peristaltic waves that move swallowed food and nutrients. In the eye smooth muscle changes the shape of the lens to bring objects into focus. Artery walls include smooth muscle that relaxes and contracts to move blood through the body.
The heart wall is composed of three layers. Cardiac muscle, found only in the myocardium, contracts in response to signals from the cardiac conduction system to make the heart beat. Cardiac muscle is made from cells called cardiocytes. Like skeletal muscle cells cardiocytes have a striated appearance, but their overall structure is shorter and thicker. Cardiocytes are branched, allowing them to connect with several other cardiocytes, forming a network that facilitates coordinated contraction.
At the other end of the tendon, it fuses with the periosteum coating the bone. The tension created by contraction of the muscle fibers is then transferred though the connective tissue layers, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton.
In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis , or to fascia, the connective tissue between skin and bones. Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal.
In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system. Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers or myofibers.
Having many nuclei allows for production of the large amounts of proteins and enzymes needed for maintaining normal function of these large protein dense cells. In addition to nuclei, skeletal muscle fibers also contain cellular organelles found in other cells, such as mitochondria and endoplasmic reticulum. 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. These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium , which allows a muscle to contract and move powerfully while maintaining its structural integrity.
The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently. Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle , by a middle layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle.
Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium.
The endomysium contains the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue. In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layers the mysia intertwines with the collagen of a tendon. At the other end of the tendon, it fuses with the periosteum coating the bone. The tension created by contraction of the muscle fibers is then transferred though the mysia, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton.
In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis , or to fascia, the connective tissue between skin and bones.
Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system. Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers.
During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers. Multiple nuclei mean multiple copies of genes, permitting the production of the large amounts of proteins and enzymes needed for muscle contraction. As will soon be described, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin thin filament and myosin thick filament , along with other support proteins.
The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other. Each group of these microfilaments is called a sarcomere and forms the functional unit of a muscle fiber. Watch this video to learn more about macro- and microstructures of skeletal muscles. The sarcomere itself is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end.
As myofibrils contract, the entire muscle cell contracts. Because myofibrils are only approximately 1. Because the actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere form strands that are thinner than the myosin, it is called the thin filament of the sarcomere.
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