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These blood vessels help in the transportation of the blood to and from the muscle and also supply oxygen and other nutrients and also remove the unnecessary carbon dioxide and other waste from the blood. Through the motor nerves, there are signals that are sent from the central nervous system to the muscles that initiate the contraction of the muscles.
There is also a response that the muscle gives to the hormones that are produced by the endocrine glands. These hormones also interact with the complementary receptors which are present on the surface of the cells which helps in initiating the specific reaction. There are important sensory structures associated with each muscle called the stretch receptors that help in monitoring the state of the muscle and also return all the information to the central nervous system. The change in the length of the muscle and the velocity of the movement of the muscle are the two things that the stretch receptors are sensitive to.
The stretch receptors are responsible for completing a feedback system that allows the central nervous system to assess the movement of the muscles and also to adjust the motor signals in the light of the movement. The myofibril is a component of the animal skeletal muscle. Myofibrils are very fine contractile fibers and many groups of myofibrils are extended in the parallel columns along the length of the striated muscle fibers.
The myofibrils and the resulting myofibers may be several centimeters in length. The myofibrils are composed of many thick and thin myofilaments that help in giving the muscle its striated appearance. The thick filaments are composed of myosin and the thin filaments are composed of actin and also have other muscle proteins such as tropomyosin and troponin. The myofibrils are made up of repeated subunits which are called the sarcomeres.
The muscular contraction happens due to the interaction between the actin and the myosin filaments when they are temporarily bound and released. The muscle fibres are single multinucleated cells that usually combine to form a muscle. Thin Filaments: The thin filaments primarily consist of the protein actin which is coiled with the nebulin filaments. When the polymerization of the actin filament happens, there is a formation of a ladder along which the myosin filament climbs to generate the motion.
The Thick Filament: The thick filament is primarily composed of the protein called myosin. The responsibility of the protein myosin is the forced generation. Myosin is composed of a globular head that has both Adenosine triphosphate ATP and actin-binding sites and also a long tail which is involved in its polymerization into the myosin filaments.
Acrinomyosin is the name given to the protein complex which is composed of both actin and myosin. The striated muscle such as that of the skeletal and the cardiac muscle have actin and myosin filaments that are of specific and constant length which are of the order of a few millimetres. These are very small when compared to the elongated muscle cells which are of the length in the centimetres.
The myofilaments are organized into repeated subunits along the length of the myofibrils. These subunits are called the sarcomeres. Myofibrils fill the muscle cells which run parallel to each other on the long axis of the cell. The sarcomeric subunits of one myofibril are in perfect alignment with the myofibrils which are next to it and this alignment causes the cells to look striated or striped.
In the case of smooth muscle cells, there is no alignment, hence there are no striations and hence the cells are called smooth.
The skeletal muscle cells are long and cylindrical and they are also referred to as the muscle fibres or the myofibers. Skeletal muscles are made up of parallel fibers called skeletal muscle fibers. Each fiber is a multinuclear cell having hundreds to thousands of myofibrils.
Myofibrils are the thin fibers made up of acti9n and myosin filaments and are present among the cytoplasm of skeletal muscle fibers. They are the contractile units that are responsible for the contraction of skeletal muscles. Each skeletal muscle fiber is innervated by a separate axon and can undergo contraction independent of other cells.
In this article, we will discuss the general structure of a skeletal muscle fiber, the structure of myofibrils, their major components, the mechanism of contraction, and some major diseases associated with them. So, keep reading. Skeletal muscle fibers are the cylindrical cells with some special modification to help them undergo contraction.
Like other animal cells, they have a plasma membrane, and other organelles like ribosomes, mitochondria, endoplasmic reticulum, etc. They have multiple nuclei within one cell. Some of the important features of these cells are as follows.
The cytoplasm of skeletal muscle cells is called sarcoplasm. It is surrounded on outside by a plasma membrane called sarcolemma. Sarcoplasm has abundant mitochondria that provide energy in the form of ATP needed for muscle contractions. They have multiple nuclei that lie flat in the periphery of the fiber. Nuclei are present adjacent to the sarcolemma of cells. Myofibrils are arranged in a parallel fashion in the skeletal muscle fibers separated by the sarcoplasm.
Mitochondria lie close to the myofibrils so that energy can be provided to these contractile fibers. It is a network of a rich network of endoplasmic reticulum found only in the skeletal muscle fibers. Its function is to store calcium ions and release them upon excitation for the contraction of skeletal muscles.
It appears as a complex network of tubules present among the myofibrils. These tubules have terminal dilations or sacs called terminal cisternae. The invaginations of the sarcolemma are present in between these cisternae. The action potential is carried by these invaginations to the sarcoplasmic reticulum so that they are excited to release calcium ions.
These are the invaginations of sarcolemma having the same concentration of ions as is present in the extracellular fluid. An action potential generated at any point along the length of the sarcolemma is carried by these tubules to the core of muscle fibers. They are connected to the terminal cisternae of sarcoplasmic reticulum and are responsible for releasing calcium ions and coupling contraction of skeletal muscles with excitation.
After understanding the general structure of a skeletal muscle fiber, let us now study the structure of myofibrils. As mentioned earlier, each skeletal muscle fiber consists of hundreds to thousands of myofibrils extending throughout the length of the fiber.
They are the contractile structures in skeletal muscles. The contraction of these myofibrils causes the contraction of skeletal muscle. The number of these filaments varies in individual myofibrils. Each myofibril contains around thin filaments and thick filaments. These light and dark filaments are arranged within a myofibril in such a way that they interdigitate and give a striated appearance to it. These striations of myofibrils are responsible for the overall striated appearance of skeletal muscles.
As myofibrils are made up of interdigitating thick and thin filaments, the parts containing only thin filaments appear as light bands called the I bands.
They are named I bands because they are isotropic to polarized light. Actin is a contractile filamentous protein that makes the backbone of actin filaments. This filamentous actin protein is made up of F-actin filaments, each of which consists of polymerized G-actin molecules.
Two F-actin filaments are wrapped around each other to form a helix in thin filaments. A molecule of ADP is associated with each G-actin molecule. These ADP molecules act as active sites for the formation of cross-bridges formed by the association of thick and thin filaments.
Tropomyosin another filamentous protein present in the thin filaments of myofibrils. Two tropomyosin filaments are wrapped around the double helix formed by F-actin. The tropomyosin filaments are wrapped in such a way that they lie in the grooves of F-actin double helix.
The tropomyosin filaments cover the active sites on F-actin molecules and prevent the formation of actin-myosin cross-bridges in resting state. Troponin is a non-filamentous protein associated with the thin filaments. It is found attached to the tropomyosin molecules at periodic sites along the length of a myofibril.
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. Troponin and tropomyosin run along the actin filaments and control when the actin binding sites will be exposed for binding to myosin. Thick myofilaments are composed of myosin protein complexes, which are composed of six proteins: two myosin heavy chains and four light chain molecules. The heavy chains consist of a tail region, flexible hinge region, and globular head which contains an Actin-binding site and a binding site for the high energy molecule ATP.
The light chains play a regulatory role at the hinge region, but the heavy chain head region interacts with actin and is the most important factor for generating force. Hundreds of myosin proteins are arranged into each thick filament with tails toward the M-line and heads extending toward the Z-discs. Other structural proteins are associated with the sarcomere but do not play a direct role in active force production. Titin, which is the largest known protein, helps align the thick filament and adds an elastic element to the sarcomere.
Titin is anchored at the M-Line, runs the length of myosin, and extends to the Z disc. The thin filaments also have a stabilizing protein, called nebulin, which spans the length of the thick filaments. Watch this video to learn more about macro- and microstructures of skeletal muscles.
The arrangement and interactions between thin and thick filaments allows for the shortening of the sarcomeres which generates force. It is important to note that while the sarcomere shortens, the individual proteins and filaments do not change length but simply slide next to each other. This process is known as the sliding filament model of muscle contraction Figure Tropomyosin winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin.
The troponin-tropomyosin complex uses calcium ion binding to TnC to regulate when the myosin heads form cross-bridges to the actin filaments. Cross-bridge formation and filament sliding will occur when calcium is present, and the signaling process leading to calcium release and muscle contraction is known as Excitation-Contraction Coupling.
Skeletal muscles contain connective tissue, blood vessels, and nerves. There are three layers of connective tissue: epimysium, perimysium, and endomysium. Skeletal muscle fibers are organized into groups called fascicles.
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