Skeletal Muscle | Anatomy & Physiology
The connective layers (epimysium, perimysium, and endomysium) merge or surgery, it is a mistake not to consider these close relationships of anatomy of managing patients with various types of tendon injuries include. The connective tissue merges with both the origin and the insertion tendons of the . fibers through the endomysium, but also the perimysium has been shown to the skeletal muscle.5 Interestingly, the difference between individuals ( animals) onset muscle soreness is developed and several days with muscle pain and. reducible cross-link content to give reproducible results. It was shown muscle from the endomysium to the tendon. In this tion of epimysium, perimysium and endomysium. . treatment involved two extractions of the pellet.
Describe the connective tissue layers surrounding skeletal muscle Define a muscle fiber, myofibril, and sarcomere List the major sarcomeric proteins involved with contraction Identify the regions of the sarcomere and whether they change during contraction Explain the sliding filament process of muscle contraction Each skeletal muscle is an organ that consists of various integrated tissues.
These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each skeletal muscle has three layers of connective tissue that enclose it, provide structure to the muscle, and compartmentalize the muscle fibers within the muscle Figure 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. Bundles of muscle fibers, called fascicles, are covered by the perimysium.
Muscle fibers are covered by the endomysium. Inside each skeletal muscle, muscle fibers are organized into bundles, called fascicles, surrounded 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 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. In skeletal muscles that work with tendons to pull on bones, the collagen in the three connective tissue layers 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 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.
Skeletal Muscle Fibers 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. It is the shortening of these individual sarcomeres that lead to the contraction of individual skeletal muscle fibers and ultimately the whole muscle. A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells.
A muscle fiber is composed of many myofibrils, which contain sarcomeres with light and dark regions that give the cell its striated appearance. The Sarcomere A sarcomere is defined as the region of a myofibril contained between two cytoskeletal structures called Z-discs also called Z-linesand the striated appearance of skeletal muscle fibers is due to the arrangement of the thick and thin myofilaments within each sarcomere Figure 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. 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 lengthbut 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.
Myofilament Components 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. The tenocytes have an elongated shape, while the tenoblasts have an ovoid shape. In the tendon healing phase, the tenoblasts are more involved in the tissue repair process, depositing collagen fibers.
Finally, in the last repair phase, the tenoblasts are transformed into tenocytes. In the compression areas we can find endothelial cells and chondrocytes. These specialized fibroblasts produce extracellular matrix, such as collagen, proteoglycans, and other proteins. The prevalent collagen in the tendons is that of type I, next to minor proportions of type III collagen, present in the epitenonium and in the endotenonium and of type II, identifiable in the fibrocartilaginous areas of the osteo-tendinous junction.
The bundles of collagen fibers show a wavy pattern, with periodic changes of direction, known in the literature as crimps. In the same tendon, the crimps differ in size and geometry, appearing as isosceles or scalene triangles of variable size.
The single crimp, observed by scanning electron microscope, appears to consist of rectilinear fibrillary segments tightly packed together joined by nodes or hinges, in correspondence of which all the fibrils of each fascicle change simultaneously direction.
In changing their course, the fibrils do not describe a loop but are deformed as a hollow cylindrical structure would do. Collagen bends but does not break. Mechanical Properties The biomechanical behavior of a tendon is related not only to the magnitude of tension stress but also to the shape of the tendon itself.
Skeletal Muscle Fibers
Muscles used to perform delicate and precise movements, such as the flexors of the fingers, possess long and thin tendons, while those that perform actions of power and endurance, such as the quadriceps femoris and the triceps sural, have shorter and more robust tendons.
A short tendon has a greater tensile strength than a long tendon because the load required to produce the break is much larger in the short tendon with the same diameter. A long tendon can undergo a greater deformation than that of a short tendon before going to rupture.
Strength and resistance of a tendon are therefore 2 different entities and depend on the diameter and length of the tendon itself. The biomechanical properties of the tendon are related to the diameter and arrangement of collagen fibrils; tendons subjected to high stress are fibrils of a large diameter, less flexible than those of small caliber. The ability of the tendons to amortize and transmit the force of muscle contraction is also closely related to the tendon crimp.
They act as a shock absorber in the tendon during the early stages of pulling, and allow the tendon to recover the form at the cessation of the applied force.
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The crimps do not disappear simultaneously flatbut gradually, from the periphery to the center. The same biochemical and cellular structure of the tendon contribute to the mechanical function of the tissue: Adaptation to Mechanical Stress There is a difference between a man and a woman. After a training that undergoes the tendon to mechanical stress, the female has less tendon strength than the male. Women have a greater tendon lengthening; this could explain why men are more susceptible to tendon inflammation.
The tendon tissue adapts itself to the mechanical environment that surrounds it. In the presence of mechanical tension contraction and muscle releasethe tendon increases its diameter, thanks to an increase in the synthesis of collagen.
Adaptation is specular to the imposed stress. Finally, it increases the stiffness and the Young's modulus. The latter defines the relationship between mechanical stress force per unit area and strain deformation.
Epimysium : Anatomy of Muscle Structure
Adaptation to Age The tendon of older people is less adaptable. If in a young person, the tension of the Achilles tendon is within the basic values after 1 hour, in the elderly subjects there is a loss of tension for more time. In an older person's tendon, there is an alteration of the cellular structure less anisotropywith a diminished response to the regenerative capacity.
The tendon is less able to adequately pilot the force expressed by the muscle toward the bone tissue. Collagen fibers are less organized; calcification phenomena can occur; there is a lower number of fibroblasts and senescent cells and a decrease in the amount of water and the number of proteoglycans, with reduced viscoelastic properties.
The tendon is weakened and becomes more susceptible to trauma. The decrease of estrogen in women makes the tendon tissue looser there are estrogenic receptors on the tendon. This metabolic event would make the tendon more prone to trauma and inflammation. Embryology The tendon is derived from the ectoderm such as ligaments.
Progenitor cells of the tendon tenocytes originate from the ventral and lateral ectodermal area. In the cranial and limb region animal model the tendon is formed in the absence of the muscle; subsequently, the contractile tissue will be indispensable for their maintenance.
Usually, the progression of cartilage and muscle tissue in the development period occurs more quickly, compared to the tendon. Blood Supply and Lymphatics The blood circulation of the tendon is ensured, in small part, by the vessels coming from the muscular belly and the periosteum surrounding the osteotendinous junction and, for the rest, from the vascular network of the peritendinous sheets and the synovial sheath, in the sites where this is present.
In some cases, in fact, the primary trunks are arranged to form a rather regular mesh structure; in other cases, they form concentric arches and are arranged in a completely irregular manner.
The vascular network consists of small and medium-caliber arteries, each of which is accompanied by one or two satellite veins extensively anastomosed to each other. Based on the characteristics of the capillaries, 3 types of microvascular units can be distinguished.
Another type of microvascular unit, less specialized than the previous one, presents capillaries with an irregular course. The presence of multiple microvascular units would facilitate the diffusion of gases and metabolites within individual tendon bundles.
Lymphatic drainage affects the connective tissue sheath of the tendon, forming a network.
The lymphatic contents will go towards the tendon veins or to other neighboring venous structures When the tendon is subjected to mechanical stress, the flow of blood that reaches the tendon tissue increases.
Lymphatic drainage under stress does not increase. Nerves Tendons are innervated by nerve branches coming from both the muscular belly and from the sensitive branches that are distributed to the skin.
The innervation is localized in the paratenon, endotenon, and epitenon. In the context of the tendon, the nervous branches, which are relatively scarce, form trunks with a course parallel to the major axis of the tendon itself, anastomosed from branches with a transverse and oblique course. The nerve endings of these branches are of various types: The blood branches accompany the nerve branches. The sympathetic innervation of the tendon is found in the vicinity of the perivascular areas.
At the peritendinous level, sensory nerve fibers can be found, similar to parasympathetic fibers. Generally, in a healthy tendon, the orthosympathetic fibers stimulate the vasoconstriction, the parasympathetic fibers allow vasodilation.
Finally, there are small-diameter sensory fibers, which can stimulate vasodilation. Muscles The tendon adapts to the morphology of the muscle. A flat muscle will have a flattened tendon or aponeurosis, while a muscle with a larger diameter will have a cordiform tendon.