Skeletal muscle is a form of striated muscle In practice, the term "striated muscle" is sometimes used to refer exclusively to skeletal muscle when distinguishing it from smooth muscle. However, different medical dictionaries report different usages of the terms. Cardiac muscle is a different type of muscle, but has almost the same structure as skeletal muscle tissue existing under control of the somatic nervous system. It is one of three major muscle types, the others being cardiac Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle. The cells that comprise cardiac muscle are called cardiomyocytes and are sometimes seen as an intermediate and smooth muscle Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by gap junctions to neighboring cells such that the. As its name suggests, most skeletal muscle is attached to bones by bundles of collagen Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. Collagen, in the form of elongated fibers known as tendons A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fascia connect muscles to other muscles. Tendons and muscles work together and can.
Skeletal muscle is made up of individual components known as muscle fibers. These fibers are formed from the fusion of developmental myoblasts A myoblast is a type of embryonic progenitor cell that gives rise to myocytes. The myofibers are long, cylindrical A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given straight line, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder. The surface area and the volume of a cylinder have been known since, multinucleated Multinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control (e.g. in metastazing tumor cells and certain mutants of S. cerevisiae) but also commonly cells composed of actin Actin is a globular, roughly 42-kDa protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans. Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of and myosin Myosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility myofibrils Myofibrils are cylindrical organelles. They are found within muscle cells. They are bundles of actomyosin filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end repeated as a sarcomere A sarcomere is the basic unit of a muscle's cross-striated myofibril(cylindrical organelles found within muscle cells). Sarcomeres are multi-protein complexes composed of three different filament systems, the basic functional unit of the cell and responsible for skeletal muscle's striated appearance and forming the basic machinery necessary for muscle contraction. The term muscle refers to multiple bundles of muscle fibers held together by connective tissue.
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Muscle fibers
Individual muscle fibers are formed during development from the fusion of several undifferentiated immature cells known as myoblasts into long, cylindrical, multi-nucleated cells. Differentiation into this state is primarily completed before birth with the cells continuing to grow in size thereafter. Skeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of cytoskeletal elements The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton. It is a dynamic structure that maintains cell shape, in the cytoplasm of the muscle fibers. The principal cytoplasmic proteins are myosin Myosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility and actin Actin is a globular, roughly 42-kDa protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans. Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of (also known as "thick" and "thin" filaments, respectively) which are arranged in a repeating unit called a sarcomere A sarcomere is the basic unit of a muscle's cross-striated myofibril(cylindrical organelles found within muscle cells). Sarcomeres are multi-protein complexes composed of three different filament systems. The interaction of myosin and actin is responsible for muscle contraction.
There are two principal ways to categorize muscle fibers: the type of myosin (fast or slow) present, and the degree of oxidative phosphorylation Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all carry out oxidative phosphorylation to produce ATP, the molecule that supplies energy to metabolism. This pathway is that the fiber undergoes. Skeletal muscle can thus be broken down into two broad categories: Type I and Type II. Type I fibers appear red due to the presence of the oxygen binding protein myoglobin Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the bloodstream is when it is released following muscle injury. These fibers are suited for endurance and are slow to fatigue because they use oxidative metabolism Cellular respiration is the set of the metabolic reactions and processes that take place in organisms' cells to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve the oxidation of one molecule and the reduction of to generate ATP. Type II fibers are white due to the absence of myoglobin and a reliance on glycolytic enzymes. These fibers are efficient for short bursts of speed and power and use both oxidative metabolism and anaerobic metabolism Fermentation is the process of deriving energy from the oxidation of organic compounds, such as carbohydrates, and using an endogenous electron acceptor, which is usually an organic compound, as opposed to Respiration where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain. Fermentation does depending on the particular sub-type. These fibers are quicker to fatigue.
Skeletal muscle fibers are not all the same. Traditionally, they were categorized depending on their varying color.
Red Fibers: Those containing high levels of myoglobin and oxygen storing proteins had a red appearance. Red muscle fibers tend to have more mitochondria and blood vessels than the white ones.
White Fibers: Those with a low content had a white appearance.
To further confuse the issue skeletal muscle fibers are also classified, depending on their twitch capabilities, into fast and slow twitch.
Fast Twitch: Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly.
However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well developed, short term, glycolytic system for energy transfer and can contract and develop tension at 2-3 times the rate of slow twitch fibers.
Slow Twitch: The slow twitch fibers generate energy for ATP re-synthesis by means of a long term system of aerobic energy transfer. They tend to have a low activity level of ATPase, a slower speed of contraction with a less well developed glycolytic capacity. They contain large and numerous mitochondria and with the high levels of myoglobin that gives them a red pigmentation they have been demonstrated to have high concentration of mitochondrial enzymes, thus they are fatigue resistant.
The 2 main categories of muscle fibers become 3 when we split the white muscle fibers into 2 sections. So we expand further:
Type I Red fibers. Slow oxidative (also called slow twitch or fatigue resistant fibers). Contain:
- Large amounts of myoglobin.
- Many mitochondria.
- Many blood capillaries.
- Generate ATP by the aerobic system, hence the term oxidative fibers.
- Split ATP at a slow rate.
- Slow contraction velocity.
- Resistant to fatigue.
- Found in large numbers in postural muscles.
- Needed for aerobic activities like long distance running.
Type IIa Red fibers. Fast oxidative (also called fast twitch A or fatigue resistant fibers). Contain:
- Large amounts of myoglobin.
- Many mitochondria.
- Many blood capillaries.
- High capacity for generating ATP by oxidation. Split ATP at a very rapid rate and, hence, high *contraction velocity
- Resistant to fatigue but not as much as slow oxidative fibers.
- Needed for sports such as middle distance running and swimming.
Type IIb White. Fast glycolytic (also called fast twitch B or fatigable fibers). Contain:
- Low myoglobin content.
- Few mitochondria.
- Few blood capillaries.
- Large amount of glycogen.
- Split ATP very quickly.
- Fatigue easily.
- Needed for sports like sprinting.
Individual muscles are a mixture of 3 types of muscle fibers (type 1 and type 2a and b), but their proportions vary depending on the action of that muscle. It must be remembered that skeletal muscles, although a mixture, can only have one type of muscle fiber within a motor unit. This is demonstrated if we look at contractions. E.g. If a weak contraction is needed only the type 1 motor units will be activated. These fibers are used mainly for endurance activities. If a stronger contraction is required the type 2a fibers will be activated or used to assist the type 1 fibers. Maximal contractions facilitate the use of type 2b fibers which are always activated last. These fibers are used during ballistic activities but tire easily. With advanced EMG techniques it is possible to look at which muscle fibers are recruited when performing an exercise/test. The total number of skeletal muscle fibers has traditionally been thought not to change. It is believed there are no sex or age differences in fiber distribution, however, relative fiber types vary considerably from muscle to muscle and person to person. Sedentary men and women (as well as young children) have 45% type 2 and 55% type 1 fibers. People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type 1 fibers. Sprint athletes, on the other hand, require large numbers of type 2 b fibers. Middle distance event athletes show approximately equal distribution of the 2 types. This is also often the case for power athletes such as throwers and jumpers. It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle. It is thought that if you perform endurance type events for a sustained period of time, some of the type 2b fibers transform into type 2a fibers. However, there is no consensus on the subject. It may well be that the type 2b fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes not a change in fiber type.
Structure Of Skeletal Muscle Fiber
Every organelle and macromolecule of a muscle fiber are arranged to ensure form meets function. The plasma membrane is called the sarcolemma The sarcolemma is the cell membrane of a muscle cell . It consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber, and the tendon with the cytoplasm known as the sarcoplasm The Sarcoplasm of a muscle fiber is comparable to the cytoplasm of other cells fluid of the muscle, but it houses unusually large amounts of glycosomes and significant amounts of myoglobin, an oxygen binding protein. The calcium concentration in sarcoplasma is also a special element of the muscular fiber by means of which the contractions takes. In the sarcoplasm are the myofibrils Myofibrils are cylindrical organelles. They are found within muscle cells. They are bundles of actomyosin filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end. The myofibrils are long protein bundles about 1 micrometer in diameter each containing myofilaments. Pressed against the inside of the sarcolemma are the unusual flattened nuclei. Between the myofibrils are the mitochondria. While the muscle fiber does not have a smooth endoplasmic reticulum it contains a sarcoplasmic reticulum The endoplasmic reticulum is a eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction. Periodically it has dilated end sacs known as terminal cisternae. These cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infoldings called a transverse tubule (T tubule). The T tubule A T-tubule is a deep invagination of the sarcolemma, which is the plasma membrane, only found in skeletal and cardiac muscle cells. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell are the pathway for the action potential to signal the sarcoplasmic reticulum to release calcium causing a muscle contraction. Together two terminal cisternae and a transverse tubule form a triad. [1]
Cellular physiology and contraction
In addition to the actin Actin is a globular, roughly 42-kDa protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans. Actin is the monomeric subunit of two types of filaments in cells: microfilaments, one of and myosin Myosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility components that constitute the sarcomere A sarcomere is the basic unit of a muscle's cross-striated myofibril(cylindrical organelles found within muscle cells). Sarcomeres are multi-protein complexes composed of three different filament systems, skeletal muscle fibers also contain two other important regulatory proteins, troponin Troponin is a complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle and tropomyosin Tropomyosin is an actin-binding protein that regulates actin mechanics. It is important, among other things, for muscle contraction. Tropomyosin, along with the troponin complex, associate with actin in muscle fibers and regulate muscle contraction by regulating the binding of myosin. In resting muscle, tropomyosin overlays the myosin binding, that are necessary for muscle contraction to occur. These proteins are associated with actin and cooperate to prevent its interaction with myosin. Skeletal muscle cells are excitable and are subject to depolarization In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential. Hyperpolarization is the opposite of depolarization, and inhibits the rise of an action potential by the neurotransmitter acetylcholine The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans. Acetylcholine is one of many neurotransmitters in the autonomic nervous system (ANS) and the only neurotransmitter used in the motor division of the somatic nervous system. (, released at the neuromuscular junction A neuromuscular junction is the synapse or junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract. In vertebrates, the signal passes through the by motor neurons In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons[2].
Once a cell is sufficiently stimulated, the cell's sarcoplasmic reticulum The endoplasmic reticulum is a eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945 releases ionic calcium Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust. Calcium is also the fifth most abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, (Ca2+), which then interacts with the regulatory protein troponin. Calcium-bound troponin undergoes a conformational change that leads to the movement of tropomyosin, subsequently exposing the myosin-binding sites on actin. This allows for myosin and actin ATP-dependent cross-bridge cycling and shortening of the muscle.
Physics
Muscle force is proportional to physiologic cross-sectional area (PCSA), and muscle velocity is proportional to muscle fiber length[3]. The strength of a joint, however, is determined by a number of biomechanical parameters, including the distance between muscle insertions and pivot points and muscle size. Muscles are normally arranged in opposition so that as one group of muscles contract, another group relaxes or lengthens. Antagonism in the transmission of nerve impulses to the muscles means that it is impossible to stimulate the contraction of two antagonistic muscles at any one time. During ballistic motions such as throwing, the antagonist muscles act to 'brake' the agonist muscles throughout the contraction, particularly at the end of the motion. In the example of throwing, the chest and front of the shoulder (anterior Deltoid) contract to pull the arm forward, while the muscles in the back and rear of the shoulder (posterior Deltoid) also contract and undergo eccentric contraction to slow the motion down to avoid injury. Part of the training process is learning to relax the antagonist muscles to increase the force input of the chest and anterior shoulder.
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Thu, 26 Aug 2010 18:04:55 GMT+00:00
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