skeletal muscle
| Dictionary: skeletal muscle |
n.
A usually voluntary muscle made up of elongated, multinucleated, transversely striated muscle fibers, having principally bony attachments. Also called striated muscle.
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| World of the Body: skeletal muscle |
Skeletal muscle moves the skeleton and is responsible for all our voluntary movements, as well as for the automatic movements required, for example, to stand, to hold up our head, and to breathe. (Other involuntary functions involve smooth muscle and cardiac muscle.)
As well as being the ‘motors’ of the body, muscles are also the brakes and shock absorbers. They can be used as heaters (when shivering) and also function as a store of protein if we should face malnutrition.
Individual muscles, such as the biceps in the arm, are made up of large numbers (about 100 000 in biceps) of giant cells, known as muscle fibres. Each fibre is formed from fusion of many precursor cells and therefore has many nuclei. The fibres are each as thick as a fine hair (50 μm in diameter) and 10-100 mm long. They are arranged in bundles, separated by sheets of connective tissue containing collagen. These bundles rarely run straight along the axis of the muscle, more usually at an angle, called the angle of pennation because many muscles show a pennate (featherlike) pattern of fibre bundles.
Each muscle fibre is surrounded by a cell membrane, which allows the contents of the fibres to be quite different from that of the body fluids outside them. Inside the fibre are the myofibrils, which constitute the contractile apparatus, and a system for controlling the myofibrils through changes in calcium concentration. This system, the sarcoplasmic reticulum (SR), is a closed set of tubes containing a high concentration of calcium. Each myofibril runs the whole length of the muscle fibre with a variable number of segments, the sarcomeres; it is only one or two micrometres in diameter, and is surrounded by the SR network. The myofibril consists of many much thinner and shorter protein rods, which are the myofilaments. These are of two kinds: thick filaments, which are made predominantly from a single protein, myosin, and thin filaments, which contain the protein actin. The actual contraction takes place by an interaction of the actin with projections on the myosin molecules (crossbridges). Each of the crossbridges can develop force (about 5 × 10-12 Newtons) and can pull the thin filament along past the thick filament by about 10 × 10-9 metres (10 millionths of a mm). The net effect of many of these small movements and small forces is to shorten the myofibrils, and thus the whole muscle; hence some part of the skeleton is moved, by means of the attachment of the muscle at each end to bone, directly or via tendons.
Skeletal muscle at increasing magnification: (a) the whole muscle; (b) bundle of muscle fibres; (c) a single muscle fibre, composed of myofibrils, showing nuclei and striations; (d) a single myofibril, composed of myofilaments. A single sarcomere extends between two 'Z discs'. The darker bands are where actin and myosin filaments overlap. The regular alignment of the light and dark bands of the sarcomeres across the whole muscle fibre accounts for the striped or 'striated' microscopic appearance. (See also figure under -->glycogen-->.) (Adapted from Jennett, S. (1989). Human physiology. Churchill Livingstone, Edinburgh.)
Section of myofibrils with sarcoplasmic reticulum (SR) and T-tubules (T).
Schematic diagram of a single sarcomere: (above) in relaxation; (below) shortened during contraction
When a person initiates a movement, events in the brain and the spinal cord generate action potentials in the axons of the motor neurons. Each of these axons branches to send action potentials to many muscle fibres. (A motor unit is this collection of perhaps several hundred muscle fibres controlled by one axon.) At the nerve terminals of each axon branch (neuromuscular junction) acetylcholine is liberated by the arriving action potential, and this combines with receptors on the membrane of the muscle fibre, causing it, in turn, to generate an action potential. This action potential spreads over the whole surface of the fibre and also down an extensive network of fine tubes (T-tubules), which conduct it into the interior. Here a message, the nature of which is uncertain, passes from the T-tubule to the sarcoplasmic reticulum, causing it to allow some of the calcium it contains to leak out into the interior of the muscle fibre. The thin filaments in the myofibrils contain, as well as actin, two proteins, troponin and tropomyosin; the calcium which leaks from the SR is able, for a brief period, to interact with the troponin molecule of the thin filament; this, through movements of the tropomyosin molecules, alters the thin filament so that the actin molecules are available to be joined by the crossbridges, starting the process of contraction. As soon as calcium escapes from the SR the process starts of mopping it up again. There are calcium pumps in the membranes of the SR, which are able to move the calcium back inside, thus bringing to an end the short period of muscle activity (a muscle twitch). More sustained periods of activity are the norm in the movements we make; they require a sequence of action potentials to be sent to the muscle, at perhaps 30 per second. The contractions produced in this way are stronger than a twitch.
Muscle contraction requires energy to drive the crossbridges through their cyclic interactions with actin: in each cycle the myosin molecule does work in moving the thin filament. Also, energy is used for the process of calcium pumping by the SR. Energy consumption is highest when muscles are used to do external work — for example in climbing stairs, when the body weight has to be lifted. However energy is also used when a weight is held up without doing work on it (isometric contraction). Least energy is used when muscles are used to lower weight, as when descending stairs.
The energy for muscle contraction comes from the splitting of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate. The muscle contains enough ATP to power it at maximum output for only a couple of seconds. ATP can be regenerated in muscle rapidly from phosphocreatine (PCr), and there is enough of this substance in the muscle to last perhaps 10 to 20 seconds of maximum activity. The fact that we can sustain strenuous activity beyond 10 seconds is due to the utilization of carbohydrate in the muscles, where it is stored as glycogen. This can be used to regenerate the ATP supply in two ways. If oxygen is available, glucose can be oxidized to water and carbon dioxide, with two-thirds of the energy released used to rebuild the ATP supply. If oxygen is not available, the process stops with glucose converted to lactic acid and only about 6% of the energy used for building ATP. The lactic acid leaves the muscle cells and can accumulate in the blood. In addition to carbohydrate, muscles use fat, in the form of fatty acids taken up from the blood, as a substrate for oxidation; this is important for prolonged activity, since the body's energy stored as fat is much greater than that stored as carbohydrate. The availability of oxygen depends on its delivery by the blood; when muscle becomes active, the products of its metabolism cause the vessels to dilate, and this enables a rapid increase in the blood flow.
Muscle fatigue is the effect of a set of mechanisms which ensure that muscle is not made active when there is not enough energy available for the activity. If that were to happen, theoretically the muscle could go into rigor mortis, and could fail to retain the large amount of potassium it contains, with dire consequences for the body as a whole.
The body contains several different varieties of skeletal muscle fibre, which can be seen as specialized for different purposes. The ‘slower’ muscles are more economical at holding up loads, such as maintaining posture of the body itself, and probably also more efficient at producing external work. Related to their lower energy use they are less easily fatigued. Faster muscle fibres, however, can produce faster movements and higher power outputs, and are essential for such tasks as jumping or throwing. The way different muscles are constructed also allows for specialization of function: muscles with shorter fibres hold forces more economically, muscle with longer fibres can produce faster movements. A pennate arrangement allows muscles to be built with many short fibres, increasing the force they can exert, whereas long fibres, running almost parallel to the axis of the muscle, give the fastest movements.
Some people have more muscular strength than others; they can exert larger forces, do external work more rapidly, or move faster. To a large extent this is because the stronger individuals have larger muscles, but there seem to be other factors at work as well. Training can change the properties of muscle. Strength training consists in using the muscles to make just a few very strong contractions each day. Over months and years this leads to an increase in the force that can be exerted and in increase in the size of the muscles. Force increase often precedes size increase. Endurance training consists of using the muscles less intensely but for longer periods. Again, over months of training the ability of the muscles to get energy through the oxidation of carbohydrate and fat is raised. The supply of blood to the muscle is also increased through changes in the blood vessels and also in the heart. Training can also lead to changes in the fatigue resistance of muscle fibres, and perhaps cause them to change into a slower type of fibre.
— Roger Woledge
See musculo-skeletal system. See also exercise; fatigue; glycogen; metabolism; movement, control of; muscle tone; sport; strength training.
| Food and Fitness: skeletal muscle |
Muscle attached to bone and, in some areas, skin. Contraction of the muscle moves parts of the skeleton. Skeletal muscle is sometimes called voluntary muscle because its actions are usually under conscious control. It is also called striated muscle because it contains fibres that appear under the microscope to have alternating dark and light bands. The muscle fibres may be of two main types: fast twitch fibres, adapted to produce quick, powerful movements; and slow twitch fibres, adapted to slower, endurance type movements. See also muscle and muscle fibre types.
| Sports Science and Medicine: skeletal muscle |
Voluntary muscle attached to bone or occasionally skin. When stimulated, skeletal muscle moves a part of the skeleton, such as an arm or leg. See also striated muscle.
| Wikipedia: Skeletal muscle |
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It has been suggested that Muscular system#Skeletal muscle be merged into this article or section. (Discuss) |
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system. It is one of three major muscle types, the others being cardiac and smooth muscle. As its name suggests, most skeletal muscle is attached to bones by bundles of collagen fibers known as tendons.
Skeletal muscle is made up of individual components known as muscle fibers. These fibers are formed from the fusion of developmental myoblasts. The myofibers are long, cylindrical, multinucleated cells composed of actin and myosin myofibrils repeated as a sarcomere, 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.
Contents |
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 in the cytoplasm of the muscle fibers. The principal cytoplasmic proteins are myosin and actin (also known as "thick" and "thin" filaments, respectively) which are arranged in a repeating unit called a sarcomere. 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 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. These fibers are suited for endurance and are slow to fatigue because they use oxidative metabolism 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 depending on the particular sub-type. These fibers are quicker to fatigue.
| Fiber Type | Type I fibers | Type II a fibers | Type II x fibers | Type II b fibers |
|---|---|---|---|---|
| Contraction time | Slow | Moderately Fast | Fast | Very fast |
| Size of motor neuron | Small | Medium | Large | Very large |
| Resistance to fatigue | High | Fairly high | Intermediate | Low |
| Activity Used for | Aerobic | Long-term anaerobic | Short-term anaerobic | Short-term anaerobic |
| Maximum duration of use | Hours | <30 minutes | <5 minutes | <1 minute |
| Power produced | Low | Medium | High | Very high |
| Mitochondrial density | High | High | Medium | Low |
| Capillary density | High | Intermediate | Low | Low |
| Oxidative capacity | High | High | Intermediate | Low |
| Glycolytic capacity | Low | High | High | High |
| Major storage fuel | Triglycerides | Creatine phosphate, glycogen | Creatine phosphate, glycogen | Creatine phosphate, glycogen |
| Myosin heavy chain, human genes |
MYH7 | MYH2 | MYH1 | MYH4 |
Cellular physiology and contraction
In addition to the actin and myosin components that constitute the sarcomere, skeletal muscle fibers also contain two other important regulatory proteins, troponin and tropomyosin, 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 by the neurotransmitter acetylcholine, released at the neuromuscular junction by motor neurons[1].
Once a cell is sufficiently stimulated, the cell's sarcoplasmic reticulum releases ionic calcium (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[2]. 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.
Signal transduction pathways
Skeletal muscle fiber-type phenotype in adult animals, and probably people, is regulated by several independent signaling pathways. These include pathways involved with the Ras/mitogen-activated protein kinase (MAPK), calcineurin, calcium/calmodulin-dependent protein kinase IV, and the peroxisome proliferator γ coactivator 1 (PGC-1). The Ras/MAPK signaling pathway links the motor neurons and signaling systems, coupling excitation and transcription regulation to promote the nerve-dependent induction of the slow program in regenerating muscle. Calcineurin, a Ca2+/calmodulin-activated phosphatase implicated in nerve activity-dependent fiber-type specification in skeletal muscle, directly controls the phosphorylation state of the transcription factor NFAT, allowing for its translocation to the nucleus and leading to the activation of slow-type muscle proteins in cooperation with myocyte enhancer factor 2 (MEF2) proteins and other regulatory proteins. Calcium-dependent Ca2+/calmodulin kinase activity is also upregulated by slow motor neuron activity, possibly because it amplifies the slow-type calcineurin-generated responses by promoting MEF2 transactivator functions and enhancing oxidative capacity through stimulation of mitochondrial biogenesis.
Contraction-induced changes in intracellular calcium or reactive oxygen species provide signals to diverse pathways that include the MAPKs, calcineurin and calcium/calmodulin-dependent protein kinase IV to activate transcription factors that regulate gene expression and enzyme activity in skeletal muscle.
PGC1-α (PPARGC1A), a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective ST muscle genes and also serves as a target for calcineurin signaling. A peroxisome proliferator-activated receptor δ (PPARδ)-mediated transcriptional pathway is involved in the regulation of the skeletal musclefiber phenotype. Mice that harbor an activated form of PPARd display an “endurance” phenotype, with a coordinated increase in oxidative enzymes and mitochondrial biogenesis and an increased proportion of ST fibers. Thus—through functional genomics—calcineurin, calmodulin-dependent kinase, PGC-1α, and activated PPARδ form the basis of a signaling network that controls skeletal muscle fiber-type transformation and metabolic profiles that protect against insulin resistance and obesity.
The transition from aerobic to anaerobic metabolism during intense work requires that several systems are rapidly activated to ensure a constant supply of ATP for the working muscles. These include a switch from fat-based to carbohydrate-based fuels, a redistribution of blood flow from nonworking to exercising muscles, and the removal of several of the by-products of anaerobic metabolism, such as carbon dioxide and lactic acid. Some of these responses are governed by transcriptional control of the FT glycolytic phenotype. For example, skeletal muscle reprogramming from an ST glycolytic phenotype to an FT glycolytic phenotype involves the Six1/Eya1 complex, composed of members of the Six protein family. Moreover, the Hypoxia Inducible Factor-1α (HIF-1α) has been identified as a master regulator for the expression of genes involved in essential hypoxic responses that maintain ATP levels in cells. Ablation of HIF-1α in skeletal muscle was associated with an increase in the activity of bob-limiting enzymes of the mitochondria, indicating that the citric acid cycle and increased fatty acid oxidation may be compensating for decreased flow through the glycolytic pathway in these animals. However, hypoxia-mediated HIF-1α responses are also linked to the regulation of mitochondrial dysfunction through the formation of excessive reactive oxygen species in mitochondria.
Other pathways also influence adult muscle character. For example, physical force inside a muscle fiber may release the transcription factor Serum Response Factor (SRF) from the structural protein titin, leading to altered muscle growth.
Research
Research on skeletal muscle properties uses many techniques. Electrical muscle stimulation is used to determine force and contraction speed at different stimulation frequencies, which are related to fiber-type composition and mix within an individual muscle group.
See also
References
- ^ Costanzo, Linda S. (2002). Physiology (2nd ed.). Philadelphia: Saunders. pp. 23. ISBN 0-7216-9549-3.
- ^ Quoted from National Skeletal Muscle Research Center; UCSD, Muscle Physiology Home Page - Skeletal Muscle Architecture, Effect of Muscle Architecture on Muscle Function
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