A top-down view of skeletal muscle

Skeletal muscle is a type of striated muscle, which usually attaches to tendons. "A-band" redirects here For other uses of the term see A band. Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the Skeletal muscles are used to create movement, by applying force to bones and joints; via contraction. In Physics, a force is whatever can cause an object with Mass to Accelerate. Bones are rigid organs that form part of the Endoskeleton of Vertebrates They function to move support and protect the various organs of the body produce A joint is the location at which two or more Bones make contact A muscles contraction (also known as a muscle twitch or simply twitch) occurs when a Muscle fibre generates tension through the action of Actin They generally contract voluntarily (via somatic nerve stimulation), although they can contract involuntarily through reflexes. A nerve is an enclosed cable-like bundle of peripheral Axons (the long slender projections of Neurons. A reflex action, also known as a reflex, is an involuntary and almost instant movement in response to stimulus. The whole muscle is wrapped in a special type of connective tissue, epimysium. Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, Muscle, and Nervous tissue) Epimysium is a layer of Connective tissue which ensheaths the entire Muscle.

Muscle cells (also called fibers) have an elongated, cylindrical shape, and are multinucleated (in vertebrates and flies). Skeletal muscle is a type of Striated muscle, which usually attaches to tendons 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 The shape ( OE sceap Eng created thing) of an object located in some space refers to the part of space occupied by the object as determined Multinucleate (also multinucleated, coenocytic) cells have more than one nucleus per cell, which is the result of nuclear division not being The nuclei of these muscles are located in the peripheral aspect of the cell, just under the plasma membrane, which vacates the central part of the muscle fiber for myofibrils. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Skeletal muscle is a type of Striated muscle, which usually attaches to tendons Myofibrils (obsolete term sarcostyles) are cylindrical Organelles found within Muscle cells They are bundles of actomyosin filaments (Conversely, when the nucleus is located in the center it is considered a pathologic condition known as centronuclear myopathy. Centronuclear myopathies (CNM are a group of Congenital Myopathies where cell nuclei are abnormally located in skeletal muscle cells )

Skeletal muscles have one end (the "origin") attached to a bone closer to the centre of the body's axis and this is often but not always a relatively stationary bone (such as the scapula) and the other end (the "insertion") is attached across a joint to another bone further from the body's axis (such as the humerus). In Anatomy, the scapula, omo, or shoulder blade, is the Bone that connects the Humerus (arm bone with the Clavicle (collar A joint is the location at which two or more Bones make contact The humerus is a Long bone in the Arm or Forelimb that runs from the Shoulder to the Elbow. Contraction of the muscle causes the bones to rotate about the joint and the bones to move relative to one another (such as lifting of the upper arm in the case of the origin and insertion described here).

There are several different ways to categorize the type of skeletal muscle fibers (see below). One method uses the type of protein contained in myosin (one of the important proteins that is responsible for the ability of muscle to contract). Using this classification scheme, there are two major types of fibers for skeletal muscles: Type I and Type II. Type I fibers appear reddish. They are good for endurance and are slow to tire 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 Type II fibers are whitish; they are used for short bursts of speed and power, and use both oxidative metabolism and anaerobic metabolism depending on the particular sub-type, and are therefore quicker to fatigue. Fermentation is the process of deriving energy from the oxidation of organic compounds such as carbohydrates using an endogenous electron acceptor which is

How skeletal muscle works

Main article: Muscle contraction

The strength of skeletal muscle is directly proportional to its length and cross-sectional area. A muscles contraction (also known as a muscle twitch or simply twitch) occurs when a Muscle fibre generates tension through the action of Actin This article is about proportionality the mathematical relation The strength of a joint, however, is determined by a number of biomechanical principles, including the distance between muscle insertions and pivot points and muscle size. Biomechanics is the application of mechanical principles on living organisms Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the Muscles are normally arranged in opposition so that as one group of muscles contract, another group 'relaxes' (in fact simply stretched) or lengthens. Antagonism in the transmission of nerve impulses (epsp and ipsp lateral balance) 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. The Pectoralis major (popularly known as pecs) is a thick fan-shaped muscle situated at the upper front ( Anterior) of the Chest wall In Human anatomy, the deltoid muscle is the Muscle forming the rounded contour of the Shoulder. The latissimus dorsi (plural latissimi dorsi is the large flat dorso-lateral muscle on the trunk posterior to the arm and partly covered by the Trapezius on its median A muscles contraction (also known as a muscle twitch or simply twitch) occurs when a Muscle fibre generates tension through the action of Actin Part of the training process is learning to relax the antagonist muscles to increase the force input of the chest and anterior shoulder.

Skeletal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons. The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central A neuromuscular junction ( NMJ) is the Synapse or junction of the Axon terminal of a Motoneuron with the motor end plate, the In Vertebrates the term motor neuron (or motoneuron) classically applies to Neurons located in the Central nervous system (or CNS that project ^[1] Once the cells are "excited", their sarcoplasmic reticulum will release ionic calcium (Ca²⁺) which interacts with the myofibrils to induce muscular contraction (via the sliding filament mechanism). The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 The sliding filament theory describes a process used by Muscles to contract. This process also requires adenosine triphosphate (ATP). Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy The ATP is produced by metabolizing creatine phosphate and glucose (stored as glycogen or absorbed from blood) within the muscle cells by mitochondria, as well as by metabolizing fatty acids obtained from the blood and within the cell. Metabolism is the set of Chemical reactions that occur in living Organisms in order to maintain Life. Phosphocreatine, also known as creatine phosphate or Pcr, is a phosphorylated Creatine molecule that is an important energy store in skeletal Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. Glycogen is a Polysaccharide of Glucose (Glc which functions as the secondary short term energy storage in Animal cells Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells ��such as nutrients and oxygen—and transports Waste products In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. In Chemistry, especially Biochemistry, a fatty acid is a Carboxylic acid often with a long unbranched Aliphatic tail ( chain) which Each motor neuron activates a group of muscle cells, and collectively the neurons and muscle cells are known as motor units. A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates. When more strength is required than can be obtained from a single motor unit, more units will be stimulated; this is known as motor unit recruitment. Physical strength is the ability of a person or animal to exert Force on physical objects using muscles. Motor unit recruitment is the progressive activation of a Muscle by successive recruitment of contractile units (motor units to accomplish increasing gradations of contractile This is spatial summation. Spatial summation is a way of achieving Action potential in a Neuron which involves input from multiple cells If more strength is required than can be obtained from the current number of motor units, the motor neurons continue to recruit more motor units. When all the motor units are recruited, there will be no further increase in contraction strength. To increase the force of contraction, it is necessary to increase the frequency of neuronal firing. This results in tetanic contraction, which is a smooth contraction. A tetanic contraction occurs when a Motor unit has been maximally stimulated by its Motor neuron. This is temporal summation. Temporal summation is an effect generated by a single Neuron as a way of achieving Action potential.

Skeletal muscle fiber types

Skeletal muscles contain two main types of fibers, which differ in the primary mechanisms they use to produce ATP, the type of motor neuron innervation, and the type of myosin heavy chain expressed. In Vertebrates the term motor neuron (or motoneuron) classically applies to Neurons located in the Central nervous system (or CNS that project The proportions of each type of fiber varies from muscle to muscle, from animal to animal, and from person to person.

• Slow-twitch, or type I, fibers (sometimes referred to as "Red") have more mitochondria, store oxygen in myoglobin, rely on aerobic metabolism, have a greater capillary to volume ratio and are associated with endurance; these produce ATP more slowly. In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Myoglobin is a single-chain globular Protein of 153 Amino acids containing a Heme ( Iron -containing Porphyrin) Prosthetic Cellular respiration is the set of the metabolic reactions and processes that take place in Organisms cells to convert biochemical energy from Capillaries are the smallest of a body's Blood vessels measuring 5-10 μm in diameter which connect Arterioles and Venules and enable the interchange Endurance (also called sufferance) is the ability for humans to exert themselves through aerobic or Anaerobic exercise for relatively long periods of time Marathon runners tend to have more type I fibers, generally through a combination of genetics and training. The marathon is a long-distance foot race with an official distance of 42
• Fast-twitch, or type II, fibers (sometimes referred to as "White") have fewer mitochondria, are capable of more powerful (but shorter) contractions, metabolize ATP more quickly, have a lower capillary to volume ratio, and are more likely to accumulate lactic acid. Lactic acid ( IUPAC Systematic name: 2-hydroxypropanoic acid) also known as milk acid, is a Chemical compound that plays a role Weightlifters and sprinters tend to have more type II fibers. Weightlifting, also called Olympic weightlifting or Olympic-style weightlifting, is a sport in which participants attempt a maximum weight single lift of a barbell Sprints are short running races in athletics. They are roughly classified as events in which top runners will not have to "pace themselves" but can run as fast as Type II fibers are distinguished by their primary sub-types, IIa, IIx, and IIb, as described below.

Type II fibers come in three primary sub-types, called type IIa, IIx, and IIb. Recent studies (see ^[2]) show that human skeletal muscle contains type I, IIa, and IIx fibers, though confusingly, human IIx fibers used to be referred to as type IIb. Types IIa, IIx, and IIb fibers are found in skeletal muscle of other mammals (e. g. , rodents and cats).

Characteristics of skeletal muscle fiber types

Genes that define skeletal muscle phenotype

Skeletal muscle fiber-type phenotype in adult animals, and probably people, is regulated by several independent signaling pathways. (more properly known as, TAG or triacylglyceride) is Glyceride in which the Glycerol is Esterified with three Fatty acids It is the Phosphocreatine, also known as creatine phosphate or Pcr, is a phosphorylated Creatine molecule that is an important energy store in skeletal Glycogen is a Polysaccharide of Glucose (Glc which functions as the secondary short term energy storage in Animal cells 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.

Exercise-Included Signaling Pathways in Skeletal Muscle That Determine Specialized Characteristics of ST and FT Muscle Fibers

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. Peroxisome proliferator-activated receptor gamma coactivator 1 alpha, also known as PPARGC1A or PGC-1α, is a human Gene. 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 byproducts 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.

External links

• Microscopic stains of skeletal and cardiac muscular fibers.
• bovine myology

See also

• Myopathy
• Muscle atrophy

References

1. ^ Physiology, 2nd Ed. In Medicine, a myopathy is a neuromuscular Disease in which the Muscle fibers do not function for any one of many reasons resulting in Muscular weakness Muscle atrophy is defined as a decrease in the mass of the muscle it can be a partial or complete wasting away of muscle , Saunders, 2002, ISBN 0-7216-9549-3, page 23.
2. ^ Spangenburg & Booth. Molecular regulation of individual skeletal muscle fibre types. Acta Physiol Scand. 2003 Aug;178(4):413-24.