See Also

Skeletal muscle

Skeletal muscle is a type of striated Sarcomere

A sarcomere is the basic unit of a cross striated muscle [i]'s myofibril [i]. ... 

 muscle Muscle

Muscle is contractile [i] tissue [i] of the body and is derived from the mesodermal layer [i] ... 

, attached to the skeleton Skeleton

In biology [i], the skeleton or skeletal system is the biological system providing physical suppor ... 

. Skeletal muscles are used to facilitate movement, by applying force to bone Bone

Bone, also called osseous tissue, is a type of hard [i] endoskeletal [i] connective tissue [i] ... 

s and joints; via contraction Muscle contraction

A muscle contraction occurs when a muscle cell shortens.... 

. They generally contract voluntarily Volunteer

The term volunteer is contested — there is no one agreed-to definition, and the term is frequently... 

 , although they can contract involuntarily. Muscles have an elongated, cylindrical shape Shape

In geometry [i], two sets have the same shape if one can be transformed to another by a combination of translations [i] ... 

, and are multinucleated Cell nucleus

In cell biology [i], the nucleus is an organelle [i] found in most eukaryotic [i] cells [i] ... 

 . The nuclei Cell nucleus

In cell biology [i], the nucleus is an organelle [i] found in most eukaryotic [i] cells [i] ... 

 of these muscles are located in the peripheral aspect of the cell, just under the plasma membrane Cell membrane

A cell membrane, plasma membrane or plasmalemma is a selectively permeable [i] ... 

, which vacates the central part of the muscle fiber Muscle fiber

A muscle fiber or muscle fibre is a single cell of a muscle [i]. ... 

 for myofibril Myofibril

Myofibrils are cylindrical [i] organelle [i]s, found within muscle [i] cell [i]s. ... 

s.

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Encyclopedia



Skeletal muscle is a type of striated Sarcomere

A sarcomere is the basic unit of a cross striated muscle [i]'s myofibril [i]. ... 

 muscle Muscle

Muscle is contractile [i] tissue [i] of the body and is derived from the mesodermal layer [i] ... 

, attached to the skeleton Skeleton

In biology [i], the skeleton or skeletal system is the biological system providing physical suppor ... 

. Skeletal muscles are used to facilitate movement, by applying force to bone Bone

Bone, also called osseous tissue, is a type of hard [i] endoskeletal [i] connective tissue [i] ... 

s and joints; via contraction Muscle contraction

A muscle contraction occurs when a muscle cell shortens.... 

. They generally contract voluntarily Volunteer

The term volunteer is contested — there is no one agreed-to definition, and the term is frequently... 

 , although they can contract involuntarily.

Muscles have an elongated, cylindrical shape Shape

In geometry [i], two sets have the same shape if one can be transformed to another by a combination of translations [i] ... 

, and are multinucleated Cell nucleus

In cell biology [i], the nucleus is an organelle [i] found in most eukaryotic [i] cells [i]... 

 . The nuclei Cell nucleus

In cell biology [i], the nucleus is an organelle [i] found in most eukaryotic [i] cells [i]... 

 of these muscles are located in the peripheral aspect of the cell, just under the plasma membrane Cell membrane

A cell membrane, plasma membrane or plasmalemma is a selectively permeable [i]... 

, which vacates the central part of the muscle fiber Muscle fiber

A muscle fiber or muscle fibre is a single cell of a muscle [i]. ... 

 for myofibril Myofibril

Myofibrils are cylindrical [i] organelle [i]s, found within muscle [i] cell [i]s.... 

s. This unique arrangement of the nuclei allows for higher efficiency.

Skeletal muscles usually have one end attached to a relatively stationary bone, and the other end is attached across a joint, to another bone .

There are two 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

Cellular respiration is the process in which the chemical bond [i]s of energy [i]-rich molecule [i]s suc ... 

. Type II fibers are whitish; they are used for short bursts of speed and power, use anaerobic metabolism, and are therefore quicker to tire.

How skeletal muscle works

The strength of skeletal muscle is directly proportional to its length and cross-sectional area. The strength of a joint, however, is determined by a number of biomechanical Biomechanics

For further understanding and deeper study of Biomechanics, please check
... 

 principles, including the distance between muscle insertions and pivot points and muscle Muscle

Muscle is contractile [i] tissue [i] of the body and is derived from the mesodermal layer [i] ... 

 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  contract to pull the arm forward, while the muscles in the back and rear of the shoulder 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 output of the chest and anterior shoulder.

Skeletal muscle cells are stimulated by acetylcholine Acetylcholine

The chemical compound [i] acetylcholine, often abbreviated as ACh, was the first neurotransmitter [i] ... 

, which is released at neuromuscular junction Neuromuscular junction

A neuromuscular junction is the synapse [i] or junction of the axon [i] terminal of a motoneuron [i] wit ... 

s by motor neurons. Once the cells are "excited", their sarcoplasmic reticulum Sarcoplasmic Reticulum

Sorry, no overview for this topic 

s will release ionic calcium , this interacts with the myofibrils and, thus, induces muscular contraction . Besides calcium, this process requires adenosine triphosphate Adenosine triphosphate

Adenosine 5'-triphosphate , discovered in 1929 by Karl Lohmann, is a multifunctional nucleotide [i] prim ... 

 . The ATP is produced by metabolizing Metabolism

[i]s in [[life|living]... 

 creatine phosphate and glycogen Glycogen

Glycogen is a polysaccharide [i] that is the principal storage form of glucose [i] in animal and human cells [i] ... 

, which are stored within the muscle cells; as well by metabolizing glucose Glucose

Glucose , a monosaccharide [i] , is one of the most important carbohydrate [i]s in biology [i]. ... 

 and fatty acid Fatty acid

In chemistry [i], especially biochemistry [i], a fatty acid is a carboxylic acid [i] , often with a long ... 

s, obtained from blood Blood

Blood is a highly specialised circulating [i] tissue [i] consisting of se ... 

 and within the cell.

Each motor neuron "controls" a group of muscle cells, known as "motor units". 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". If more strength is required than can be obtained from the current degree of unit contraction, the motor neurons will send additional stimuli; this causes a process of contractile summation, which increases the degree of contraction. If a muscle is maximally contracted, it is said to be in a state of tetanic contraction.

Red and white fibers

Skeletal muscles contain two types of fibers, which differ in the mechanism they use to produce ATP; the amount of each type of fibre varies from muscle to muscle and from person to person.

  • Red fibers have more mitochondria Mitochondrion

    In cell biology [i], a mitochondrion is an organelle [i], variants of which are found in most eukaryotic [i] ... 

    , store oxygen Oxygen

    Oxygen is a chemical element [i] with the chemical symbol O and atomic number [i] 8.... 

     in myoglobin Myoglobin

    Myoglobin is a single-chain [i] protein [i] of 153 amino acid [i]s, containing a heme [i]... 

    , rely on aerobic metabolism Cellular respiration

    Cellular respiration is the process in which the chemical bond [i]s of energy [i]-rich molecule [i]s suc ... 

    , have a greater capillary Capillary

    Capillaries, are the smallest of a body's blood vessel [i]s, measuring 5-10 m [i]. ... 

     to volume ratio and are associated with endurance; these produce ATP more slowly. Marathon runners Marathon

    The marathon is a long-distance road running [i] event of 42.195 km . ... 

     tend to have more red fibers, generally through a combination of genetics and training.
  • White fibers have fewer mitochondria, are capable of more powerful contractions, metabolize ATP more quickly, have a lower capillary to volume ratio , and are more likely to accumulate lactic acid Lactic acid

    Lactic acid, also known as milk acid, is a chemical compound [i] that plays a role in several biochemical [i] ... 

    . Weightlifters Weightlifting

    Weightlifting is a sport where competitors attempt to lift heavy weights mounted on steel [i] bars, the ... 

     and sprinters tend to have more white fibers.

Characteristics of muscle types

Fibre Type Type I fibres Type II a fibres Type II b fibres
Contraction time Slow Fast Very fast
Size of motor neuron Small Large Very large
Resistance to fatigue High Intermediate Low
Activity Used for Aerobic Long-term anaerobic Short-term anaerobic
Force production Low High Very high
Mitochondrial density High High Low
Capillary density High Intermediate Low
Oxidative capacity High High Low
Glycolytic capacity Low High High
Major storage fuel Triglyceride Triglyceride

is glyceride [i] in which the glycerol [i] is ester [i]ified with three fatty acid [i]s. ... 

s
Creatine phosphate, glycogen Glycogen

Glycogen is a polysaccharide [i] that is the principal storage form of glucose [i] in animal and human cells [i] ... 

 
Creatine phosphate, glycogen

Genes that define skeletal muscle phenotype

Skeletal muscle fiber-type phenotype is regulated by several independent signaling pathways. These include pathways involved with the Ras/mitogen-activated protein kinase , calcineurin, calcium/calmodulin-dependent protein kinase IV, and the peroxisome proliferator ? coactivator 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 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-a, 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 d -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-1a, and activated PPARd 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 a ST glycolytic phenotype to a FT glycolytic phenotype involves the Six1/Eya1 complex, composed of members of the Six protein family. Moreover, the Hypoxia Inducible Factor-1a 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-1a 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-1a responses are also linked to the regulation of mitochondrial dysfunction through the formation of excessive reactive oxygen species in mitochondria.

See also

  • Myopathy—muscle pathology/diseases, Muscle atrophy, Muscle Muscle

    Muscle is contractile [i] tissue [i] of the body and is derived from the mesodermal layer [i] ...