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Encyclopedia > Sarcomere
Image of sarcomere
Image of sarcomere

A sarcomere is the basic unit of a cross striated muscle's myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems. Image File history File links Sarcomere. ... Image File history File links Sarcomere. ... A top-down view of skeletal muscle Muscle (from Latin musculus little mouse[1]) is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ... A diagram of the structure of a Myofybril Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cells. ...

  • The thick filament system is composed of myosin protein.
  • The thin filaments are assembled by actin monomers.
  • The elastic filament system is composed of the giant protein titin (also called connectin).

A muscle cell, from a biceps, may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres. Myosin is a motor protein filament found in muscle tissue. ... G-Actin (PDB code: 1j6z). ... Sliding filament model of muscle contraction. ... Structure of a skeletal muscle Muscle is one of the four tissue types. ... Look up Biceps in Wiktionary, the free dictionary. ... Cultured Smooth muscle of the aorta. ...

Contents

Bands

The sarcomeres are what give skeletal and cardiac muscles their striated appearance.

  • A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs, or Z bodies). In electron micrographs of cross striated muscle the Z-line (from the German "Zwischen", between the I bands) appears as a series of dark lines.
  • Surrounding the Z-line is the region of the I-band (for isotropic).
  • Following the I-band is the A-band (for anisotropic). Named for their properties under a polarizing microscope.
  • Within the A-band is a paler region called the H-band (from the German "Heller", bright). Named for their properties under a polarizes microscope.
  • Finally, inside the H-band is a thin M-line (from the German "Mittel", middle of the sarcomere).

The relationship between the proteins and the regions of the sarcomere are as follows: Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... A 1915 Bausch and Lomb Optical microscope. ... A 1915 Bausch and Lomb Optical microscope. ...

  • Actin filaments are the major component of the I-band and extend into the A-band.
  • Myosin filaments extend throughout the A-band and are thought to overlap in the M-band.
  • The giant protein titin (connectin) extends from the Z-line of the sarcomere, where it binds to the thin filament system, to the M-band, where it is thought to interact with the thick filaments. Titin (and its splice isoforms) is the biggest single protein found in nature. It provides binding sites for numerous proteins and is thought to play an important role as sarcomeric ruler and as blueprint for the assembly of the sarcomere.
  • Several proteins important for the stability of the sarcomeric structure are found in the Z-line as well as in the M-band of the sarcomere.
  • Actin filaments and Titin molecules are cross-linked in the Z-disc via the Z-line protein alpha-Actinin.
  • The M-band proteins myomesin as well as M-protein crosslink the thick filament system (myosins) and the M-band part of titin (the elastic filaments).
  • The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for the muscle contraction (sliding filament model).

Sliding filament model of muscle contraction. ... A top-down view of skeletal muscle A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) lengthens or shortens. ...

Contraction

See main article: Muscle contraction A top-down view of skeletal muscle A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) lengthens or shortens. ...


Upon muscle contraction, the A-bands maintain their length (1.6 micrometer in mammalian skeletal muscle) whereas the I-bands shorten.


The A-band, I-band and Z-line are the only components visible at the light-microscope level.


The protein tropomyosin covers the myosin binding sites of the actin molecules in the muscle cell. To allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross bridge binding site on the actin. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum. Muscle contraction ends when calcium ions are pumped back out of the sarcomere. This article or section is in need of attention from an expert on the subject. ... ...


Skeletal muscle only contracts when an impulse is received from a motor neuron. During stimulation of the muscle cell, the motor neuron releases the neurotransmitter acetylcholine which travels across the neuromuscular junction (the synapse between the terminal bouton of the neuron and the muscle cell). The action potential then travels along T (transverse) tubules until it reaches the sarcoplasmic reticulum; the action potential from the motor neuron changes the permeability of the sarcoplasmic reticulum, allowing the flow of calcium ions into the sarcomere. The outflow of calcium allows the myosin heads access to the actin cross bridge binding sites, permitting muscle contraction. The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ...


Rest

At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is unable to access the cross bridge binding sites on the actin. However, the myosin head can hydrolyze ATP into ADP and an inorganic phosphate ion. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and inorganic phosphate ion, changing its configuration back to one of low energy. As the filament of actin moves away from the myosin head and back toward the center of the sarcomere, the myosin head is unable to preserve its bond with the actin. After cross bridge dissociation, ATP binds with the myosin head and the head is ready for another cycle of muscle contraction. Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide that is most important as a molecular currency of intracellular energy transfer. ...


Storage

Most muscle cells only store enough ATP for a small number of muscle contractions. While muscle cells also store glycogen, most of the energy required for contraction is derived from phosphagens. One such phosphagen is creatine phosphate, which is used to provide ADP with a phosphate group for ATP synthesis in vertebrates. The phosphagens are energy storage compounds, also known as high energy phosphate compounds, are chiefly found in muscular tissue in animals. ... Creatine, or creatine monohydrate [NH2-C(NH)-NCH2(COOH)-CH3], is a naturally occurring compound that helps to supply energy to the muscle cells. ...


External links


  Results from FactBites:
 
BioMed Central | Full text | Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal ... (4772 words)
Due to this variability of sarcomere length during contractions, all results presented herein are related to sarcomere length readings made prior to the contractions (passive sarcomere length), at one position along the length of the fiber.
The sarcomere length was established in a clear region of the bundle, close to the force transducer.
An amplifier was used to produce an analog signal, the voltage of which was proportional to the sarcomere length, based on the median intensity profile of the first-order diffraction pattern [12].
Gray, Henry. 1918. Anatomy of the Human Body. Page 375 (752 words)
The sarcomere is situated between two membranes of Krause and consists of (1) a central dark part, which forms a portion of the dark band of the whole fiber, and is named a sarcous element.
When the sarcostyle is extended, the clear intervals are well-marked and plainly to be seen; when, on the other hand, the sarcostyle is contracted, that is to say, when the muscle is in a state of contraction, these clear portions are very small or they may have disappeared altogether (Fig.
The sarcous element does not lie free in the sarcomere, for when the sarcostyle is stretched, so as to render the clear portion visible, very fine lines, which are probably septa, may be seen running through it from the sarcous element to the membrane of Krause.
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