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Encyclopedia > Myofibril
A diagram of the structure of a Myofybril
A diagram of the structure of a Myofybril

Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cells. They are bundles of filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end. The filaments of myofibrils, myofilaments, consist of 2 types, thick and thin. Thin filaments consist primarily of the protein, actin; thick filaments consist primarily of the protein, myosin; the protein complex composed of actin and myosin is sometimes referred to as "actomyosin." In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length of the myofibril. These subunits are called sarcomeres. The muscle cell is nearly filled with myofibrils running parallel to each other on the long axis of the cell. The sarcomeric subunits of one myofibril are in nearly perfect alignment with those of the myofibrils next to it. This alignment gives rise to certain optical properties which cause the cell to appear striped or striated. In smooth muscle cells, this alignment is absent. Hence there are no apparent striations and the cells are called smooth. Diagram of a Myofibril Created by →Raul654 File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... A right circular cylinder In mathematics, a cylinder is a quadric, i. ... Schematic of typical animal cell, showing subcellular components. ... A top-down view of skeletal muscle Muscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ... Cells in culture, stained for keratin (red) and DNA (green) The cell is the structural and functional unit of all living organisms, and are sometimes called the building blocks of life. ... The filaments of myofibrils constructed from proteins, myofilaments consist of 2 types, thick and thin. ... Actin (red) profilin (blue) complex. ... Myosin is a motor protein filament found in muscle tissue. ... A top-down view of skeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton. ... Cardiac muscle is a type of striated muscle found within the heart. ... A sarcomere is the basic unit of a cross striated muscles myofibril. ...


The photo below is a high power picture (electron micrograph) of a small region of a skeletal muscle cell. The long axis of the cell is indicated by the direction of the Red double arrow. That arrow begins and ends on the boundaries of a sarcomere. The Green arrows demarcate the width of 2 adjacent, parallel myofibrils. The Cyan colored arrow, labeled TK, indicates the length of a portion of a sarcomere made up of thick filaments, which can be seen running in the long axis of the myofibril. The thin filaments extend from the dark boundary of a sarcomere into the region occupied by the thick filaments. They interdigitate with the thick filaments there. In this photo, thin filaments do not show up well. They are in the white or clear band between the sarcomere boundary and the region occupied by the thick filaments, but continue into the region where you can see the thick filaments. An electron micrograph is a micrograph made with an electron microscope. ... Cyan is a pure spectral color, but the same hue can also be generated by mixing equal amounts of green and blue light. ...



The names of the various sub-regions of the sarcomere are based on their relatively lighter or darker appearance when viewed through the light microscope. Each sarcomere is delimited by two very dark colored bands called Z-discs or Z-lines (from the German zwischen meaning between). These Z-discs are dense protein discs that do not easily allow the passage of light. The area between the Z-discs is further divided into two lighter colored bands at either end called the I-bands, and a darker, grayish band in the middle called the A band.


The I bands appear lighter because these regions of the sarcomere mainly contain the thin actin filaments, whose smaller diameter allows the passage of light between them. The A band, on the other hand, contains mostly myosin filaments whose larger diameter restricts the passage of light. (Note for the insatiably curious: A stands for anisotropic and I for isotropic, referring to the optical properties of living muscle as demonstrated with polarized light microscopy.) This article is being considered for deletion in accordance with Wikipedias deletion policy. ... 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 treats polarization in electrodynamics. ...


The parts of the A band that abut the I bands are occupied by the both actin and myosin filaments (where they interdigitate as described above). Also within the A band is a relatively brighter central region called the H-zone (from the German helle, meaning bright) in which there is no actin/myosin overlap when the muscle is in a relaxed state. Finally, the A band is bisected by a dark central line called the M-line (from the German mittel meaning middle).


When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible (as in the photo above). Note that the actin and myosin filaments themselves do not change length, but instead slide past each other. This is known as the sliding filament theory of muscle contraction. A diagram of the structure of a Myofybril Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cells. ...


External links

  • Key Histology Features-Skeletal muscle Histology-World!

http://www.nismat.org/physcor/muscle.html

Organelles of the cell
Acrosome | Chloroplast | Cilium/Flagellum | Centriole | Endoplasmic reticulum | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle

  Results from FactBites:
 
Reference.com/Encyclopedia/Myofibril (622 words)
Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cells.
In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells).
The sarcomeric subunits of one myofibril are in nearly perfect alignment with those of the myofibrils next to it.
Myofibril - Academic Kids (708 words)
Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cells.
In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells).
The sarcomeric subunits of one myofibril are in nearly perfect alignment with those of the myofibrils next to it.
  More results at FactBites »

 
 

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