Everything about Myofibril totally explained
Myofibrils (obsolete term:
sarcostyles) are
cylindrical organelles, found within
muscle cells. They are bundles of
actomyosin filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end.
Actomyosin motors are important in
muscle contraction (relying in this case on "classical myosins") as well as other processes like retraction of membrane blebs, filiopod retraction, and uropodium advancement (relying in this case on "nonclassical myosins").
Structure
The filaments of myofibrils,
myofilaments, consist of two types, thick and thin.
- Thin filaments consist primarily of the protein actin, coiled with nebulin filaments.
- Thick filaments consist primarily of the protein myosin, held in place by titin filaments.
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.
Appearance
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 don't easily allow the passage of light. The T-tubule is present in this area. 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.)
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's 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).
Action
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 bottom diagram, left). Note that the actin and myosin filaments themselves don't change length, but instead slide past each other. This is known as the sliding filament theory of muscle contraction.
Further Information
Get more info on 'Myofibril'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://myofibril.totallyexplained.com">Myofibril Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
