Muscle fibrils

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Muscle fibrils under the electron microscope

A muscle fibril , also called myofibril , is a functional unit in the muscle cells ( muscle fibers ) at the level of a cell organelle , which enables the cell to actively shorten (contraction).

Muscle fibrils form bundles of muscle fibrils

Many muscle fibrils are grouped together within the muscle fiber by the intermediate filament desmin to form fibril bundles that are wrapped in a special form of the endoplasmic reticulum , the sarcoplasmic reticulum .

A muscle fibril is made up of sarcomeres

Striation of the muscle fibrils (schematic)
Muscle fibrils are composed of sarcomeres

A muscle fibril consists of hundreds of components connected in series with the same internal structure, the sarcomeres . If one looks at a muscle fibril with a light microscope, a typical horizontal striation becomes visible, which is caused by the regular distribution of the muscle filaments. A typical band pattern is created.

The sarcomeres consist essentially of parallel, thick protein fibers, the myosin . The thinner actin filaments protrude between the myosin filaments . The whole complex is stabilized by titin , the largest protein in the human body. α-actinin forms firm attachment discs at regular intervals from which thin titin threads extend. The myosin molecules lie between these threads. Their ends overlap with the ends of the actin threads.

The muscle fibrils shorten as the filament slides

During a contraction, calcium ions are released from the sarcoplasmic reticulum , which diffuse between the myosin and actin filaments . The calcium ions (Ca 2+ ) trigger interactions between the myosin and actin filaments, so that the actin filaments are pushed between the myosin filaments (sliding filament mechanism of muscle contraction). This shortens the individual sarcomere and with it all the muscle fibrils of a muscle fiber. The muscle fiber twitches. The shortening of many muscle fibers leads to a powerful contraction of the muscle.

See also

literature

  • Jeremy M. Berg, John L. Tymoczko, Lubert Stryer : Biochemistry. 6 edition, Spektrum Akademischer Verlag, Heidelberg 2007. ISBN 978-3-8274-1800-5 .
  • Donald Voet, Judith G. Voet: Biochemistry. 3rd edition, John Wiley & Sons, New York 2004. ISBN 0-471-19350-X .
  • Bruce Alberts , Alexander Johnson, Peter Walter, Julian Lewis, Martin Raff, Keith Roberts: Molecular Biology of the Cell , 5th Edition, Taylor & Francis 2007, ISBN 978-0815341062 .