Here you will find introductory knowledge about the structure of a muscle cell and how it works during contraction. A richly illustrated contribution to the anatomical structure of the musculature can be found in the article Musculature Structure.
Procedure of a muscle contraction – from stimulus to contraction
- The command “muscle contract” comes from the brain via a motor nerve.
- Here the electrical signal (the so-called action potential) of the nerve is converted into a chemical signal for the muscle cell. This connection is called the motor end plate.
- spreading of excitation along the muscle cell membrane (=plasmalemm) and the T-system (see Fig.3).
- Excitation to the L-system (see Fig.3). Opening Ca++ Channels.
- Ca++ diffuses from the L-system into the cytoplasm of the muscle cell and to the microfilaments. The Ca++ concentration in the filaments increases by a factor of 1000(!). The Ca++ works as a messenger in the muscle cell (= intracellular messenger substance) and is responsible for the information transfer from the cell membrane to the microfilaments.
First through the release of Ca++ into the filaments of the muscle, the muscle starts to contract.
Operation of muscle cells – course of a contraction cycle of actin/myosin
Stand of rest
At rest, troponin binds to actin and thus inhibits the binding of actin to myosin (actin-myosin binding). One ATP adheres to each of the myosin heads.
Appropriate action potential
When an action potential arrives, the CA++ concentration increases by a thousand times. The Ca++ binds to the troponin and pulls it away from the binding site (actin).
Thus the troponin is no longer in the way and the myosin can now bind to the action. Immediately the ATP disintegrates into ADP and phosphate at the myosin heads, releasing energy.
Turn over myosin
Due to the released energy of the phosphate release of the ATP, the myosin heads turn over and hang along the actin. The muscle fibre shortens.
Dispensing of ADP
When the myosin heads are turned over, the ADP is released from the myosin heads and the end position of the heads is reached.
Detach and reattach
The binding of the fresh ATP with the myosin head has a “softening effect”. The actin-myosin bond is released and the myosin head is retightened. Without ATP, a so-called “stable rigor complex” would develop, the muscle would then be as hard as stone and the joints could no longer be moved (e.g. rigor mortis).
Restored resting state
Now the resting state of actin, myosin, and troponin is reached again.
For a renewed tension, CA++ must release the binding between troponin and actin again (see Fig.6) and pull the troponin away from the action. Only then can the myosin recombine with the action.
As long as an action potential exists and CA++ pulls the troponin away from the action, the myosin works under consumption of ATP.
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