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What is a Myofilament?

By Greer Hed
Updated May 17, 2024
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A myofilament is a chain of protein molecules found in the myofibrils of a striated muscle. There are two types of myofilament, thin and thick, with the thin filaments being made primarily of the protein actin and the thick ones primarily comprised of the protein myosin. In striated muscle tissue, the filaments are arranged within the myofibrils in repeating polypeptide complexes called sarcomeres. The protein molecules play an important role in muscle contraction and relaxation.

Striated muscle is found throughout the body in the form of skeletal muscle, which is a type of muscle whose contractions and movements can be consciously controlled. Cardiac muscle is also striated, and is found only within the walls of the heart. Within the cells of striated muscles are tubular organelles called myofibrils. The myofibrils are made up of bundles of protein in the form of thick and thin myofilaments.

In striated muscle tissue, a myofilament has a set length that doesn't change, even when the muscle shortens as it contracts or elongates as it relaxes again. A thin myofilament is primarily made of a protein called actin, which assembles itself into a ladder-like scaffold during muscle contraction that the myosin filaments can then use to generate force. With the thin structures, actin protein molecules are bound to two other types of protein called nebulin and troponin. A thick myofilament is primarily made of the motor protein myosin, which is attached to the actin network of proteins with another protein called titin.

These structures play a role in muscle contraction, using a model called the sliding filament model. Action potential impulses from the central nervous system trigger the release of calcium ions from within skeletal muscle tissue. Calcium ions bind with the protein tropomyosin, which obscures myosin binding sites on the actin myofilaments. The calcium bond changes the structure of the tropomyosin molecules, allowing the myosin filaments to form chemical bonds with the actin filaments at the myosin binding sites.

Once myosin and actin filaments are bound together, the process of contraction really begins. Muscles shorten, or contract, as myosin myofilaments slide over the passive network of actin proteins. Once the muscle is done contracting, a nucleotide called adenosine triphosphate (ATP) binds with the myosin filaments, weakening their bond with the actin filaments. The myosin filaments use ATP to gain energy so they can detach from the actin filaments and move back to their original position. This causes the muscle to lengthen and return to a relaxed position.

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Discussion Comments
By allenJo — On Dec 04, 2011

@Charred - It’s interesting that you talk about tearing the muscles. The human muscle system is delicate indeed and can be damaged, in ways that are not good.

I suffered from tendinitis years ago and some shoulder bursitis because of poor ergonomic postures. I noticed that as a result, any weight lifting I did afterwards was especially painful and exacerbated muscle tremors that I would normally get from weight lifting.

Sometimes it didn’t even have to be heavy lifting. It could be something as simple as ten pound dumb bells. Anyway, you have to be careful with your posture and strength training so that you don’t throw your muscles out of whack.

By Charred — On Dec 03, 2011

@nony - You don’t necessarily have to know the technicalities of the whole process of muscle contraction, but I do agree that nutrition is important. Also, if you’re doing body building, it’s useful to know how muscles actually build through resistance training.

People talk about getting “ripped.” What happens is that the weights you lift cause a shock to your muscles, tearing them, and new muscle cells rush in to fill in and repair the damage.

These new cells are the actin and mysin the article talks about. These are proteins and in bodybuilding, it’s all about proteins. I used to do the heavy protein shakes when I was weightlifting but I stopped and just got my proteins from food alone.

It’s important to realize that just as importantly as you can gain new muscle tissue, if you stop, that tissue will convert to fat again, and you’ll be in worse shape than you were to begin with.

By nony — On Dec 02, 2011

Muscle fiber contraction is a complex process it seems. I’ve always wondered what causes muscle twitches or spasms. I suffered from these for many years.

These weren’t the kinds of twitches or spasms you get from exercising, like muscle tremors or things like that. They were just random twitches throughout my body and they would happen even as I was sitting still.

For years I thought I was suffering from “nerves” (even though I was relaxed). Finally, I discovered what the problem was – it was nutrition. I had not been getting any vegetables in my diet. As a result, I was deficient in important minerals that muscles needed; that’s what was causing the twitching.

Once I started loading up on vegetables the twitching stopped.

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