What is Actin and Myosin? Easy Explanation of Movement

What is actin and myosin cover image

Humans and all the other motile animals are constantly moving their bodies, using the “simple” contraction and relaxation of muscles. While for you, picking up an object or scrolling through this article seems very simple, inside your muscle fibers, a complex process takes place. These processes involve the contraction of muscles and the two elementary components of it; actin and myosin. So what is actin and myosin and how does it help in movement?

Actin and Myosin are two protein elements that are responsible for the contraction of your muscles. But before we get into details about these two, it is necessary to understand the basics of muscle contraction. We’ll keep it brief and to the point.

The basics of muscle contraction

Animals move when their muscles contract; shorten and then relax. This simple movement allows us to run, dance, jump, write, etc. Muscles need calcium ions to contract, and the ions are present in abundance in the sarcoplasmic reticulum. However, they are not in contact with the muscle fibers. The calcium ions are released when there’s a neural impulse; supposed if you want to lift your hand. This generates an impulse and the calcium ions are released.

This is how a muscle contraction is initiated, with the release of calcium ions. From here, we’ll have to understand what actin and myosin are to see how movement occurs. Here’s the easiest explanation for the anatomy of muscle and the structure for movement.

Skeletal muscle is made of muscle fibers, which are bundles of muscle cells. These cells are elongated and look like cables. Imagine a wire that has multiple, tiny copper wires inside it. That is how a muscle cell looks.

These muscle cells are made of tiny filaments called myofibrils. Think about that wire with tiny copper filaments, those copper filaments are the myofibrils. The two protagonists of this article, actin, and myosin, live in these myofibrils. Let’s look at their address.

You must have heard the term “sarcomere” when reading about muscles. The myofibrils are divided into equal segments. Take the example of copper filaments. Divide the long copper filaments into segments of 1 cm. These segments in the myofibrils are called the sarcomere. The sarcomere is the address of myosin and actin.

Actin and myosin, the functional units of muscle.

The segments in a myofibril are called sarcomere and this is where actin and myosin reside. The sarcomere has two filaments; thin and thick filaments. Don’t worry, these are not new elements, but just a different name for actin and myosin.

The thin filament is actin with other proteins (will be explained later) and the thick filament (also called myofilament) is a series of myosin protein. These two filaments slide over each other, shortening the sarcomere and causing contraction. To understand the Z and M lines, think of the segments we created in the copper filament. The lines dividing the segments (sarcomere) are the Z line. The sarcomere has a central line called the M line and this is the myosin line.

The M-line pulls the Z-line towards itself during a muscle contraction. This pull is just the effect. The real process occurs inside the sarcomere and the way actin and myosin interact. This is the most complex part of muscle contraction and here’s how that happens.

What is actin and myosin pictorial representation
The functional units of a sarcomere, actin and myosin

The sliding filament theory

The thin filaments (actin) sits above the thick filaments (myosin). Myosin has multiple bulb-like projections which act like hands. Why hands? This will be explained later. This is all about myosin. Now let’s look at actin.

The thin filament has three components; actin fiber, troponin, and tropomyosin. Troponin covers the binding sites for myosin, tropomyosin connects all the troponin. Now here’s how motion occurs.

Note: Proteins change their shape when anything compatible (chemical) attaches to them. This change of shape also results in a change of the function. This is important because myosin and actin are proteins.

How thin and thick filaments slide

When calcium is released due to the action potential generated by neurons (when you think of moving), these calcium ions attach themselves to troponin. Since proteins change their shape and function when something attaches to them, the troponin moves from the binding site, exposing the site to myosin.

Since the tropomyosin connects all the troponin, all the binding sites are exposed. Now let’s see how myosin acts when these binding sites are open for it to attach.

The bulb part of myosin gets ATP (which is the energy source for cells). The ATP reacts and turns into ADP and phosphate (the reaction that releases energy). The energy from this reaction causes the myosin to move ahead, stretching beyond its normal configuration. Then, with the binding site nearby, it attaches itself to the site.

As with proteins, if they change their shapes, their function changes. When myosin attaches to the binding site, the ADP and phosphate molecules are released and the energy stored is used to pull the actin filament. This is where myosin acts like a hand, holding a rope and pulling it towards itself.

After myosin pulls actin filament and relaxes, it releases the binding site. Then another molecule of free-floating ATP attaches itself to the myosin and the same process repeats with myosin attaching itself to the next binding site. This is how the muscle contracts.

Muscle contraction occurs as long as there’s action potential and calcium ions are being released. When the action potential dies (no calcium ions releasing), the entire process stops and the muscles are relaxed.

The Z line and M line of a sarcomere
The Z line and M line of a sarcomere

Limits in muscle contraction

Your muscles cannot keep on contracting. There is a limit to this contraction. When the Z line and the M line are closest, which means that the myofibrils have contracted to their limits, that’s the limit of your muscular strength as well.

There are also other layers of covering over your muscle fibers. These layers are there to provide strength to the muscle. When the muscles contract, they bugle. And to protect them from bursting, these fibers enclose the fibers. This is also the reason why sometimes too much strain on muscle results in damage and even tearing (ouch!) of muscle fibers.

Also, after sustained contraction, the ATP and calcium ions start depleting. This results in the relaxation of muscles since there are no ions to attach with the troponin and no source of energy for the myosin head to move. This is why it is so difficult to make a T-pose and use your hands to crumble some newspaper (or do any physical movement for a longer period)

General questions about actin and myosin

Actin and myosin are present in all types of muscle since these proteins are the main component of movement. These two proteins are also present in involuntary smooth muscles. The only difference is here the impulse is generated automatically and involuntarily.

Actin and myosin make up the thick and thin filaments that make up the myofibrils of a muscle cell. Another common question about actin and myosin is whether they shorten or elongate during muscle movement. The answer is no, these proteins don’t shorten or elongate, but they slide over each other.

With the change in the shape of protein (when ADP and Phosphate are released) the myosin head releases the binding site on the actin filament along with ATP molecule attaching again to the myosin. When there is no calcium floating nearby, the troponin and tropomyosin cover the binding sites. This is why actin and myosin don’t bind by themselves.

This concludes the article. But there are many more interesting articles to know more about your body. Take a look at these related articles;

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