Actin and Myosin

Myofibrils

Breaking down a muscle into individual cellsThe basic structural and functional units of life. called muscle fibers is just one level of organizationThe structured arrangement of biological systems. in skeletal muscles. Muscle fibers are full of large cylindrical equal proteinsLarge molecules made of amino acids with various functions in the body. called myofibrils. MyofibrilsCylindrical structures within muscle fibers that contain myofilaments. have the ability to shorten during a contraction. When all of the myofibrils within one muscle fiber shorten, they pull on the sarcolemma. This happens during a contraction.

This generates tension on the endomysium. This tension is transferred through the perimysium to the epimysium. The epimysium is in turn attached to a bone. This shortening or tension creates force to move the bone.  As you can see in this picture, this myofibril has alternating regions. Some regions are more densely packed with color than others. It’s these myofibrils that have this striated appearance giving skeletal muscle the stripes we see under the microscope.

We called the light and dark areas on a myofibril the eye bands and the a bands. The eye bands are areas that are light in color and the a bands are darker in color. In the light bands, we will find only actin as a small myofilament. In contrast, the dark bands contain both actin and myosin. Before examining this picture on the bottom, let’s understand what a thick filament is. It is also important to know what a thin filament is and what actin and myosin are. The picture is of a circle mirror.

Thick filament

The thick filament is a myofilament which is a very tiny protein within the myofibrils of a muscle fiber.  The thick filament is composed of a small protein called myosin. Myosin is the combination of two smaller proteins intertwined together like you see here on the top. Myosin proteins have a headRounded proximal end that fits into the acetabulum of the hip bone. a tail and a hinge. The hinge ratchets the myosin heads back and forth.

A thick filament is a combination of many hundreds of myosins. You can see their tails all aggregated together. These little nubs coming off those are all of the myosin heads. have you ever used a ratcheting tool?  You might be tightening a bolt onto something. You would use a ratcheting tool to apply torque. This forces the bolt in a clockwise pathway.

And then your tool would ratchet itself back into the starting position. This is somewhat similar to myosin and its relationship to the elastic protein that is present in myofibrils. Myosin moves into a new position. Then, the elastic protein applies force to bring it back to its starting place.

Thin filament

The thin filament is full of a smaller protein called actin. Actin doesn’t have tails in a head actin has subunits. These subunits and their spherical shape in this picture here. Each of the subunits has an active siteThe specific region of an enzyme where a substrate binds and a reaction occurs.. Back in chapter 2, we discussed how enzymesProteins that speed up chemical reactions in the body. are proteins. They have a very specific active site where another molecule connects to them. The active site is covered with a purple strap. This strap is called tropomyosin. In the picture at the very bottom choose tropomyosin has twisted to reveal all of those action sites. Although you can’t see it there is another rod like protein running down the core called nebulin.

Thin filament

To make the thin filament even more complicated there is also this structure called troponin. I have a picture here of troponin. One picture shows troponin without calcium, indicated by the teal colored sphere. Another picture shows troponin with calcium attached. Troponin is something like a lock and calcium is its key. In the picture in the center, you can see my thin filament again. All of the active sites on actin are covered by tropomyosin. You can also see that troponin does not have calcium attached.

Calcium attaches to troponin much like a key is inserted into a lock. Troponin allows the tropomyosin to twist and reveal the active sites on actin. Take a moment and think about how these thin filaments are embedded in those myofibrils. Myofibrils are cylinder-shaped proteins that are shoved into one muscle fiber. There can be many muscle fibers in one muscle. If you can flood that muscle with calcium, you can have a contraction. This occurs because of the complex structure you see right here.  

Myofibrils

Now coming back to this picture. Let’s look at the bottom of this picture. It shows the arrangement of actin in myosin. This arrangement forms what we like to call a sarcomere. A sarcomere is basically the smallest structure in a muscle that’s capable of creating a contraction. Take a look at the myofibril here there are many sarcomeres in one myofibril. The complex and specific arrangement of a sarcomere is what allows contractions to happen.

Notice here how the sarcomere is bounded by a protein arrangement called the Z discThe boundary of a sarcomere that anchors actin filaments. or the Z line. There are Z discs or Z lines marking the boundary of a sarcomere. In the very center of the sarcomere we have the M lineThe middle line of a sarcomere that stabilizes thick filaments.. You can see the alternating arrangement of actin and myosin. You can almost imagine all of these little myosin heads attaching to the active site on the actin subunitsthe small spheres of actin in the thin filament. What happens when this structure becomes smaller and myosin can no longer ratchet past actin? At that point, the muscle must relax back to its original shape. Elastic filaments allow that to happen.