The Sarcomere

The Sarcomere

Every time we look at different tissues in this class, we discover something interesting. Textbooks and teachers like to refer to this as a functional unit. These tissues include skeletal tissue and muscle tissue. In the skeleton an osteon was a functional unit of support and strength. Here in contractile muscle tissue the tiniest unit that can create a contraction is called a sarcomere. I like this figure right here. It shows you one big organ or skeletal muscle. Then, it also shows you one individually cylinder shaped muscle fiber contained in the entire organ. We know that muscle fibers are packed full of cylinder shaped myofibrils which are basically just big proteinsLarge molecules made of amino acids with various functions in the body.. And now we’re discovering that there are distinct areas in a myofibrilCylindrical structures within muscle fibers that contain myofilaments. called a sarcomere.

It’s in these distinct areas that a contraction occurs. If you think of one cylinder-shaped myofibril, you can envision these sarcomeres lined up on it. They are aligned kind of like headRounded proximal end that fits into the acetabulum of the hip bone. to toe. How did we ever determine what a sarcomere is? We know that it is the smallest division of a myofibril that contains an overlapping area of actin and myosin. We are going to find that it is an interaction between actin and myosin that causes a contraction. And therefore having them overlap like in this area here is incredibly important.  if they don’t overlap then there’s no ability to create a contraction.

Sarcomere Anatomy

A sarcomere is defined at its outer limits by these zigzaggy lines here which we conveniently call Z lines.  If we were to look at a myofibril we would see sarcomere after sarcomere divided by these Z lines. Equidistant from each Z line is the middle of a sarcomere which is also conveniently called the M lineThe middle line of a sarcomere that stabilizes thick filaments..   Before we go any further we should name some of the other things on this diagram. This thickened purple strip sure here is the protein named myosin. This thin red twisty structure here is the protein actin. These coiled-up little proteins have different names. It depends on the textbook you reference, but they can be called elastin proteins, elastic proteins, or Titan.

The H zoneThe middle of the sarcomere where only thick filaments are present. is the area from the M line up until the area where actin and myosin overlap. It’s this area right here in the sarcomere. You can also see three H zones in the myofibril pictured up top. I bandsThe part of the sarcomere containing only thin filaments can also be known as light bands. These bands or areas on a sarcomere are the strips where actin is present, along with the elastic coiled-up protein. There is no myosin in these strips. A bandsThe part of the sarcomere containing thick filaments. are also called dark bands. They account for the remaining strip that we have not highlighted in here on my terrible picture. A bands are the area where myosin and actin overlap. I strongly suggest that you try to make your own picture of a sarcomere. Follow this picture here. You will realize that the H zone, the I band, and the A bands are all defined by whether they contain actin, myosin, or elastin. They can also be defined by any combination of the two.

Sarcomere Cross Sections

This is always very difficult for students to visualize. This requires you to use all the experience we’ve had from lab looking at 2D pictures. You must take a 2D image and consider what the corresponding 3D picture would look like. For example, you may see a structure in the body that looks like a triangle on a picture. It could also appear as a cross section A cut or slice of the body or an organ for study.. However, that structure is most likely a pyramid shape when found in the body.

What we’re doing here in this diagram is we are cutting this myofibril right here, right here, and right here. We’re showing you the cross section of that myofibril. This is what it would look like if we were to take the cut face and look at it. Before you examine the diagram on the bottom, check the right end of the myofibril. It is up in the top right corner. You can see some purple dots and you can also see some red dots. And the relationship between those thoughts shows you the orientation that these structures have in a 3D cylinder shaped myofibril. And I guess I should just remind you that a myofibril is one cylinder shaped protein. It is in one cylinder shaped muscle fiber. This fiber is in one muscle.

Now take a look at all of the cross sections on the bottom. They contain a visual representation of your definitions for the I band, the a band, and the H zone. In the I band, we said that we had only actin and this elastic protein. Here in the cross section, we see that it’s true. In the H zone, we stated there is only myosin. We can see here in this picture of the M line that it is true. In the zone of overlap or in a band, we said there was actin and myosin. This picture of the zone of overlap confirms that. When we look at these cross sections, they appear very different from the drawings of the sarcomere. They also look different from the a bands, the i bands, and the H zone. I think this is one of the first times in this class. Observing these structures from a longitudinal or a transverse perspective shows you how differently a structure can look.