2.2 Muscle Structure and Function.pdf

[00:00:00] >> Muscle structure and function. These are the objectives for this lecture. A muscle is surrounded by a thin connective tissue called the epimysium. The epimysium helps to keep each muscle separate from adjacent muscles. The paramecium is another connective tissue that subdivides the mus...

[00:00:00] >> Muscle structure and function. These are the objectives for this lecture. A muscle is surrounded by a thin connective tissue called the epimysium. The epimysium helps to keep each muscle separate from adjacent muscles. The paramecium is another connective tissue that subdivides the muscle into sections within the entire muscle. [00:00:28] Each subsection of a muscle is called a fasciculus. Each of the muscle's fasciculi is made up of lots of muscle fibers. The muscle fibers are the basic structure of the muscle, muscle fibers or muscle cells. Each muscle cell is made up of multiple rod like myofibrils spanning the entire length of the muscle fiber. [00:00:55] Each myofibril has a covering or membrane called the sarcolemma, and is composed of a gelatin-like substance sarcoplasm. Hundreds of muscle fibers and other vital structures such as the mitochondria and the sarcoplasmic reticulum are embedded in the sarcoplasm. Mitochondria is where the metabolic processes occur. Let's take another look at the details of muscle fibers or muscle cells. [00:01:24] Here's your muscle fiber, and as you can see, each muscle fiber is made up of multiple rod like myofibrils, which go the entire length of the muscle fiber. The length and the diameter of the muscle fibers vary. Each muscle fiber also has several nuclei. Myofibrils are bundles of filaments within a muscle fiber. [00:01:50] Myofibrils consist of multiple myofilaments. Myofilaments are made up of threads of two protein molecules, actin, which is a thin filament and myosin, which is a thick filament. The functional purpose of actin is to provide a binding site for the myosin during a muscle contraction. Again on this side, you can see that we have a myofibril, and each unit in a myofibril is called a sarcomere. [00:02:24] So there's our sarcomere. A sarcomere lies between two Z-lines. So those lines there are the Z-lines. And between those two Z-lines are the actin and myosin. Those actin and myosin create the appearance of the light and dark striations in skeletal muscle, which is why skeletal muscle is referred to as a stride and muscle. [00:02:49] So we can see over here also that we have our Z lines right here. And we can see this A band is a darker band, which includes the myosin filament. So that myosin is the red part in there. So the A band is where the myosin is at, and it also includes the area where that myosin overlaps with the actin. [00:03:15] The H band is this area in the center that only includes the myosin filaments. And then the lighter bands at the side are here on this side of the Z line and here on this side of the Z line, where it only includes actin. Each actin myofilament is anchored to the Z line. [00:03:39] The same thing is happening over on this side. I band, Z line, there's the beginning of another A band. Here's another look at the sarcomere. On this image, these lines here are z lines. And then this dark red here is our myosin. So you can see that the sarcomere has the z lines on each end. [00:04:13] And then this top picture, this muscle is at rest. So you see your I bands, where you have only the actin. And then your H band where you have only the myosin here in the center. As you can see during a contraction, the size of that myosin or the A band does not change. [00:04:40] But what does change is that I band here decreases as the muscle contracts. And that H band right here is no longer there in a contracted muscle. These observations demonstrate that the free ends of the actin filaments slide towards each other into the central H zone are the A bands when the muscle contracts. [00:05:09] As the actin filaments move towards each other, the Z lines therefore are becoming closer to one another, which is why you see those I bands shorten and that H band disappear. You can also see in D, you have a stretched muscle, your I bands get larger, your H band gets larger, and your Z lines move apart from each other. [00:05:37] All right, you can see on this picture, here's our Z line, and then these are our myosin filaments, and these are our actin filaments. Remember, actin provides the binding site for the myosin during a muscle contraction. The end of each myosin chain has a globular structure that forms two heads of a myosin. [00:06:00] Those heads are at the end of an arm portion, which is here, which are hinged to the myosin. The hinges allow the arms to project out laterally from the myosin and move during a muscle activation. These heads are called cross bridges because they bridge the thick filaments of myosin to the thin filaments of actin during muscle activity. [00:06:20] Cross bridges are not present in the central portion of the myosin filament and the cross bridges on the two halfs project in opposite directions. The Sliding Filament Theory is the concept of actin and myosin filaments sliding past each other to produce muscle contraction. Many repetitions of the cycle at a large percentage of the active sites are needed to produce a strong muscle contraction. [00:06:43] So you can see here at rest, cross bridges project from that myosin, but are not touching or coupled with that actin. ATP, or adenosine triphosphate, is attached near the head of the cross bridge. And you also have calcium stored in the sarcoplasmic reticulum. Coupling happens where they become attached with the arrival of a muscle action potential which depolarizes the sarcolemma, and you have calcium ions released which changes the shape of the complex and uncovers sites on the actin. [00:07:22] And then that cross bridge couples with the actin site, thereby linking the myosin and active filaments as you can see here. The linking of the cross bridge and an active site then triggers ATP activity of the myosin and ATP splits into adenosine diphosphate and energy. And that reaction produces a flexion of the cross bridge gene, so you can see it moves past there. [00:07:54] And that actin myofilament is pulled a short distance along the myosin myofilament. So those Z lines start moving closer together, so this is coming in. And then in the last picture you can see a recharging so that cross bridge uncouples from the active site, the ATP is replaced on it. [00:08:16] And then the recoupling flexion and uncoupling reach contraction continue to happen multiple times per second to create that muscle contraction. These are the references. Thank you.