Organization of a Skeletal Muscle Fiber

00:01 So now let us look inside the muscle fibre itself, inside the sarcoplasm and concentrate now on the structure of these myofibrils. As I've said earlier, the muscle myofibrils are made up of all the contractile proteins. They are made up of myofilaments. And these myofilaments consist of a number of different proteins, but the main ones are the myosin II, which are thick filaments and the thinner actin filaments. These generate movement and they are wrapped up in bundles within the cell, within these myofibrils. And even though I said earlier, that these myofibrils dominate the sarcoplasm, there is still a certain amount of endoplasmic reticulum wrapping around each of these myofibrils but it's very difficult to see. It appears very sparse because as I said, the sarcoplasm is dominated by these myofibrils. I used the word endoplasmic reticulum, as we do when we talk about the endoplasmic reticulum in most cells. But in muscle cell, we use the term sarcoplasmic reticulum.

01:36 Now this is a diagram of a myofibril showing details of its banding pattern. This banding pattern is reflected in the light microscopic picture of muscle we can see when we look at skeletal muscle using a microscope. But I really want to explain this particular slide rather slowly and carefully because there's a number of concepts here that are important for you to understand. Remember that the myofibrils are made up of myofilaments, the contractile units.

02:20 And these myofilaments consists of actin and myosin and also lots of other proteins that hold these myofilaments in a regular pattern. And that regular pattern is repeated across the myofibril and repeated along the myofibril. So you see this banding pattern that you can see in the diagram. Have a look at the banding pattern in some detail yourself before I start to explain its detail. The muscle fibre here is filled with these myofibrils. That is the first concept that is important for you to understand. That muscle fibre is wrapped up by connective tissue, the perimysium as a muscle bundle. And that muscle bundle then is packaged together in groups and wrapped up by the perimysium and then the epimysium and then you have the whole muscle. So let us just go back in the order of magnitude because I think it is very important for you to understand the concept and orders of magnitude of this muscle structure. The muscle shown in the diagram is surrounded by epimysium.

03:55 Within the muscle itself, there are muscle bundles, muscle fascicles, each wrapped up by perimysium. And then within each muscle bundle or muscle fascicle, there are individual muscle fibres that have endomysium around them as well. And the really important part of the concept now is to have a look within this muscle fibre and appreciate as I pointed out the dominance of individual myofibrils that make up as I have said a number of times already the bulk of the muscle fibre. So have a look at this banding pattern.

04:44 The diagram shows you a muscle fibril in detail. When you look at the banding pattern, some of the bands are dark, some are light, some are thin and some bands are thick. So have a look at the banding pattern yourself now and just make sure you can pick up some dark bands, some light bands, some thick bands and some thin bands. And then we can start to call these bands different names. What I want you to first of all look at, is have a look at the diagram of this myofibril and find the Z-disc. It is that thin dark band you can see labelled on the myofibril. And have a look along towards the end of the diagram of the fibre and you can see a light band labelled the I band. I, the letter I in light reminds me that the light band you see is the I band. Again think of the letter I and think of the letter I in light. And in the middle of that I band is another Z disc. Well, the distance between one Z-disc and another is called the sarcomere and that sarcomere, as I explained at the very start of the lecture is a basic unit of contraction in muscle fibre. And that sarcomere if you look all the way along in the diagram of this myofibril, is repeated, time and time again along the myofibril for the whole length of the muscle fibre. So make sure that you are really now understanding the concept of what a sarcomere is? It is the structure running between two Z discs. Sometimes we use the term Z line. Now the I band is really the part of the sarcomere that contains the thin actin filaments. The dark band you see there is labelled A band. Think of the letter A for dark and that reminds me that the dark band is called the A band, the letter A. Well that A band represent a region of sarcomere, that's towards the centre of the sarcomere, that is occupied by the thick myosin filaments and also the darkness is because there is an overlap between the thin actin filaments and the thick myosin filaments. And right in the middle, you can see another very lightish band with little dark line running through it, that is the H band. And the line you see is the M line that will be labelled later on. So again carefully look at the sarcomere, carefully appreciate the light band or the I band, the dark band or the A band and the central H band with a little line going through the M line. And again, make sure you understand that the I band is the thin actin filaments. The dark band, the A band is the heavier myosin filaments with overlapping of the actin filaments because they interdigitate. The H band is actually where the heavy or the myosin II thick filaments are but it is lightish there because the actin filaments have not extended through to that part of the sarcomere.

08:54 Now, if you look carefully at the H&E section you see there, have a look and see if you can find the banding patterns. You can see the dark band and the light band. Have a look in the middle of the light band and just see if you can start to identify a Z line. I know it is very hard when you look at sections of skeletal muscle fibres, but if you look very carefully I am sure you can pick out a Z disc and you can pick out an A band. Well, this is the very important concept for you to understand the banding pattern and the structure of the sarcomere as I've said it is the functional, most important part of skeletal muscle fibre.

09:42 It is the contractile unit. Well, this is an electron micrograph of a skeletal