How Is Muscle Tissue Classified?

by Geoffrey Meyer, PhD

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    00:01 Well, let us have a look and see how the muscle is classified. We can classify muscle either because it is striated or that it is not striated. When you look at skeletal muscle, when you look at visceral striated muscle and when you look at cardiac muscle, you can see in the tissue little tiny stripes or striations. And these striations reflect the contractile proteins within the muscle fibre or muscle cell. These contractile proteins are arranged in a very regular pattern and that is why you see stripes along the tissue, along the muscle fibre. Now here is a muscle, a large piece of muscle shown in the middle of the image and then on the right-hand side is a section through skeletal muscle viewed with a light microscope. The section at the top looks at skeletal muscle cut transversely.

    01:10 The section down the bottom looks at striated muscle cut longitudinally. Now I have used the word muscle cell and muscle fibre. But when we talk about skeletal muscle, we really mean muscle fibre because they are very long. Some muscle fibres extend a long way, a huge distance.

    01:32 For instance, the sartorius muscle in the thigh goes close to the hip down towards the knee, a very long muscle. So the muscle cells are long. So it is more appropriate to call that muscle fibres. Well let us look at the gross anatomy view of the piece of the muscle in the centre of the image. When you look at pieces of muscle, they are separated from each other by a connective tissue capsule called the epimysium. This epimysium wraps around the entire muscle, as you see here, and it separates that muscle from adjacent muscles, because sometimes these muscles need to contract or relax independent of their neighbors, so that epimysium separates the muscles apart. It also will blend with more connective tissue that carries blood vessels and nerves down through the body that are going to innervate the muscles and also supply those muscles with oxygen and nutrients. Now if you look into the muscle itself, you can see little white lines. They are connective tissue septa that penetrate into the muscle itself from the epimysium. And these connective tissue fibres called the perimysium, wrap the muscle up into individual bundles of fibres.

    03:12 There are many many bundles of fibres in these piece of muscle wrapped up each by perimysium.

    03:19 We often call a muscle bundle, a muscle fascicle. Now, if you look at the individual cell and the best view is to look at the longitudinal section of muscle you see at the bottom of the right-hand side image, you can see that there are some very fine greeny blue stained connective tissue fibres. Well thye wrap up individual muscle fibres. So every single muscle fibre is surrounded by endomysium connective tissue collagen. And then what separates the muscle fibre from this endomysium is the external lamina of the muscle. So muscle is separated from connective tissue but is wrapped up by connective tissue. Now if you have a look at that gross picture of the muscle as well, you are looking through a section of that muscle in its thickest part.

    04:24 I want you to imagine that muscle tapering down towards the tendon or tapering towards the point of attachment of the muscle on bone. But what I really want you to understand is that when the muscle tapers down to insert onto the bone via the tendon so that its contraction force can be imparted onto the bone via that tendon, that tendon is formed because all those connective tissue fibres from the epimysium, from the perimysium and from the endomysium, they all come together as the muscle tapers down. So really a tendon consists of all those three layers of connective tissue in the muscle, plus a little bit more collagen added at the myotendinous junction. So that means that when the individual muscle fibres contract, their force of contraction is then imparted onto that endomysium, onto the perimysium and via the epimysium as well onto the tendon. And that is an important concept to understand.

    05:46 Well, let us have a look at the skeletal muscle fibre in a little bit more detail. I have said that they are very long, they can be short in certain muscles, but they can also be very long in other muscles. Then when you do gross anatomy, you'll realize the difference in muscle length and muscle types in fact. The shape of different gross anatomy muscles varies depending on what part of the body you will look at these different muscle that moves the limbs.

    06:15 So now let us look at the skeletal muscle fibre in more detail. I have already explained that some of these fibres are very long in muscles that are long themselves in extent long distances across joints. Well, these long muscle fibres also vary in their width or their thickness.

    06:38 They can arrange from 10 to over 100 microns in thickness. The reason why they are long fibres is because during their development, they resulted from the fusion of little cells called myotubes that would derive from myoblasts. And because of that, the fibres are not only very long, but as we'll see later on, it is multinucleated. Many many nuclei are situated all the way along the skeletal muscle fibre and around the periphery of the fibre, which is an important point to remember. Now there are some special terms that we use when we talk about skeletal muscle fibres or muscle in general. First of all, we use the term sarcoplasm to describe the muscle cytoplasm. Sarco is pertaining to muscle or flesh. So we use this term. When we talk about the cell membrane or the plasma mambrane of the muscle fibre, we call it the sarcolemma.

    07:52 And you see here that these two muscle fibres are slightly seperated, but on the edge of each of these muscle fibres, there is the cell membrane, but we call it the sarcolemma.

    08:07 Well, as I said the nucleus sits on the periphery of the skeletal muscle fibre and there are many of them all the way along the fibre. The sarcoplasm is dominated by the contractile factory of the cell. And these contractile factories are loaded together or packaged together in long structures called myofibrils. And these myofibrils as you see in this section dominate the sarcoplasm because they are there to affect contraction. And if you look very very closely in this image, in the cross section of the skeletal muscle fibre, you can see all these myofibrils appearing as tiny little red dots when they're cut perfectly transversely.

    09:03 Well, when you look at fresh muscles or sections of muscle and using certain stains, you can identify three different muscle fibre types in skeletal muscle. And these reflect the functional capacity or the contractile strength and energy usage of these different muscle fibre types.

    09:28 The red fibres are the most abundant. They have the properties of slow twitch, which means a twitch means really a single muscle contraction. So these slow twitch muscle fibres contract rather slowly. They are also fatigue resistant. They don't tire very quickly and so these muscle fibres are ideally found in locations of the body where you want to have a rather slow prolonged contraction such as in the back where you really want muscle to adapt to being slow, but prolonged contraction to maintain the erect posture. Now other fibres are medium-sized, they are fast-twitch fibres. So they can contract very quickly and they can also reach maximum contraction tension. They can also be very fatigued resistant because they derive their energy from oxidative glycolytic pathways. They store glycogen and they can undergo anaerobic glycolysis. Now these types of muscle fibres are ideal where you want a reasonably long high contraction tension and muscle that is not going to tire very easily. So middle distance runners, 400 or 800-metre runners, or even middle distance swimmers would like to have a lot of these fibres, that don't tire easily but they generate high contractile strength and tension. And the last muscle fibre type is the fast glycolytic fibre.

    11:24 These store enormous amounts of glycogen. They are large fibres. They contract very fast and they generate very high contractile tension, but they are prone to fatigue very quickly. And that is because of the build-up of lactic acid in these fibres during usage.

    11:47 So you see these fibres in sprinters or athletes that have that sort of sprinting type role.

    About the Lecture

    The lecture How Is Muscle Tissue Classified? by Geoffrey Meyer, PhD is from the course Muscle Tissue.

    Included Quiz Questions

    1. Smooth muscle
    2. Skeletal muscle
    3. Cardiac muscle
    1. fast glycolytic fibres
    2. slow oxidative fibres
    3. fast oxidative glycolytic fibres
    4. slow twitch fibres
    5. suitable for middle distance running events

    Author of lecture How Is Muscle Tissue Classified?

     Geoffrey Meyer, PhD

    Geoffrey Meyer, PhD

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