Sarcomere

00:00 Well, this is an electron micrograph of a skeletal muscle fibre showing you a number of myofibrils. And the myofibrils are actually almost lined up in parallel here. There's a slight different because of shrinkage during processing, but on this particular picture, you can see maybe 1, 2, 3, 4, 5, 6 or so myofibrils separated by a tiny little space you sometimes see which is going to be the sarcoplasmic reticulum and I am going to talk about that in the bit more detail in a moment. But again look at this picture or image, it's a section of a skeletal muscle fibre viewed with an electron microscope and the red band you see, demarcates a sarcomere. The Z line or Z disc running along the middle of the light or I band and then the dark band consisting of myosin and overlapping filaments of actin. And then there's the H band and the little M line in the middle. Again let me stress the importance of really recognising the components of the sarcomere. Well, you know I am smiling and having a little laugh here because I want to tell you a story. I put there the size or the length of the sarcomere when it is relaxed, when it is stretched or when it is contracted. Now you know when I was a student studying histology, I have to sit for an exam as part of my basic tissue exam in histology. And one of the questions I can always remember, it was a multiple choice question and I do not really like multiple choice question, but anyway the question was what is the length of the sarcomere? And the choices I cannot remember the details of all the four choices, but that were something like 2 microns, 2.3 microns, 2.6 microns or 3 microns. And I thought well I do not really understand this question, so I guessed. I set off and ticked the box for 3 microns. When I got my exam paper back, it was marked wrong. And I went to my professor and I said professor you have marked this wrong and my professor said well that's because the answer is wrong. The answer is 2.6 microns.

02:41 And I looked at my professor and said but the question is confusing. You should have asked the question what is the length of a sarcomere when it is relaxed or when it is stretched or when it is contracted? And he smiled at me and he said ok Geoff, I will change the question. I will change your mark. I will delete the question and amend your mark and the mark for all the other classmates. So I smiled and thanked my professor.

03:13 And you know today, he remains one of my best friends. He supported my position at my university, but I've always remembered the little contest we had when our professor about being marked wrong on this question about the length of the sarcomere. Well, after the story, let us get back to looking at skeletal muscle. I spoke a minute ago about sarcoplasmic reticulum.

03:43 It is just like the endoplasmic reticulum in other cells and make sure you now are aware that this sarcoplasmic reticulum is very closely associated with myofibrils. Well, here is a rather complicated picture of the sarcomere. It is important to understand that when muscle contracts, the sarcomere changes in dimensions and the previous slide show those dimensions.

04:24 I think it is important to appreciate that really during contraction, the Z lines move together towards each other. The dark A band remains the same in length. What changes is the length of the H band and the length of the I band, the light band. And that is because as I have said before, the actin filaments slide in between the darker myosin filaments.

04:58 And therefore, the I band changes its dimension. it starts to disappear. It gets smaller and so to these H band because now the actin filaments extend further along the myosin filaments that zone, where they did not extend before is now occupied with actin filaments and so the H bands get smaller. It is important that you can realize how the contraction occurs.

05:28 It is important to realize that these Z lines come together and because of that, some of the bands will change their dimension. Well, I am not going to talk about the physiology of muscle contraction. You know the sliding filament hypothesis of muscle contraction or theory now of muscle contraction is something that I'll the physiologist brag about or show off about. I have got plenty of other things to show off about when I talk about histology.

06:04 So I have just mentioned this slide here to really explain to you I have mentioned before myosin filaments and actin filaments and I have really concentrated on those two contractile proteins. But the very ordered structure you see here, the very ordered array of these proteins and the ordered repeated pattern of the sarcomere along the myofibril is because all that information, all those molecules are held together by very special molecules, some are named there, but do not forget that many many different molecules hold the structure of the sarcomere together as it appears here. Some of those molecules anchor the myofibrils of the muscle through the sarcolemma of the muscle cell into the extracellular matrix on the outside of the muscle cell. One of those molecules is dystrophin and when dystrophin disappears or has a defect and is not produced by the cell, then the person suffers from muscular dystrophy. Let us now look at some of the regulation of how muscle contracts. The physiologist will explain to you that it is very important to have calcium present for muscle contraction. Calcium is very important for the actin and the myosin filaments to interact with each other and for the actin filaments to slide alongside the myosin filaments, and therefore shorten a sarcomere. Well that calcium is stored within the sarcoplasmic reticulum and this diagram shows you a picture of a number of myofibrils, but also it shows you two very very important structures. One is the terminal cisterna. In this diagram, you can see a very light bluish component wrapping around each myofibril. That is called the terminal cisterna. That is part of the sarcoplasmic reticulum.

08:28 It is a very expanded portion of the sarcoplasmic reticulum. That is where the calcium is stored.

08:37 And running down the middle of that terminal cisterna, that component of the sarcoplasmic reticulum storing calcium, is what we call a transverse tubule or a T system.

08:53 Together the transverse tubule and the terminal cisterna are very important components that bring about contraction. The transverse tubule is really an invagination of the sarcolemma, the cell membrane. And what really happens is that, when an action potential comes down from a neuron and depolarizes the sarcolemma membrane because of the action of neurotransmitters, then the wave of depolarization passes very quickly along the cell. But it also passes very quickly into the cell, through these transverse tubules. So that means that myofibrils, deep inside the middle of the skeletal muscle fibre get the same impulse, the same wave of depolarization at the same time as myofibrils receive it at the very periphery of the cell.

10:00 And that wave of depolarization passing down through that transverse tubule to all levels of the myofibrils in the middle of the cell causes the cisterna, the terminal cisterna storing calcium to release that calcium and that brings about muscle contraction. And when that contraction is completed, then that calcium return back into those terminal cisterna.

10:27 So this is really a very important outer structure specialisation that skeletal muscle fibres have to make sure that muscle contraction occurs instantly along the length of the fibre and also right through to the centre of the fibre.