Sarcomere

00:01 So that's how cells connect one to another what's within the individual cells.

00:05 And there are a number of these structures, these are the sarcomeres.

00:10 These are the actual elements that are going to be responsible for allowing the cell to contract.

00:16 So this is some of the most beautiful electron microscopy that you will see these kind of alternating light and dark lines.

00:25 And then perpendicular to those, we have Z-Bands and A-Bands, we'll talk more about them.

00:30 This is a very metabolically active structure, the sarcomeres of the cardiac myocyte.

00:35 And so we have to have mitochondria, which are in profusion, which are very high levels within the myocardium, as we'll see in a minute.

00:42 And they're cranking out ATP, like crazy to allow the movement of various ions that will facilitate the contraction or relaxation of the sarcomeric proteins.

00:55 So general organization, we have these lines in the middle that are called Z-Bands.

01:00 So you can actually see this by light microscopy if you have a perfectly organized bit of tissue.

01:08 And the Z-Bands are going to be those that connect with the actin filaments, and they demarcate a particular sarcomere.

01:17 So from Z-Band to Z-Band indicated here in red, that's one sarcomere.

01:22 So on this slide, we have a couple sarcomeres side by side.

01:27 This is showing you the sarcomere.

01:30 Again, highlighted in green between the two Z-Bands.

01:33 Also, what came up on the slide is glycogen, we have little tiny bits of dark particulate matter that are electron dense by electron microscopy.

01:42 And again, heart muscle is very metabolically active.

01:45 So we have a lot of fat metabolism and glycogen, metabolism and glycogen storage that is happening within cardiac myocytes.

01:53 Myosin, so myosin are the thicker filaments.

01:57 So you see this alternating band of dark and light, dark, light, dark light from kind of top to bottom there, indicated in purple, the darker bands are going to be the myosin heavy chain bands.

02:10 Okay.

02:11 And then, in between, the lighter bands are the actin, we're gonna see it schematically better in just a moment.

02:18 In fact, here we are seeing it schematically in just a moment.

02:21 So we see the Z-Bands demarcating a single sarcomere.

02:27 Got that? on each side.

02:28 And then we have the thicker, which would have been the darker bands present on electron microscopy, those are going to be our myosin heavy chains.

02:37 And then we have the sinner bands, that's going to be actin.

02:40 So we have myosin and we have actin.

02:43 And those are going to be the major elements that allow us to contract this sarcomere.

02:51 Now there are going to be other proteins that are also very important.

02:54 So there are proteins that make up the center.

02:59 So how do we hold the myosin heavy chains together that's through the M-Band.

03:04 And we talk about the total thickness, this is by electron microscopy, the total thickness from end to end of the myosin heavy chain, that's the A-Band.

03:15 You do not need to know that detail for your boards, you just don't.

03:21 But if you are interested in muscle physiology, then this becomes a very interesting thing because there are certain limits that you need to understand.

03:30 But you need to be able to identify Z-Bands, and you need to know that they're actin and myosin that are the major elements of the sarcomere.

03:38 The other elements of the sarcomere that are also present that allow the interaction between myosin heavy chains and actin light chains include tropomyosin, and troponin.

03:49 These are important because when we have injury to the cardiac muscle, these proteins get released into the circulation and we can measure them as an indication of the degree of myocardial damage.

04:02 So we now measure myocardial infarct by measuring troponin levels, for example.

04:08 Okay, so this is what the action of the sarcomere does.

04:12 The myosin heavy chains in the presence of calcium bind to the actin and bring it together, that's a contraction.

04:18 And then at the end of the cycle, we pump the calcium ions back into the smooth endoplasmic reticulum, sarcoplasmic reticulum.

04:28 And that allows relaxation of the muscles so you have contraction and relaxation, driven by the movement of the light chains over the heavy chains.

04:39 And we see that the sarcomeres will shorten during contraction and will relax during relaxation, so they will expand.

04:50 Okay, some other elements of the sarcomere or other elements of cardiac muscle that's the mitochondria.

04:56 Remember I said it's very metabolically active.

04:58 But yeah, let's look on the left hand side that's cardiac muscle.

05:01 Roughly a third of the volume of the cardiac muscle is going to be mitochondria, somewhere between 20 to 30%.

05:11 Skeletal muscle, on the other hand, is about 2 to 3%.

05:14 So even in the best, most robust marathon runner, the number of mitochondria in the skeletal muscle is about tenfold less than what you see in cardiac muscle.