Myocardium

00:01 Welcome back to some of the conduction system in a bit.

00:04 But let's work our way back up here to the myocardium.

00:07 So myocardium really a great structure too.

00:10 It's got to contract and relax, contract and relax in a recurrent fashion.

00:15 How is this happening? So we have myocardium.

00:17 Here's what it looks like in real life.

00:19 And histologically, which I get to look at everyday down the microscope, it's there.

00:24 So the pinkness that's there, the vast majority of that is myocytes.

00:30 You see the blue dots, those are the nuclei of the myocytes.

00:34 And the cardiac myocytes constitute about 25-30% of the total cellularity of the heart.

00:41 They comprise 80-90% of the volume of the heart.

00:45 But if we count the individual cells, they're only about 20-30%, they're just big compared to...

00:52 Oh and then, the cardiomyocytes are connected one to another.

00:56 This is how we get a coordinated contractile wave.

00:59 And they're connected physically, mechanically, and also biochemically through the intercalated disc.

01:07 That allows us to have a nice contractile wave from cell to cell to cell.

01:11 So we can see that disk lining up.

01:15 Now the other cells that are present within the heart are endothelial cells.

01:19 There are roughly three capillaries for each cardiomyocyte.

01:22 That's how metabolically active that cell is the cardiomyocyte, we have to have a lot of capillaries to keep it happy.

01:29 So actually, the greatest number of cell type within the heart is probably endothelium.

01:35 But also with the fibroblasts, you can't just have muscle, you actually have to have an extracellular matrix to kind of pull against.

01:42 So there are a lot of fibroblasts that are in there as well.

01:46 And then, what's shown on the slide is kind of the wear and tear pigment associated with being a heart muscle or heart cell for a lifetime.

01:55 And we get oxidized lipid, oxidized glycoprotein that can no longer be degraded accumulates in the lysosomes of the cardiac myocytes.

02:04 And it's a recognizable entity called lipofuscin.

02:07 It's not particularly pathologic, but it does mean that the heart has been going through a bit of wear and tear.

02:14 Alright, here's a schematic of what we're dealing with.

02:17 And cardiac myocytes are not boxcars, they're actually kind of branching structures that are tightly intimately associated one with another.

02:26 Again, they have to actually maintain a signal cell to cell to cell.

02:30 So they have to have a lot of gap junctions that connect laterally as well as the intercalated discs at the end of cardiac myocytes connecting cardiac myocyte to cardiac myocyte.

02:40 So I have just shown you the intercalated disc in this particular cell.

02:44 And again, facilitating cell-cell mechanical and electrical and biochemical coupling.

02:51 Here's what's at the intercalated disc.

02:53 So, you see two cardiac myocytes, one in the upper right hand corner, one in the lower left hand corner and there is a gap between them.

03:01 And that is connected by a number of proteins that are responsible for mechanically linking but also electrically linking cell to cell.

03:12 So if there is a signal on the left hand side that says contract, there is a calcium current that gets into the right hand cell through the gap junctions that says contract.

03:23 And now we have a cell linked to the earlier contraction wave and we get a nice wave of contractility up along the ventricles.

03:34 What does this look like in real life? So this is a transmission electron micrograph that is showing you the intercalated disc.

03:42 Other rather dense structures within them are desmosomal proteins.

03:46 There are also within them gap junction proteins that allow the transmission of a calcium current.