Hi. It's Mr. Andersen. And in this podcast I'm going to talk about the muscular
system. When I was growing up one of my favorite books to read was the Guinness Book of World
Records. And one of the reasons I liked that is this guy was in there. Paul Anderson. Which
is my name. He was the world's strongest man back in the 1950s. That's changed quite a
bit. We have power lifers now can, I think the record for bench press is somewhere over
1000 pounds which is just ridiculous. But that's not all the muscles there are. There
are also what we call slow twitch muscles. And so even though some people maybe really
explosive and very fast, there are other people who have a huge amount of endurance. So this
is the world record holder in the mile which is like 3:43, Hicham El Guerrouj. And so that's
another type of muscle. And that's really showing amazing aerobic respiration. But basically
your muscular system is made up of three types of muscles. We have skeletal or sometimes
we refer to those as striated muscle. And so this right here would be like a bicep muscle
or the muscle in my arm that allow my fingers to move. We have smooth muscle. Smooth muscle
is going to be involuntary muscle. So like the muscles that operate the lower portion
of my esophagus, that operate my small intestine, large intestine. They have a different structure
and they work a little bit differently. But they're involuntary. I don't have to think
about it. And then something, kind of a combination between the two is the cardiac muscle. It
looks striated, but cardiac muscle is only going to be found in the heart. And so it
has these nice little inter collated disks which allow the transmission of electrical
signals kind of to wash over them. And that's how a heart is able to contract. And so basically,
I want to talk about skeletal striated muscles. How they're organized. And so if we were to
look at this. This would be a tendon which is simply attaches muscle to bone. But basically
we have a muscle which is a muscle fiber, but those are fibers within fibers. But we
eventually get to what's called a sarcomere. A sarcomere is going to be one unit of a muscle.
And if we even zoom inside this sarcomere what we'll find is that there are two chemicals.
And those chemicals are actin and myosin. And so to really understand how a muscle works
you need to understand how these two molecules work together. So actin is going to look like
this. And then myosin is going to have all these heads. So they have these double heads.
And they're trying to grab on to the actin. And so before I get to the whole idea of how
a muscle is able to contract, I want to talk about the story of actin and myosin. And so
if we look at those two, we'll look right up here. Myosin has these double heads. But
basically what it would do is myosin would like nothing more than to grab on to actin.
So it want's to grab on to actin. And to do that it needs essentially two things. It needs
ATP. And that's going to provide the energy. But it also needs to have access. And to get
access, it can't get access because there's this tropomyosin which is acting as a block.
So here's myosin, myosin is very very sad because myosin would love to get to actin.
But actin is over here. It's blocked. So we have the tropomyosin in the middle. And so
what's the other things that we need? We need ATP. We need access to actin. And to get that
access we need something called calcium. And so basically that calcium is going to be provided
by, released from action potential nerve signals that are coming down to fire off that muscle.
And so basically what the calcium will do is the calcium will grab on to the tropomyosin.
And basically it's going to move that block out of the way. Once it moves that block out
of the way, then the myosin head can lose it's phosphate group. It will grab on to the
actin, let me try to do this with this. So basically myosin would love to get on to the
actin. But it can't. But once calcium is there it can get through that block. It will let
go of a phosphate and as it does that, it will creep along the actin. So as long as
we have ATP present and as long as we have calcium present, the myosin is simply going
to slide that actin past it. And there's not only one myosin head. There's numbers of heads
that are all doing this. And so basically this actin is sliding past the myosin. And
as it does that, that's contracting the muscle. And so once the calcium is gone, once the
ATP is gone, then we can't do that. And so basically you should understand that myosin
heads are trying to get on to the actin and slide the actin. So what does this look like
then? So wrap your head around this. Basically we have bands. And those are going to be the
bands that you see in that striated muscle. But the big things that we should understand
is on the end we have what are called the Z disks. Z disks are going to be these long
disks that go down the end. So we have one there. We have one there. We know have the
actin. Actin is going to be blue in this diagram. So that's the actin that goes down the middle.
And then we have myosin in the middle. So the myosin is all attached to the middle.
They're attached to myosin on either side. And so this myosin can't go anywhere. So basically
it's stuck. You have all these myosin heads and then we have the actin in the middle.
And so basically what does myosin do if ATP is present? If calcium is present? All of
these heads are going to start grabbing on to the actin and they're going to start pulling
the actin in this direction. And so they'll be pulling it on this side, pulling it on
this side. And so what's going to happen? Well if you look down here they're going to
move that actin pass the myosin. What does that do? Well if you look, now our Z disks
are much closer together. And so we've contracted the muscle. When we relax the muscle, then
they're going to slide back to where they were. And so when you contract a muscle, what's
doing that? It's all these myosin heads that are grabbing on to the actin. They're pulling
it past it. And then when you relax it they fall back to their normal position. And so
that's how a muscle works. It's called the sliding filament theory. And I hope that's
all helpful.