So let's move in now to more specifically talking about skeletal muscle,
where we've already said that the muscle itself is going to be made up
of many muscles, cells, or fibers.
You can use that term interchangeably.
So here's a cross-section of a muscle, and you see that we are going to zoom in
on a single muscle cell or fiber, and then that cell or
fiber is composed of many other units called myofibrils.
And that's what's show here, where we can zoom in and look at a single myofibril,
that is going to be composed of many molecules of actin and myosin.
So it's going to be the number of myofibrils in a muscle cell,
that is going to determine the force that it can generate.
So the myofibril is kind of the functioning unit within the cell
that is contracting, and along its length,
myofibrils can be divided into sarcomeres, which is what's shown in here.
So that a muscle cell is made up of many myofibrils that are in parallel,
and then a myofibril is composed of many sarcomeres that are in series.
So there'll be hundreds, if not thousands, of sarcomeres that are making up
a single myofibril, and it's going to be each sarcomere that is going to contract.
And so if you've got many sarcomeres that are contracting at the same time,
then you're going to have a shortening of the muscle,
which will cause an action, which we'll be talking more about.
So it's the number of myofibrils that are going to determine the force generating
capability of the fiber, and we can divide the myofibril into sarcomeres.
And the sarcomere pattern is going to be what's
going to cause the striations, or the banding, of the skeletal muscle cells,
because the myofibrils will be aligned in a very organized way.
So that's what we're going to start talking about now, where at the top of
this image we have an electron micrograph of a portion of a myofibril,
that's showing you a single sarcomere.
And you can see that it's got a light portion, and
a dark portion, and that those are alternating.
So that if we had many sarcomeres, you would see alternating light and dark, and
light and dark, and this is going to be responsible for the striations.
Then below that image, we can see what are the fibers or
filaments that are causing that pattern where we have
these Z lines which are the ends of the sarcomere,
they determine the borders between the sarcomeres.
And sticking out of each Z line are a set of thin filaments, which are actin.