Well, let us have a look and see how the muscle
is classified. We can classify muscle either
because it is striated or that it is not striated.
When you look at skeletal muscle, when you
look at visceral striated muscle and when
you look at cardiac muscle, you can see in
the tissue little tiny stripes or striations.
And these striations reflect the contractile
proteins within the muscle fibre or muscle
cell. These contractile proteins are arranged
in a very regular pattern and that is why
you see stripes along the tissue, along the
muscle fibre. Now here is a muscle, a large piece
of muscle shown in the middle of the image
and then on the right-hand side is a section
through skeletal muscle viewed with a light
microscope. The section at the top looks
at skeletal muscle cut transversely.
The section down the bottom looks at striated
muscle cut longitudinally. Now I have used the word
muscle cell and muscle fibre. But when we talk
about skeletal muscle, we really mean muscle
fibre because they are very long. Some muscle
fibres extend a long way, a huge distance.
For instance, the sartorius muscle in the
thigh goes close to the hip down towards the
knee, a very long muscle. So the muscle cells
are long. So it is more appropriate to call
that muscle fibres. Well let us look at the gross
anatomy view of the piece of the muscle
in the centre of the image. When you look
at pieces of muscle, they are separated from
each other by a connective tissue capsule
called the epimysium. This epimysium wraps
around the entire muscle, as you see here, and
it separates that muscle from adjacent muscles,
because sometimes these muscles need to contract
or relax independent of their neighbors, so
that epimysium separates the muscles apart.
It also will blend with more connective tissue
that carries blood vessels and nerves down
through the body that are going to innervate
the muscles and also supply those muscles
with oxygen and nutrients. Now if you look
into the muscle itself, you can see little
white lines. They are connective tissue septa
that penetrate into the muscle itself from
the epimysium. And these connective tissue
fibres called the perimysium, wrap the muscle
up into individual bundles of fibres.
There are many many bundles of fibres in these
piece of muscle wrapped up each by perimysium.
We often call a muscle bundle, a muscle fascicle.
Now, if you look at the individual cell and the
best view is to look at the longitudinal section
of muscle you see at the bottom of the right-hand
side image, you can see that there are some
very fine greeny blue stained connective tissue
fibres. Well thye wrap up individual muscle fibres.
So every single muscle fibre is surrounded
by endomysium connective tissue collagen. And
then what separates the muscle fibre from
this endomysium is the external lamina of
the muscle. So muscle is separated from connective
tissue but is wrapped up by connective tissue.
Now if you have a look at that gross picture
of the muscle as well, you are looking through
a section of that muscle in its thickest part.
I want you to imagine that muscle tapering
down towards the tendon or tapering towards
the point of attachment of the muscle on bone.
But what I really want you to understand is
that when the muscle tapers down to insert
onto the bone via the tendon so that its contraction
force can be imparted onto the bone via that
tendon, that tendon is formed because all
those connective tissue fibres from the epimysium,
from the perimysium and from the endomysium,
they all come together as the muscle tapers
down. So really a tendon consists of all those
three layers of connective tissue in the muscle,
plus a little bit more collagen added at the
myotendinous junction. So that means that
when the individual muscle fibres contract,
their force of contraction is then imparted
onto that endomysium, onto the perimysium and
via the epimysium as well onto the tendon.
And that is an important concept to understand.
Well, let us have a look at the skeletal muscle fibre
in a little bit more detail. I have said that they
are very long, they can be short in certain
muscles, but they can also be very long in
other muscles. Then when you do gross anatomy,
you'll realize the difference in muscle length
and muscle types in fact. The shape of different
gross anatomy muscles varies depending on
what part of the body you will look at these
different muscle that moves the limbs.
So now let us look at the skeletal muscle fibre
in more detail. I have already explained that
some of these fibres are very long in muscles
that are long themselves in extent long distances
across joints. Well, these long muscle fibres also
vary in their width or their thickness.
They can arrange from 10 to over 100 microns in
thickness. The reason why they are long fibres
is because during their development, they
resulted from the fusion of little cells called
myotubes that would derive from myoblasts.
And because of that, the fibres are not only
very long, but as we'll see later on, it is multinucleated.
Many many nuclei are situated all the way along
the skeletal muscle fibre and around the periphery
of the fibre, which is an important point
to remember. Now there are some special terms that
we use when we talk about skeletal muscle
fibres or muscle in general. First of all,
we use the term sarcoplasm to describe the
muscle cytoplasm. Sarco is pertaining to muscle
or flesh. So we use this term. When we talk
about the cell membrane or the plasma mambrane
of the muscle fibre, we call it the sarcolemma.
And you see here that these two muscle fibres
are slightly seperated, but on the edge of
each of these muscle fibres, there is the
cell membrane, but we call it the sarcolemma.
Well, as I said the nucleus sits on the periphery
of the skeletal muscle fibre and there are
many of them all the way along the fibre.
The sarcoplasm is dominated by the contractile
factory of the cell. And these contractile
factories are loaded together or packaged
together in long structures called myofibrils.
And these myofibrils as you see in this section
dominate the sarcoplasm because they are there
to affect contraction. And if you look very
very closely in this image, in the cross section
of the skeletal muscle fibre, you can see
all these myofibrils appearing as tiny little red
dots when they're cut perfectly transversely.
Well, when you look at fresh muscles or sections
of muscle and using certain stains, you can
identify three different muscle fibre types
in skeletal muscle. And these reflect the functional
capacity or the contractile strength and energy
usage of these different muscle fibre types.
The red fibres are the most abundant. They
have the properties of slow twitch, which
means a twitch means really a single muscle
contraction. So these slow twitch muscle fibres
contract rather slowly. They are also fatigue
resistant. They don't tire very quickly and
so these muscle fibres are ideally found in
locations of the body where you want to have
a rather slow prolonged contraction such as
in the back where you really want muscle to
adapt to being slow, but prolonged contraction
to maintain the erect posture. Now other fibres
are medium-sized, they are fast-twitch fibres.
So they can contract very quickly and they
can also reach maximum contraction tension.
They can also be very fatigued resistant because
they derive their energy from oxidative glycolytic
pathways. They store glycogen and they can
undergo anaerobic glycolysis. Now these types
of muscle fibres are ideal where you want
a reasonably long high contraction tension
and muscle that is not going to tire very
easily. So middle distance runners, 400 or 800-metre
runners, or even middle distance swimmers would
like to have a lot of these fibres, that don't
tire easily but they generate high contractile
strength and tension. And the last muscle
fibre type is the fast glycolytic fibre.
These store enormous amounts of glycogen.
They are large fibres. They contract very
fast and they generate very high contractile
tension, but they are prone to fatigue very
quickly. And that is because of the build-up
of lactic acid in these fibres during usage.
So you see these fibres in sprinters or athletes
that have that sort of sprinting type role.