00:01
So now let us look inside the muscle fibre
itself, inside the sarcoplasm and concentrate
now on the structure of these myofibrils.
As I've said earlier, the muscle myofibrils are
made up of all the contractile proteins. They
are made up of myofilaments. And these myofilaments
consist of a number of different proteins,
but the main ones are the myosin II, which
are thick filaments and the thinner actin
filaments. These generate movement and they
are wrapped up in bundles within the cell,
within these myofibrils. And even though I
said earlier, that these myofibrils dominate
the sarcoplasm, there is still a certain amount
of endoplasmic reticulum wrapping around each
of these myofibrils but it's very difficult
to see. It appears very sparse because as
I said, the sarcoplasm is dominated by these
myofibrils. I used the word endoplasmic reticulum,
as we do when we talk about the endoplasmic
reticulum in most cells. But in muscle cell,
we use the term sarcoplasmic reticulum.
01:36
Now this is a diagram of a myofibril showing details
of its banding pattern. This banding pattern
is reflected in the light microscopic picture
of muscle we can see when we look at skeletal
muscle using a microscope. But I really want
to explain this particular slide rather slowly
and carefully because there's a number of
concepts here that are important for you to
understand. Remember that the myofibrils are
made up of myofilaments, the contractile units.
02:20
And these myofilaments consists of actin and
myosin and also lots of other proteins that
hold these myofilaments in a regular pattern.
And that regular pattern is repeated across
the myofibril and repeated along the myofibril.
So you see this banding pattern that you can
see in the diagram. Have a look at the banding
pattern in some detail yourself before I start
to explain its detail. The muscle fibre here
is filled with these myofibrils. That is the
first concept that is important for you to
understand. That muscle fibre is wrapped up
by connective tissue, the perimysium as a
muscle bundle. And that muscle bundle then
is packaged together in groups and wrapped
up by the perimysium and then the epimysium
and then you have the whole muscle. So let
us just go back in the order of magnitude
because I think it is very important for you
to understand the concept and orders of magnitude
of this muscle structure. The muscle shown
in the diagram is surrounded by epimysium.
03:55
Within the muscle itself, there are muscle
bundles, muscle fascicles, each wrapped up
by perimysium. And then within each muscle
bundle or muscle fascicle, there are individual
muscle fibres that have endomysium around
them as well. And the really important part
of the concept now is to have a look within
this muscle fibre and appreciate as I pointed
out the dominance of individual myofibrils
that make up as I have said a number of times
already the bulk of the muscle fibre. So
have a look at this banding pattern.
04:44
The diagram shows you a muscle fibril in detail. When
you look at the banding pattern, some of the bands
are dark, some are light, some are thin and
some bands are thick. So have a look at the
banding pattern yourself now and just make
sure you can pick up some dark bands, some
light bands, some thick bands and some thin
bands. And then we can start to call these
bands different names. What I want you to first
of all look at, is have a look at the diagram
of this myofibril and find the Z-disc. It
is that thin dark band you can see labelled
on the myofibril. And have a look along towards
the end of the diagram of the fibre and you
can see a light band labelled the I band.
I, the letter I in light reminds me that the light
band you see is the I band. Again think of
the letter I and think of the letter I in
light. And in the middle of that I band is
another Z disc. Well, the distance between one Z-disc
and another is called the sarcomere and that
sarcomere, as I explained at the very start
of the lecture is a basic unit of contraction
in muscle fibre. And that sarcomere if you
look all the way along in the diagram of this
myofibril, is repeated, time and time again
along the myofibril for the whole length of
the muscle fibre. So make sure that you are
really now understanding the concept of what
a sarcomere is? It is the structure running
between two Z discs. Sometimes we use the
term Z line. Now the I band is really the
part of the sarcomere that contains the thin
actin filaments. The dark band you see there
is labelled A band. Think of the letter A
for dark and that reminds me that the dark
band is called the A band, the letter A. Well
that A band represent a region of sarcomere,
that's towards the centre of the sarcomere,
that is occupied by the thick myosin filaments
and also the darkness is because there is
an overlap between the thin actin filaments and
the thick myosin filaments. And right in the
middle, you can see another very lightish
band with little dark line running through it, that
is the H band. And the line you see is the
M line that will be labelled later on. So
again carefully look at the sarcomere, carefully
appreciate the light band or the I band, the
dark band or the A band and the central H
band with a little line going through the
M line. And again, make sure you understand
that the I band is the thin actin filaments.
The dark band, the A band is the heavier myosin
filaments with overlapping of the actin filaments
because they interdigitate. The H band is
actually where the heavy or the myosin II
thick filaments are but it is lightish there
because the actin filaments have not extended
through to that part of the sarcomere.
08:54
Now, if you look carefully at the H&E section you see
there, have a look and see if you can find
the banding patterns. You can see the dark
band and the light band. Have a look in the
middle of the light band and just see if you
can start to identify a Z line. I know it
is very hard when you look at sections of skeletal
muscle fibres, but if you look very carefully
I am sure you can pick out a Z disc and you
can pick out an A band. Well, this is the
very important concept for you to understand
the banding pattern and the structure of the
sarcomere as I've said it is the functional, most
important part of skeletal muscle fibre.
09:42
It is the contractile unit.
Well, this is an electron micrograph of a skeletal