It is the contractile unit.
Well, this is an electron micrograph of a skeletal
muscle fibre showing you a number of myofibrils.
And the myofibrils are actually almost lined
up in parallel here. There's a slight different
because of shrinkage during processing, but
on this particular picture, you can see maybe
1, 2, 3, 4, 5, 6 or so myofibrils separated
by a tiny little space you sometimes see which
is going to be the sarcoplasmic reticulum
and I am going to talk about that in the bit
more detail in a moment. But again look at
this picture or image, it's a section of a skeletal
muscle fibre viewed with an electron microscope
and the red band you see, demarcates a sarcomere.
The Z line or Z disc running along the middle
of the light or I band and then the dark band
consisting of myosin and overlapping filaments
of actin. And then there's the H band and the little
M line in the middle. Again let me stress
the importance of really recognising the components
of the sarcomere. Well, you know I am smiling
and having a little laugh here because I want
to tell you a story. I put there the
size or the length of the sarcomere when it
is relaxed, when it is stretched or when it
is contracted. Now you know when I was a student
studying histology, I have to sit for an exam
as part of my basic tissue exam in histology.
And one of the questions I can always remember,
it was a multiple choice question and I do
not really like multiple choice question,
but anyway the question was what is the length
of the sarcomere? And the choices I cannot
remember the details of all the four choices,
but that were something like 2 microns, 2.3
microns, 2.6 microns or 3 microns. And I thought
well I do not really understand this question,
so I guessed. I set off and ticked the box
for 3 microns. When I got my exam paper back,
it was marked wrong. And I went to my professor
and I said professor you have marked this
wrong and my professor said well that's because
the answer is wrong. The answer is 2.6 microns.
And I looked at my professor and said but
the question is confusing. You should have
asked the question what is the length of a
sarcomere when it is relaxed or when it is
stretched or when it is contracted? And he
smiled at me and he said ok Geoff, I will
change the question. I will change your mark.
I will delete the question and amend your
mark and the mark for all the other classmates.
So I smiled and thanked my professor.
And you know today, he remains one of my best
friends. He supported my position at my university,
but I've always remembered the little contest
we had when our professor about being marked
wrong on this question about the length of
the sarcomere. Well, after the story, let us
get back to looking at skeletal muscle. I
spoke a minute ago about sarcoplasmic reticulum.
It is just like the endoplasmic reticulum
in other cells and make sure you now are aware
that this sarcoplasmic reticulum is very closely
associated with myofibrils. Well, here is
a rather complicated picture of the sarcomere.
It is important to understand that when muscle
contracts, the sarcomere changes in dimensions
and the previous slide show those dimensions.
I think it is important to appreciate that
really during contraction, the Z lines move
together towards each other. The dark A band
remains the same in length. What changes is
the length of the H band and the length of
the I band, the light band. And that is because
as I have said before, the actin filaments
slide in between the darker myosin filaments.
And therefore, the I band changes its dimension.
it starts to disappear. It gets smaller and
so to these H band because now the actin filaments
extend further along the myosin filaments
that zone, where they did not extend before is
now occupied with actin filaments and so the
H bands get smaller. It is important that
you can realize how the contraction occurs.
It is important to realize that these Z lines
come together and because of that, some of
the bands will change their dimension. Well,
I am not going to talk about the physiology
of muscle contraction. You know the sliding
filament hypothesis of muscle contraction
or theory now of muscle contraction is something that
I'll the physiologist brag about or show off
about. I have got plenty of other things to
show off about when I talk about histology.
So I have just mentioned this slide here to
really explain to you I have mentioned before
myosin filaments and actin filaments and
I have really concentrated on those two contractile
proteins. But the very ordered structure you
see here, the very ordered array of these
proteins and the ordered repeated pattern
of the sarcomere along the myofibril is because
all that information, all those molecules
are held together by very special molecules,
some are named there, but do not forget that
many many different molecules hold the structure
of the sarcomere together as it appears here.
Some of those molecules anchor the myofibrils
of the muscle through the sarcolemma of the
muscle cell into the extracellular matrix
on the outside of the muscle cell. One of
those molecules is dystrophin and when dystrophin
disappears or has a defect and is not produced
by the cell, then the person suffers from
Let us now look at some of the regulation
of how muscle contracts. The physiologist
will explain to you that it is very important
to have calcium present for muscle contraction.
Calcium is very important for the actin and
the myosin filaments to interact with each
other and for the actin filaments to slide
alongside the myosin filaments, and therefore
shorten a sarcomere. Well that calcium is stored
within the sarcoplasmic reticulum and this
diagram shows you a picture of a number of
myofibrils, but also it shows you two very
very important structures. One is the terminal
cisterna. In this diagram, you can see a very
light bluish component wrapping around each
myofibril. That is called the terminal cisterna.
That is part of the sarcoplasmic reticulum.
It is a very expanded portion of the sarcoplasmic
reticulum. That is where the calcium is stored.
And running down the middle of that terminal
cisterna, that component of the sarcoplasmic
reticulum storing calcium, is what we call
a transverse tubule or a T system.
Together the transverse tubule and the terminal cisterna
are very important components that bring about
contraction. The transverse tubule is really
an invagination of the sarcolemma, the cell
membrane. And what really happens is that, when
an action potential comes down from a neuron
and depolarizes the sarcolemma membrane because
of the action of neurotransmitters, then the
wave of depolarization passes very quickly
along the cell. But it also passes very quickly
into the cell, through these transverse tubules.
So that means that myofibrils, deep inside
the middle of the skeletal muscle fibre get
the same impulse, the same wave of depolarization
at the same time as myofibrils receive it
at the very periphery of the cell.
And that wave of depolarization passing down through
that transverse tubule to all levels of the
myofibrils in the middle of the cell causes
the cisterna, the terminal cisterna storing
calcium to release that calcium and that brings
about muscle contraction. And when that contraction
is completed, then that calcium return
back into those terminal cisterna.
So this is really a very important outer structure
specialisation that skeletal muscle fibres
have to make sure that muscle contraction
occurs instantly along the length of the fibre
and also right through to the centre of the
fibre. Well, let us have a look at the neuromuscular