Well let us look and see how muscle can repair
itself after injury. Remember this slide,
we explained the components of muscle earlier
on the lecture. Focus on the nucleus or said
earlier that muscle fibres are multinucleated.
Well, some of these nuclei are in fact stem
cells. They are called satellite cells.
You cannot tell the difference between the satellite
cells and the true skeletal muscle nucleus,
but these satellite cells sit on the periphery
of the skeletal muscle fibre and they have
a nucleus also. As I've said you cannot tell the
difference between the nucleus of the satellite
cells and the true nucleus of the skeletal
muscle fibre. But if skeletal muscle is damaged,
these satellite cells can lay down more contractile
proteins. The muscle cell cannot divide, it
is fully differentiated. It can only rely
on the satellite cells to repair the damaged
fibre to lay down more contractile proteins
that will allow the fibre to get thicker and
thicker. If you do weightlifting exercises
and you increase muscle mass, you do not get
more muscle cells because muscle cells cannot
divide, at least the skeletal muscles cannot
divide. The muscle fibre gets bigger, thicker,
hypertrophies because of these satellite cells
laying down more contractile proteins.
I want to briefly just explain to you the
difference between striated skeletal muscle
and striated cardiac muscle. I am going to
deal with cardiac muscle in more detail when
we look at the cardiovascular system.
But cardiac muscle fibres are smaller and they
are joined end to end at special locations
called intercalated discs. These consists
of very tight adherent junctions and also
gap junctions. And that allows you wave of
contraction or depolarization to pass along
the length of the cells along, the length of
the cardiac muscle components, and therefore
bring about a sequential contraction of the
cardiac muscle that is important in pumping
blood out of the heart. Cardiac muscle
is striated, but it has a central nucleus
and only one nucleus. So the fact that it
has a central nucleus and only one nucleus
and it has these intercalated discs is a sort
of specialized structural details that separate
it from a normal striated skeletal muscle
fibre. Let us quickly look at the smooth muscle
fibre. They are not striated. They're joined together
by gap junctions, and those gap junctions allow
again the wave of contraction or the wave
of depolarization to move along the length
of the massive smooth muscle and bring about
again a sequential contraction.
They have one central nucleus like cardiac muscle,
but they do not have striations. And that again
is enough criteria for you to now to be able
to distinguish between skeletal muscle fibres,
cardiac muscle fibres and smooth muscle fibres.
Smooth muscle can respond to injury.
In the case of the artery wall, smooth muscle cells
can divide and the wall can get thicker and
thicker if need be. In the endometrium of
the uterus, the muscle cells can divide and
also hypertrophy. We see that during pregnancy.
And the muscular external layers of smooth
muscle in the gut tube can also divide and
hypertrophy. So smooth muscle has the ability
to grow and repair because they can divide.
They can undergo regular mitotic activity.
While in summary then, make sure you can distinguish
the three muscle types, skeletal, cardiac
and smooth muscle and those that are striated
and those that are not. Make sure you understand
particularly with skeletal muscle about innervation
of these muscle fibres. Understand the wrappings
around the skeletal muscle and how those
wrappings come together to form a myotendinous
junction. Make sure you are aware of the different
muscle fibre types, the red, the intermediate
and the light whiter pink fibres. And also understand
the structure of the sarcomere and be able
to identify the sarcomere in the skeletal
muscle fibre. And finally be aware of the
function of both the neuromuscular spindle
and the Golgi tendon organ. They allow us
to perceive our position in space, the position
of our limbs because these receptor organs
tell the central nervous system about the
stretch and length change in muscle and also
about muscle tension.
So thank you for listening to this lecture.
I hope you have enjoyed learning a little
bit about skeletal muscle in particular and
I hope you look forward to learning more about
smooth muscle and cardiac muscle when I deal
with those very special muscles when we look
at the cardiovascular system and also when
we look at other parts of the organ systems
and look at the role of smooth muscle in those
systems. So once again thank you very much
for listening to this lecture.