hello anatomy friends this is Dr. Alsup and we are going to start exploring the wonderful world
of skeletal muscle tissue at a more microscopic level so we're going to focus on skeletal muscle
fibers which goes by many names the the scientific name is going to be myofibers but we'll talk about
some others as we go through so we have much much to do this is going to be one of the longer videos
as we're going to discuss first of all how they even form but then we're going to spend a little
bit more time in terms of the structures within a myofiber and last but not least we're just
going to introduce sliding filament mechanism very physiology heavy so we're not going to get
into the details here but it's super important in terms of understanding how muscles actually
contract all right excellent so during embryonic development hundreds of myoblasts which are just
these small mesodermal cells remember mesoderm is one of the germ layers that so you'll have these
mesodermal cells that will collect as you can kind of see here they start off individually they start
to collect and fuse to form these long muscle fibers like you can see here and that is why
each mature muscle cell or myofiber will have hundreds of nuclei associated with them because
they are from these different cells that have kind of formed together so we have multinucleate cells
but once a muscle cell is fully fused and considered a mature cell
cell division is not possible after that point so in but we do know that muscles grow right we know
that muscles get larger so how do they do that well in order for a muscle cell to get larger
it has to what's referred to as hypertrophy which is the enlargement of the existing muscle fibers
so this hypertrophy is possible by the increased production of structures in the cell
most noticeably what often gets larger or what often increases are the amount of myofibrils and
we'll talk about what that is in a moment but you can have other components of the cells that will
increase you'll have more of so that could be the sarcoplasm, the sarcoplasmic reticulum, mitochondria
other organelles those can also have increased production therefore making the cell get larger
now hypertrophy not hyperplasia okay so hyperplasia would be more cells hypertrophy
is the increase in size of the existing cells during childhood certain hormones like human
growth hormone like we talked about before or sex hormones can lead to hypertrophy of those
existing cells but after the juvenile period muscle hypertrophy really generally can occur
due to repeated forceful muscular activity like weight training that can lead to a larger size
of existing muscle cells so that's a little bit about how muscles will actually grow
now let's return back to this concept of the myofiber in order to better understand the
structures within so we mentioned some of those names so let's better understand what they are
now as i mentioned at the beginning muscle cells go by other names i personally like the name
muscle fiber you'll hear me say that quite a bit because i think it's indicative of this
long size of one muscle cell it's a really long cell compared to other types of cells
again myofiber is the more scientific name and sometimes you just hear it referred to
simply as the muscle cell and as you can see in this image there's a lot going on
in one cell it's like packed full of stuff so what is that stuff so let's talk about that
most of the structures within a muscle cell have slight variations to their names to indicate that
they are part of a muscle cell so there are a lot of the same components that you have in all
cells but oftentimes you'll see it with this sarc- associated with it which means flesh
so if you see this sarc- generally you can think muscle cells are part of a muscle cell so let's
start with the sarcolemma and the sarcolemma has two parts sarc- means flesh -lemma means sheath
and basically in essence a sarc- the sarcolemma is the plasma membrane of a muscle cell all right so
you can see that kind of in this cartoonish image here surrounding the contents of the muscle cell
now in certain areas of the sarcolemma the sarcolemma will invaginate or tunnel into
the surface and go into the inner contents of the cell and this is referred to as a transverse or t-
tubule and these things are super cool very important in terms of contraction because these
invaginations allow muscle action potentials to propagate or spread widely and importantly
quickly throughout the muscle fiber so not only will that action potential kind of be
traveling along here it will actually go down into the inner portion of the cell
and one thing let me go back to the slide and you'll see in this other image it's really
closely related to this other structure within the cell called the sarcoplasmic reticulum so a little
bit about what the sarcoplasmic reticulum is or the SR these are fluid filled membranous sacs that
encircle each of the myofibrils now myofibrils are the the major structure within the myofiber
and that's going to be those contractile structures more to come on that in a moment
the sarcoplasmic reticulum is similar to the smooth endoplasmic reticulum in non-muscle cells
and key to the function of the SR is that it stores and where do i have it here right here
it stores calcium in relaxed muscle that's a huge part of its importance functionally
now note I said it's in a relaxed muscle it stores in a relaxed and in
a muscle cell that has been activated this will be released and that's very important
at the ends of the sarcoplasmic reticulum there are these dilations of the reticulum called
terminal cisternae you can see one here you can see one here and what is it really closely related
to it's related to that transverse t tubules right next to it in fact one t tubule and two terminal
cisterns equal a triad so you'll hear a triad associated with the muscle cell
and what can happen is that action potential will travel down that T tubule and it will trigger the
release of that calcium that we know is stored in the sarcoplasmic reticulum in a relaxed muscle
but that action potential is allowing for the muscle to be activated and that will allow the
calcium to release into the sarcoplasm of the cell to allow muscle contraction to occur okay
so we are going to get into the other components so we talked about the sarcoplasmic reticulum we
talked about the sarcolemma and then we're going to have this we're going to have
portions of the other contents as well so you have the sarcoplasm which is basically the cytoplasm
of the muscle cell and there's a lot going on in there there's that sarcoplasmic reticulum
as i mentioned you're also going to have other components in there as well
the bulk of the contents of the sarcoplasm are these structures called myofibrils and that's what
these are all over the place right here so it's these threads basically that are filling up this
one has been pulled out and then magnified right here we're going to come back to the myofibrils
in the next slide because it's this is really pretty important but let's let's talk about these
other structures there's obviously going to be mitochondria associated with the sarcoplasm very
important in terms of ATP production there is also myoglobin not really shown in this image myoglobin
these are red colored proteins that are only found in muscle and these proteins bind oxygen molecules
that can diffuse into muscle fibers from the interstitial fluid and this oxygen will be
released from myoglobin when it's needed by the mitochondria for ATP production
so very important in terms of ATP production the sarcoplasm also has a substantial amount
of glycogen again not really shown in these images here and glycogen is a molecule that
stores linked glucose molecules that also can be broken down when needed to help produce
ATP ATP keeps coming up over and over again so ATP if you haven't heard of it before
and this is this is going to play this is an organic compound that plays a role
in energy transfer and it's vital to allowing muscle contraction to occur
so it's allowing muscle contractions to actually happen but it also allows for
muscle contraction to stop you need ATP for both so hugely hugely important so say particularly
if you have like sustained contractions over time you need a lot of ATP for that to happen
okay returning to those myofibrils so all these little things that are dotting throughout the
entirety of that sarcoplasm these are the contractile elements of the muscle cell and
one myofibril will extend the entire length of a muscle fiber okay so they're long they're long
like the muscle fiber as well and they have these prominent striations as you can see
and this kind of close-up illustration right here that gives that very prominent striped appearance
of the muscle cell so when we talk about striated muscle myofibrils are playing a role here
now these myofibrils can be broken into even smaller structures called myofilaments
and these myofilaments are proteins and there's three main types associated with a muscle
muscle cell you're going to have contractile which we'll spend a little bit more time on
regulatory which will kind of turn this on and off
and structural and we're not going to spend as much time on the structural components of the
myofilaments but they are very important in terms of allowing for the elasticity and
the extensibility that's very vital as well but as we don't have you know a whole course
to talk about these things we're going to focus in a little bit more on these other structures
all right there are two main types of contractile protein there is actin and there is myosin
and what these proteins do is convert ATP's chemical energy so it's converting chemical energy
into the actual mechanical energy of motion myosin which you can see a myosin molecule right here
is going to be the main component of thick filament I should say
thick filament here instead of protein actin is going to be the main component of thin film or
thin the thin filament or the thin a bunch of proteins making up the filament so all these
little things all these little balls right here basically are supposed to be representing actin
and this there you're going to have two thin filaments for every thick filament and these thin
filaments will overlap with the thick filament during various phases of muscle contraction also
as you can see a little bit better on this this image here each actin protein is going to have
a myosin binding site so that's what these are representing right here and that will
allow the myosin head to actually attach to the actin during the the contraction process
another component of the thin filament so obviously actin is going to be the major component
but you're also going to have regulatory proteins that are playing a role as well
so in a relaxed muscle or a muscle that's not contracting
myosin is blocked from the myosin binding sites on actin as you can see here by tropomyosin
okay so tropomyosin is going to be this fairly large protein you can see it
kind of sitting right here and blocking the myosin from being able to bind to the actin
now also very closely associated to the tropomyosin are these troponin molecules
it's going to keep that tropomyosin in place but when you have a calcium ion that kind of moves in
from that sarcoplasmic reticulum and binds
to the troponin it will cause there to be a change in shape and the movement of that tropomyosin away
from those those myosin binding sites to allow for the process of contraction to occur
when there's not calcium in this region that kind of moves out of the way that tropomyosin
will move back into place and block these sites so that's why these are called regulatory proteins
so the organization of these structures are into these basic functional units that are called
sarcomeres on each end of the sarcomere you will have these z discs so you can see one here
you can see one here and basically that space in between that is one sarcomere
and how much overlap there is between the thin and the thick
filament depends on whether the muscle is relaxed stretched or contracted
don't worry about these H zones A bands I bands they're super cool if you ever take a
physiology class you're going to spend a lot more time in these regions here but we're not
doing that deep dive but you can see in this illustration you see that representation of
the thin filament and the thick filament so there's two thin filaments per thick filament
for our purposes in this course i will be happy if you understand that skeletal muscle can
shorten during contraction but that the thick and the thin filaments don't necessarily shorten they
just slide past one another so the myofilaments don't shorten or contract they slide but you can
see in this image that the thin filament is going to slide over that thick filament and it causes
the sarcomere to shorten so here's a relaxed muscle here's a contracting muscle you can see
here's a thin filament here's a thick filament right here and there's not that level of overlap
right here but during the contraction that thin filament will slide over and what that does is it
brings the z disc with it causing causing the sarcomere to shorten and generally sarcomeres
will shorten at the same time to cause for overall shortening of the muscle of the whole muscle
okay so obviously that is a very simplistic version of the physiology of a skeletal muscle
but some of these concepts will come up again particularly in some of these these proteins but
basics i want you to understand are that obviously we see that skeletal muscle can shorten with
concentric contraction but it's not necessarily these structures that are it's they're kind of
sliding over one another but the connective tissues and such can play a role as well
all right excellent my favorite time is question time right all right so the question is
which structure of a myofiber stores calcium in a relaxed muscle all right so pause the video if you
need time and when you're ready let's look at this question stem again so myofiber that's that muscle
cell what stores so stores is a key word calcium and another key here is in a relaxed muscle
all right well we didn't talk about dystrophin that's actually one of those
structural proteins that we didn't get into so it's not dystrophin generally if you don't know
if you've never heard of a term then generally it's not one you need to to focus on in terms
of the question the sarcoplasm certainly can have calcium within it but generally it it is
going to be in the sarcoplasm and affecting the myofilaments and attaching to particular proteins
in a muscle that is beginning to contract sarcolemma is that plasma membrane around
there's calcium going on there but it's not so much stored there and then actin is going to be
a major component of the thin filament but not so much that it is going to be the sarcoplasmic
reticulum that is going to be hugely important in terms of storage of calcium in a relaxed muscle
if it gets the message or the action potential allows for the release
of calcium into the sarcoplasm that's generally when you begin the process of muscle contraction
all right excellent thank you so much for this big picture
view of the myofiber and some of its smaller contents within this is really interesting
area now we're going to move into what are the different types of muscle contraction
oftentimes we think of concentric contraction and shortening but there's others as well that are
as important this has been a lot of fun and i look forward to seeing you in the next video thanks