hello anatomy friends this is dr alsup and this is my favorite topic when talking skeletal
muscle cells and this is the composition of the structures within the cell and it's really through
these structures that you can actually begin to understand how muscles are able to contract so you
have to have all of these small scale structures working in order to get the larger scale
contraction that you can actually see and that will cause the movements that we we can observe
now as mentioned in another video muscle cells go by some other names now i personally like
the name muscle fiber which is indicative of the relatively long size of a muscle cell
you may also hear the term myofiber so all three of these terms muscle cells muscle fibers
and myofibers mean the same thing and as you can see in this illustration there is a lot going on
within a muscle cell there's a lot of structures within so let's talk about
some of these structures in a bit more detail most of the structures within a muscle cell
have slight variations to their name to indicate they are part of a muscle cell
so if you see this sarc this means flesh and you will see this prefix affixed to many
of these muscle cellular structures the second part of this name lemma means sheath with the
sarcolemma basically forming a sheath around the muscle cell so it will go around the entirety
of the muscle cell this one's kind of showing you what's going on deep here but just think of
in essence the sarcolemma is the plasma membrane of the muscle cell
all right so in certain areas of the sarcolemma it will invaginate or kind of tunnel its
way deep in from the surface toward the center of the muscle and you can see that right here as well
these invaginations are called transverse or t tubules and
this allows these structures will allow the muscle action potential to propagate
or spread widely and quickly throughout the muscle fiber so not only say if you have a
action potential running along the sarcolemma not only will it get the superficial portion
but it will head down towards through these t tubules to the deeper portions of the cell as well
note that these t tubules are very closely related to the sarcoplasmic reticulum
so a bit about the sarcoplasmic reticulum or the sr these are going to be fluid filled
membranous sacs that are going to encircle myofibrils
myofibrils are different from the myofiber myofiber recall is the whole cell
a myofibril is going to be the smaller contract contractile structures within the sarcoplasm
the sarcoplasmic reticulum is similar to the smooth endoplasmic reticulum and non-muscle cells
and this is kind of a key here the key to the function of the sr or part of the function is
that it stores calcium in a relaxed muscle alright so note that i said relaxed here and then i'm
underlining it so it's in a relaxed muscle that the calcium will stay now at the ends of the
sarcoplasmic reticulum there'll be these dilations which you can see here and here here and here
on these images and these dilations of the sr are called terminal cisterns
or cisternae if you're talking plural and these will butt up against the t tubules so they'll
be right next to the t tubules of the sarcolemma and when you combine the two cisterns so here is
a cistern here is a sister and here's a t tubule when you have those three together that's referred
to as a triad and when an action potential travels down that t tubule from the sarcolemma
it will trigger the release of calcium it will travel trigger the release of calcium from those
terminal cisterns of the sarcoplasmic reticulum into the sarcoplasm and it's
that calcium release that is vital in allowing muscle contraction to occur
now the sarcoplasm like i said
there will be release of calcium into the sarcoplasm the sarcoplasm is basically just
the cytoplasm of a skeletal muscle cell and it's surrounded by the sarcolemma
there is a lot going on in the sarcoplasm some of which we've already discussed like the sr
the bulk of the sarcoplasm it there's going to be quite a bit of these small thread like that's
what this is and this is and you're seeing this one's kind of pulled out right here these thread
like structures called the myofibril all right so the myofibril and this is a close-up of myofibril
microscopically this is what it looks this is what the sarcoplasm looks stuffed with
and we'll come back to the myofibrils in the next slide and talk a little bit more
about its composition but that's what looks like is making up the bulk of the muscle cell
there are obviously going to be mitochondria like most other cells associated with the muscle cell
and there is something also called myoglobin which these are red colored proteins only found
in muscle and you have that myo in there that's kind of leading you to that these proteins bind
oxygen molecules that diffuse into the muscle fibers from the interstitial fluid and this
oxygen so it's binding to the myoglobin and it can be released from the myoglobin when needed by the
mitochondria for atp production the sarcoplasm also has a substantial amount of glycogen which
is a molecule that stores linked glucose molecules that can be broken down when needed to produce atp
and atp keeps coming up here over and over again right so atp is something that's very important in
terms of muscle contraction this is atp stands for adenosine triphosphate don't worry so much about
the full name but this is an organic compound that plays a role in an energy transfer and it's vital
to allowing muscle attraction contraction to occur as well as you need atp to actually stop muscle
contraction so it's needed for both processes also sustain contractions you'll need more atp
so muscle cells need to be able to create or synthesize atp so a lot of the structures within
can can be broken down or allow for oxygen to be released and for the atp production so it makes
sense you have to have lots of structures that play a role in terms of creating atp
okay returning to myofibrils these are going to be the contractile elements of the muscle cell
and one myofibril is going to extend the entire length of the muscle fiber or the muscle cell
and these have very prominent striations and it is these that really give that very prominent striped
appearance excuse me of a muscle cell so that's what it's kind of giving you this in here and
we'll talk about what's actually composing these myofibrils in a moment specifically myofilaments
all right myofilaments are going to make up the myofibrils
for the most part and these are proteins and there are three major types associated with a
muscle cell there are contractile proteins which will generate the force during contraction there
are regulatory proteins which help kind of switch on and off the contraction process
and lastly there are structural proteins that keep structures in place and in proper alignment
as well as linking the myofibrils to the sarcolemma and also play a
very important role in terms of the overall extensibility and elasticity of the muscles
when you put these all together when you put these these proteins all together
you will have a myofibril so the smaller myofilaments do not extend the entire length of
a muscle cell but the myofibril does so myofibrils larger the myofilaments make up the myofibrils so
let's talk a little bit about the different types of proteins starting with the contractile proteins
there are two major types of contractile proteins they'll be
actin and myosin and what these proteins do will really be to convert atp's chemical
energy into the actual mechanical energy of motion so these are what's going to um
to lead to the the sliding that occurs with these filaments myosin is the main component of thick
the thick filament or excuse yeah the thick filament that's really what i meant to put here
so um each of the these myosin muscles as you can see here kind of shaped like two golf clubs
that are all twisted together towards the back and then you'll have the the prominent myosin heads
the i'm going to put this here that i meant to put filament they are proteins and that's
very true but when you kind of combine a bunch of myosin together and some other structures
you have thick filament and same thing here this should say filament so actin is the main
protein in thin filament and there are two thin filaments for every thick filament and these thin
filaments will overlap with the thick filament during the various phases of muscle contraction
also on each of the actin proteins there will be what's referred to as myosin binding sites where
a myosin head can attach during the contraction process and it'll just kind of move down the line
during the contraction process also part of the thin filament are the regulatory proteins which
as i mentioned previously will play a role in terms of switching the contraction process on and
off so in a relaxed muscle or a muscle that's not contracting myosin is blocked from the uh from the
myosin binding sites on actin because tropomyosin is kind of sitting and blocking those sides so
you can see the myosin binding sites right here and you see that the tropomyosin is just sitting
right on top of those not allowing myosin to be able to attach the actin also note that troponin
which you can see kind of illustrated right here is attached to the tropomyosin keeping tropomyosin
in its place but when a calcium ion actually binds to troponin say after the release from a
terminal cistern of the sarcoplasmic reticulum due to an action potential that troponin will actually
kind of move uh will actually actually undergo a change in shape so this part will undergo a change
in shape which will allow for the tropomyosin to move some and move away from those myosin binding
sites on the actin allowing for myosin to bind and this is where muscle contractions can really begin
so the organization of these structures are into basic functional units called sarcomeres
and there's that sarc again on each end of a sarcomere you will have what is called a z disk
i'm just going to outline these z disks in red here and z discs are plate like uh plate like
regions of protein to which the thin filament is attached so you can say a sarcomere extends
from one z disc to the next so this as what as you can see here is considered a sarcomere
and how much overlap there is between the thin filament and the thick filament so this
would be the thin right here and this larger portion would be the thick and how much overlap
there is between the thin and the thick will really depend on whether the muscle is relaxed
or stretched or contracted now don't worry about these things right here or this m line right here
h zones and a bands and i bands they're super cool but we're getting super into
physiology and histology there so we aren't going to dive quite that deep for this course
for our purposes i will be happy if you understand that skeletal muscle can
shorten during contraction because that thick and the thin filament slide past one another so
you can see here in a more relaxed muscle that the thin filament right here is not completely
overlapping with the thick right here but with muscle contraction you can see that the thin
filament will overlap completely with the thick filament and that will cause
that will cause for the muscles to contract so that it's not the myofilaments that are
shortening you can see that this is the same size as this it just has everything to do with how much
overlap there is so myofilaments slide um and that will cause the sarcomere to shorten
and the linkage of sarcomeres will shorten in a row leading to shortening of the whole muscle
before we move away from this topic i do want to mention a few of the structural proteins
but certainly not all of them there are a lot of structural proteins and they play a multitude
of roles some mentioned right here and we will start with the big one titan or titan you'll hear
either way this is going to be the third most plentiful protein in skeletal muscle just behind
actin and myosin and its name titan or titan based on its super large size it's relatively huge size
particularly in comparison to the other proteins in this area these are going to be particularly
important in terms of stabilizing the thick filament so you can see it having an attachment
with the thick filament and the z disc right here this protein can also stretch and bounce back
really well undamaged so it will account for much of the elasticity and extensibility of muscles
next we have nebulin which helps in thin filament stabilization so titan was more
important in terms of thick filament nebula is more important in terms of thin filament
and actually wrap around the entirety of the thin filament and anchor to the z disc
and lastly for our discussion are the dystrophin proteins which will link the
thin filament to proteins and the sarcolemma so many suggest that it really helps to transmit
tension generated by sarcomeres to tendons and also plays a very important role in terms
of reinforcing the sarcolemma in individuals with duchene muscular dystrophy the gene that
codes for dystrophin is mutated which will lead to little to no dystrophin
being produced and without that reinforcement that comes from the presence of dystrophin
the sarcolemma is more likely to tear during muscle contractions and that can lead to slow
and eventual muscle rupture and death alright that was a lot of information in this particular video
but a lot of really important information that leads to a better understanding of how muscles
contract so let's go over a concept together before we move to the next video so the question
is which structure of a myofiber stores calcium in a relaxed muscle oops sorry in a relaxed muscle
all right is it actin dystrophin sarcolemma sarcoplasm or sarcoplasmic reticulum
so pause the video i really want you to get this one because this is an important concept
and when you're ready to go let's talk a little bit about this stem so this is a structure of a
myofiber well all of these structures are going to be within a myofiber so not all that helpful
storing calcium okay that's important really only two of these may have calcium associated with it
to a bigger extent and that would be this one the sarcoplasm and the sarcoplasmic reticulum but this
term stores is um kind of key here because really only the sarcoplasmic reticulum is going to store
calcium and really it kind of just totally clues you in and hopefully leads you to the right answer
is that it is going to be in a relaxed muscle so we know that e sarcoplasmic reticulum is correct
calcium can certainly move into the sarcoplasm particularly once released from the sarcoplasmic
reticulum that can lead to contraction but it's really that sr that's going to store the calcium
in a relaxed muscle all right so this was uh quite a bit and i hope you have time to kind
of review this and find those areas that you still have questions on or you just need some
more clarification and you can always feel free to reach out to me with any of those questions
thank you for your time and attention here and have a great rest of your day