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MCAT
Course: MCAT > Unit 7
Lesson 15: Muscular system- Muscular system questions
- Myosin and actin
- How tropomyosin and troponin regulate muscle contraction
- Role of the sarcoplasmic reticulum in muscle cells
- Anatomy of a skeletal muscle cell
- Three types of muscle
- Motor neurons
- Neuromuscular junction, motor end-plate
- Type 1 and type 2 muscle fibers
- Calcium puts myosin to work
- Muscle innervation
- Autonomic vs somatic nervous system
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Type 1 and type 2 muscle fibers
Type 1 and type 2 muscle fibers differ in a few key ways. Type 1 (“slow twitch”) fibers contain more mitochondria, which means they can produce more energy and are better for long, aerobic activities. Type 2 (“fast twitch”) fibers, on the other hand, are suited for short, fast bursts of activity that don't require as much oxygen. Created by Raja Narayan.
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At7:35. Feher's "Quantitative physiology", 2012 (Elsevier) shows on pg 245 that fast-twitch muscle generates more power (Fig 3.4.10). This statement appears to be in opposition to yours. Would you comment?(38 votes)
you are correct, power is force produced over time, a fast-twitch muscle fiber makes more use of anaerobic glycolysis and the creatine phosphate system which can produce a lot more energy over a shorter amount of time (less chemical steps needed) Somebody like Usain Bolt will therefore will have a distribution of muscle fibers with more fast-twitch muscle fibers and will be able to run veerrrrrrrry fast! The storage of glycogen and creation phosphate (used in anaerobic phosphorisation) in the body is very limited, thats why after around 10 sec Usain would have to slow down(27 votes)
Can you expand the video to discuss Type 2a and Type 2b/x ?(16 votes)- Think about muscle fibers as a line graph. On the quick recovery/ more mitochondria/ aerobic metabolism / less strength & power / first recruited / side of the scale you have Type I muscle fibers. The Type I side is all the way to the left let's say. All the way to the right you have Type II B (sometimes X, different physiologists have called it different things). On this absolute right side of our scale Type II fibers are characterized by the ability to produce more strength / take longer to recover/ fatigue quicker/ anaerobic metabolism / producing more force quicker / recruited last (however we are talking milliseconds). These are our two absolutes. Along this graph we have a mix of all sorts of intermediate fibers. Type IIa / Type IIab / Type IIc are some of the ones I have seen in textbooks and research. These different fiber types have mixed properties of both depending where they fall at on the scale. Every human is genetically disposed to a random distribution of fiber types. Some are better are longer endurance sports and some are better at strength sports because of this. The amount you can impact your muscle fibers with stimulus is still a debated topic, but there is a consensus that you can partially convert some of your intermediary fibers down the scale in either direction with training in either endurance or strength.(7 votes)
- I thought that the Type 1 Oxidative Fibres were red because of an increased myoglobin content, not just because they use more oxygen or have more oxygen supply... ?(5 votes)
You are partially correct, Raja is simplifying dramatically here to make memorization easier.
Oxygen has no color and it is not directly responsible for the color of anything in the body. The color comes from the extensive delocalization of charge in the protoporphyrin heme rings that are interacting with an iron(II) core (the rings themselves are pigmented even without the iron). The color of these rings changes when oxygen is bound to the iron (even though the oxidation state of the iron does not), because oxygen introduces more electrons into the delocalized system.
The color differences in muscle, and of oxygenated vs deoxygenated blood, are therefore dependant both on the quantity of heme (i.e. myoglobin or hemoglobin concentration) and on the degree of oxygenation. However, the depth/intensity of the color is correlated only with the heme concentration, not with oxygen concentration. Oxygen concentration only affects color.
As Raja says, the color of the different types of muscle cells depends in part on the number of mitochondria, however he is not really correct that the reason of this is oxygen. The reason is that mitochondria have many cytochromes, which are very similar to heme rings, and that these are pigmented. As you say, the number of myoglobin also affects the color, for the same reason: they have those pigmented heme rings. However, these two factors are intimately related: higher mitochondrion concentration and higher myoglobin concentration are always (to the best of my knowledge) positively correlated, because the myoglobin provide an additional O2 source for the electron transport chain.
So in reality the greater pigmentation of Type I fibers is due to the higher concentrations of myoglobin and of mitochondria, and the color of that pigmentation depends in part upon oxygen.
For MCAT memorization, none of these things are important though, which is why Raja is simplifying so much.(9 votes)
- What type of muscle is Diaphragm Muscle?(6 votes)
- Typically muscles have a mixture of both fibers. Muscle that's are involved in multiple, slow contractions are going to have higher distribution of type I. The diaphragm, which is constantly working in order to help us take in air/O2 which would more than likely mean there is a higher distribution of type I fibers so they didn't fatigue.(4 votes)
quick question: I thought primarily Glycogen was stored in skeletal muscle not tryglycerides(7 votes)
At7:15, where Raja says that Type 1 muscle fiber is "strong" and Type 2 is "weak", there is a clarification at the bottom right-hand corner stating that "type 2 fibers generate more instantaneous force than type 1 fibers." So, which muscle fiber should be considered "weak" and which should be considered "strong"?(4 votes)- So what the note in the corner is trying to explain is that with Type 1 fibres there is less instantaneous force, but the power of the force lasts longer due to the greater supply of energy. But with Type 2 fibres, there is more instantaneous force, so the force produced is a lot stronger but that strong power doesn't last as long due to lack of energy supply in the muscle fiber.(4 votes)
- How do the types of muscle fibers (1 and 2) relate to the type of muscle (smooth, striated, cardiac)? E.g., would smooth muscle tend to have a greater percentage of type I muscle fiber as compared to striated muscle since type I is more long-lasting?(2 votes)
Smooth, skeletal, and cardiac refer to muscle tissue. Skeletal muscle tissue can be comprised of 3 different muscle fibers (sometimes called cells) which are type 1, type IIa and type IIb. Skeletal muscle is diverse in the body in that it can be used to generate rapid movements, used to maintain tension with minimal fatigue, and many other voluntary processes we recruit it to do. Because of the diversity of skeletal muscle actions, as opposed to involuntary smooth muscle and cardiac muscle only found in the heart, skeletal muscle tissue's have various structural and functional characteristics which are then classified into three types: I, IIB and IIA fibers. Cardiac muscle is involuntary and striated and comprised of cardiomyocytes only found in the heart. Smooth muscle is involuntary and non-striated. It can be divided into single-unit cells and multiunit smooth muscle tissue.
Thus, the classification of fibers in skeletal muscle is representative of the different functions and processes that typically occur in the muscle and is not related to cardiac or smooth muscle tissue.(3 votes)
My undergrad textbook mentions three, not two types of skeletal muscle fibers: slow oxidative fibers, fast oxidative fibers (kind of intermediate), and fast glycolytic fibers. Is it 2 types, 3 types, or is it arbitrary boundaries in a gradient between two extremes?(1 vote)- I think this is just a case of being a more simplified. As for the MCAT (which this video was targeted for), some topics are more superficial. The 2 vs 3 is definitely not arbitrary, however the 3 type version is just more complex partitioning. In addition, dogma is hard to kick and in biology there always seems to be an "exception to the rule"; hence why different classifications arise as new research is performed and new knowledge obtained.(5 votes)
- At4:01, what makes the type 2 fibers have a faster conduction velocity?(2 votes)
there's a lot of errors in this video as mentioned below.. Also I think it would be helpful to talk about type IIa and IIb not lump them together(2 votes)
7:35
Video transcript
What's the difference
between type 1 muscle fibers and the equally descript
type 2 muscle fibers? And I hate it when
they give things names like type 1 and type 2. That's not really descriptive. That doesn't tell me
a lot of information about these types
of muscle fibers. So what I do is I
get back at them. I come up with one golden rule. And this one golden rule will
help me go through a table, like we're about to do
right here, to differentiate between type 1 and
type 2 muscle fibers. So the golden rule I'm going
to have for this table here is that mitochondria--
whoops, look how I wrote that "i" there. Mitochondria are present in
greater quantities in your type 1 muscle fibers, type 1. So mitochondria are more
prevalent in type 1 muscle fibers than in type 2. And just based on
that knowledge alone, we should be able to go through
and fill out this table. All right, so let's
start from the top. I may have alluded to it here
through the way I wrote this out, but the color type 1 muscle
fibers are often noted as? Red. And why do you think that is? Well, what are
mitochondria used for? Mitochondria are used
in biochemical processes that help us make energy. And the main process
they function in that I'm going to
reference a couple of times is called oxidative
phosphorylation. Now, what is that
term mean to you? What does that suggest? What are the two things
that are probably involved in oxidative
phosphorylation? So just as the name suggests,
oxidative means oxygen is going to be involved here. And then phosphorylation
means that something is going to receive
a phosphate group. Phosphoryl- -ation, something
will receive a phosphate group. So the oxidative
part applies here to red, the same way that
the color red shows up in our arteries. Think about it. Why are arteries red,
and why are veins blue? Well, arteries have more
oxygen than veins do. And because of that, the color
of our type 1 muscle fibers will be red because they
produce more energy from oxygen than type 2 muscle fibers do. More oxygen is present in
type 1, so they're red. And so we could say then
that our type 2 muscle fibers will be white. All right, what about
the speed of contraction that we see here? How fast do type 1
muscle fibers contract? Well, let's think about the
process of making energy with mitochondria. If type 1 muscle fibers rely
on mitochondria for energy, think about all
the processes that have to go into
making energy through oxidative phosphorylation. You can't just do
this right away. You have to have
glycolysis happen. You have to have the
Krebs cycle occur. You need to make NADH and
FADH2, all of these cofactors that have to go into this
electron transport chain. There's a lot of things
that have to be done. So mitochondria take a
while to make energy. So that means that the
contraction speed here is also going to be slow
because it takes a while to make that energy. And so on the flip
side, that means that the contraction speed of
type 2, or white muscle fibers, is going to be fast. What about the
conduction velocity? How quickly are we going
to be able to receive a neuronal impulse or a nerve
signal to type 1 muscle fibers to contract? This is also going to be slow. And the term here is
called "slow twitch." And I remember that the same way
I remember that the contraction speed is slow in
type 1 muscle fibers. On the flip side, we call
type 2 muscle fibers fast twitch muscle fibers. All right, now, what
types of activities do you think type
1 muscle fibers are going to be involved in? Well, oxidative phosphorylation
has another name. The mitochondria is used for a
specific type of respiration. That's a cue term right there. That's something that should
cue another word in your mind. If mitochondria is involved for
a type of cellular respiration, that type of
[INAUDIBLE]-- aerobic. Aerobic respiration requires
mitochondria to be present. And so because there's more
mitochondria in type 1 muscle fibers, type 1
muscle fibers will undergo aerobic respiration. What about type 2? Well, because they don't
have as much mitochondria, they're going to have to
undergo anaerobic respiration, or respiration in the
absence of oxygen. That's what a aerobic
means-- "by using oxygen." Anaerobic means
"without oxygen." OK, how long will these
muscle fibers be contracting? Well, let's think about
how much energy we're making if we have
mitochondria versus if we don't have mitochondria. We make a lot more energy
if we have mitochondria. And so the duration of
contraction then will be long. You're going to have longer
contractions when you're able to make a lot
more energy or ATP, because you've got mitochondria
present in greater quantities. If you lack mitochondria or you
undergo anaerobic respiration because you can't undergo
oxidative phosphorylation as much, you're going to have a lot
of short-duration contractions. What's the difference
between these two? Well, if I'm having a
long-duration contraction, that's something like
the muscles in my back that I use for standing
or in my legs so that way I can walk at a certain pace. Short-duration muscle fibers,
those that contract really quickly and that's
it, are things like the ones in our arms
when we shake somebody's hand or if we flick something
with our finger. If we do that for a
long period of time, we're going to get really tired. So these muscle
fibers are not meant to be working for
the entire day. If we're in line
at Disneyland, we want to be using our
long-duration muscle fibers, the ones in our back,
in our glutes, in our legs. So that way, we can stand
for a long period of time. We don't want to be standing
on our hands or our fingers because we won't be able to last
by using type 2 muscle fibers. All right. So which of these
types of muscle fibers are most likely to fatigue? Well, if type 1
muscle fibers are able to contract for
a long period of time, we say then that they
are fatigue resistant. They are fatigue resistant
because we have enough energy for type 1 muscle
fibers to contract for a long period of time. But type 2 muscle fibers
do not have as much energy, because they don't have
as much mitochondria. And so as such,
they easily fatigue. So I'll write easily right here. They are not resistant. What about the
power of contraction that's associated with
type 1 muscle fibers? Well, I talked about some
pretty heavy muscles just now, the ones in your
glutes, your back. And if we're having long
durations of contraction, these are going to be some
pretty big, powerful muscle groups. And so I'll write strong here. Because again, noting back
to the mitochondria analogy, if we have a lot more
mitochondria present in type 1 muscle fibers, they're going
to have a lot more energy to contract and more
muscles contracting. More muscle cells
contracting at the same time elicits a stronger, more
powerful contraction overall. On the flip side,
type 2 muscle fibers-- not enough mitochondria,
not enough energy, not enough power-- so these are
going to be weak contractions. And lastly, how do we store our
energy in type 1 versus type 2? This might be a
little bit of a trick, but you can figure this out. With mitochondria, we
make a lot of energy. And if we have a
lot of energy, are we going to leave that
sitting around as ATP? No, ATP is a really labile,
or a really reactive molecule. If we have it there,
we're anticipating it to be used right away. And so what type
1 muscle fibers do is they store their
energy in triglycerides, these fatty substances. And so that's why we
put energy into fat. So that way, we can use it
later on for the stronger, longer-duration contractions. If we want it to contract right
away we'll just use raw ATP. And so the ATP that's
just sitting around is used in type 2 muscle fibers. Also, we'll use something
called creatine phosphate, which is very similar to
ATP in the sense that it has a phosphate
group that it'll be able to donate
to produce energy. And so this phosphate group
is very reactive, very labile. If it's sitting
around, that means it's going to be
used any second now for a quick,
short-duration contraction. So by using this
one golden rule here that mitochondria are more
prevalent in type 1 muscle fibers, we can figure out
this whole table here. And that's the
difference between type 1 and type 2 muscle fibers.