Janet: Hey guys, I’m Janet from the school of HRT, thanks so much for the nomination
for the lightboard challenge. And, I thought to kick it off, I would pull in a friend of
mine, Ajay, also from HRT.
So, Ajay, where are you?
Ajay: I’m right here Janet!
Janet: Awesome! So, Ajay, I’m wondering, is there anything that keeps you up at night?
Ajay: Well, yeah actually, I was in class the other day, and I put a question out to
the class and a student raised their hand, and that was surprising to me for two reasons:
one, they raised their hand – no one ever participates, and two, with a background in
kinesiology, the science of human movement, I was trying to figure out how does that actually
happen, how do they raise their arm?
Janet: Are you talking about a muscle contraction?
Ajay: Yeah, I think I am! What’s going on there?
Janet: Do you want to learn about it?
Ajay: Sure!
Janet: Okay! Alright, so today we’re going to kind of teach about how a muscle contracts,
it’s called excitation contraction coupling. So in order for us to do this, or raise our
hands, grab a cup of coffee, whatever it is – jump – we need to initiate the movement.
So, the movement might start up in our brain, we might decide that we want to move, so that
comes from this familiar guy up here. This is called our cell body of our neuron, and
it can come from the brain, or it can come from the spinal cord, so we all know that
after the cell body, we have an area of the neuron called the axon hillock.
Ajay: Who comes up with these terms?
Janet: I don’t know. Hillock, yeah I don’t know what that means, but, anyway.
Ajay: They must be nerds.
Janet: The axon hillock, and, that is where the action potential is generated, so we have
the idea to move, and then the action potential comes from axon hillock. Yeah, so then the
action potential travels down the axon of the neuron, and where does it go, you might be wondering?
Ajay: I have no idea, are you going to show me?
Janet: I will. So then, the neuron actually branches off into terminal ends, with these
cool little synaptic knobs on the end of them.
Ajay: It looks like feet.
Janet: It does. Little feet. Little “neuronal” feet.
Ajay: Okay.
Janet: So, this is essentially our nerve, so we’ve got our axon hillock, action potential
coming down, and it, branches down to our synaptic terminal, and we’ve got the synaptic
knobs down here.
So we know that synaptic knobs can attach to many things, they can attach to a gland
cell, they can attach to a muscle cell, they can attach to another knob. So in this case
let’s talk about a muscle cell, cause that’s what we’re really wondering, right? How
does this cause a muscle contraction, if this is a muscle belly? Right?
Ajay: Right!
Janet: So, let’s zoom in. So we’re going to take this little guy here, and blow it
up, and talk about what’s going on, at the synaptic knob.
Ajay: Okay.
Janet: Okay. Alright, so, we’ve got our synaptic knob, and we know that it joins up
with the muscle fibre. This whole set here is called the neural muscular junction.
Ajay: Hold on, Janet, so just to be clear, this is this.
Janet: Yes.
Ajay: So this is like a big foot.
Janet: This is like, yes, a bigfoot. Blown-up, expanded, it just blows your mind.
Ajay: Okay, so we’re zooming in.
Janet: We’re zooming in.
Ajay: This is like a real microscopic… I got you!
Janet: So at the synaptic knob, we’ve got the action potential coming down the neuron,
and, what happens as a result of the action potential coming down the neuron, we have
calcium just hanging outside the synaptic knob.
Ajay: Like, calcium from milk calcium?
Janet: Yeah, calcium from like milk calcium!
So calcium will come through voltage gated channels, and what that calcium does, is it
allows these vesicles that are hanging out in the synaptic knob, that are full of little
blobs of acetylcholine which is our excitatory neurotransmitter. Calcium will cause these
guys to be released to the muscle.
Ajay: Ah!
Janet: Cool, eh?
Ajay: How does that happen, what happens next? Tell me more!
Janet: Okay, I will.
So we have, calcium coming in, these synaptic vesicles releasing acetylcholine – and,
what colour am I on here, yellow – so, acetylcholine will come out through exocytosis, into the
synaptic cleft, and, a really cool thing happens, it actually binds to a receptor on the cell
surface.
Ajay: I’m beginning to realize why students don’t raise their hands more often,
but go on.
Janet: Is this confusing?
Ajay: No, it’s complicated, but…
Janet: It is.
Ajay: It’s making a lot more sense as you draw it.
Janet: Good.
So, acetylcholine will bind to the acetylcholine receptor, and what that does is cause the
receptor to open up, and, remember our friend sodium? We’ve learned all about sodium,
sodium will come into the cell, once acetylcholine binds the receptor. So, I’ll just put my
little acetylcholine’s on here.
so, that’s how the action potential travels down the nerve, to the muscle fibre, and then,
at the motor end plate, we’ve got, sodium coming into the cell, and then, a contraction might start.
Ajay: So this is the part I was confused about, do you know how contractions actually happens though?
Janet: No, but maybe you can tell me.
Ajay: For some reason, that’s the only part that I understand, so, sure, I can show you
what happens next. We’ll just pick it up from when the signal has gone from the neuron,
now it’s inside the muscle, right?
So let’s just pretend this pink represents the signals inside now, right?
Janet: Okay.
Ajay: And you have this weird structure… may I borrow the blue?
Janet: Yeah
Ajay: Cause blue’s cool
Janet: Blue is cool
Ajay: It kind of looks like a spiderweb-ish looking kinda thing.
Janet: What is this spiderweb-ish looking thing?
Ajay: They call it the sarcoplasmic reticulum. I’ll just put “S.R.”, Janet, I don’t
know who comes up with these words, but it is what it is.
Inside here, is also lots and lots of this calcium.
Janet: There’s that calcium again. So weird.
Ajay: So what happens is, once the signal is inside, this signal is actually telling
us S.R, hey calcium it’s time to get released.
Janet: Okay
Ajay: So now you have this calcium, just all over this muscle cell, so let’s just zoom
in, to just, if we were to pretend to zoom-into this area right here and just try to blow it up.
Janet: Let’s blow it up over here.
Ajay: Sure. Let’s do that. Here’s what it looks like if you were to
zoom into one structural contractile unit of a muscle cell. There’s two parts.
Janet: What’s that called?
Ajay: It’s called a sarcomere. Yeah, we’ll get to that later, but let me just draw you
what it would kind of look like under a very strong microscope.
Janet: Okay.
Ajay: There’s a thin part, we’ll call this the thin filament.
Janet: That has another name, doesn’t it?
Ajay: Yeah, they also call it actin, because scientists just like to have lots of terms
for the same thing, that way, it causes more confusion.
Janet: Oh yeah, students love that.
Ajay: So the thin has actin, it also has another part that kind of looks like a rope. They
call this one, tropomysin.
Janet: Again, a weird name.
Ajay: I know, and there’s actually one last part on this. I’ll put a different colour,
it’s on top here. This is called troponin. It’s just another term, they all sound the
same, but they are all different structures.
So that’s the thin part, then there’s a thick part. If there’s a thing, there’re
got to be a thick. So there’s a thicker part, and this on has heads on it, it sounds
weird, but these are actually called myosin heads.
Janet: So where does the calcium come into play?
Ajay: The calcium, oh, I’m going to tell you.
Janet: I want to get back to the calcium, I like the calcium
Ajay: Oh, it’s coming back. So if we’re over here, the signal came down, told the
sarcoplasmic reticulum to release calcium, then we took a trip across the board, or the
cell, and then what happens is – what colour is calcium, it’s blue, let’s keep blue
– calcium is looking for this spot here, one of these, troponins, so what happens is,
when this calcium is stuck here, it actually, it’s moving this tropomyosin part out of
the way, and what that’s allowing is for this thicker filament part, the myosin to
attach.
Once this thick filament, the myosin head, comes into contact with actin, and they are
in contact, that is muscle forced production.
Janet: It’s a full contact sport.
Ajay: That’s called a cross bridge, it’s called a cross bridge.
Janet: Cross bridge, okay.
Ajay: When myosin is in contact with actin it’s called it form’s a bridge, they’re
in contact. The more of these crossbridges you have, the stronger the contraction.
Janet: I’ve always wondered that. That’s what keeps me up at night.
Ajay: No way!
Janet: Yeah way!
Ajay: But you see, we teach an exercise, except I’m not really sure how this can all relate
to working out. Maybe we can call someone else.
Janet: Yeah, how do you want to call in? I don’t know any fitness people.
Ajay: I know a good guy.
Janet: Who?
Ajay: Brad!