particular-- when we say myosin II it actually has two

of these myosin heads and their tails are inter-wound

with each other-- how myosin II can use ATP to

essentially-- you can almost imagine either pulling an

actin filament or walking up an actin filament.

It starts attached.

ATP comes and bonds onto it.

That causes it to be released.

Then the ATP hydrolyzes into ADP and a phosphate group.

And when that happens, that energy's released.

It puts this into a higher energy state.

It kind of spring-loads the protein and then it attaches

up another notch on the actual actin filament and then the

phosphate group leaves and that's where the confirmation

change in this protein is enough.

It generates the power stroke to actually push on the actin

filament-- and you could imagine, either move the

myosin-- whatever the myosin is connected to-- to the left

or whatever the actin is connected to to the right.

We're going to talk a lot more about what they're connected

to in future videos.

Now, a couple of questions might have been

raising in your head.

This guy had so much effort to pull on this thing, right?

There's some tension pulling in the other direction, right?

I said this is what happens in muscles, so there must be some

weight or some other resistance.

So what happens when this releases?

At the first step when ATP joined and this released,

wouldn't the actin filament just go back to

where it was before?

Especially if there's some tension on it

going in that direction.

And the simple answer to that is, this isn't the only myosin

protein that's acting on this actin.

You have others all along the chain.

Maybe you have one right there.

Maybe you have one right there.

They're all working at their own pace at different times.

So you have so many of these that when one of them is

disengaged, another one of them might be in their power

stroke or another one might be engaged.

So it's not like you have this notion of, if all of a sudden

one lets go, that the actin filament will recoil back to

where it was.

Now the next question that you might be thinking is, how do I

turn on and off this situation?

We have command over our muscles.

What can turn on or off this system of the myosin

essentially crawling up the actin?

And to understand that, there's two other proteins

that come into effect.

That's tropomyosin and troponin.

And so I'm going to redraw the actin-- I'll do a very rough

drawing of the actin filament.

Let's say that that's my actin filament right there with its

little grooves.

It's actually a helical structure.

And actually, these grooves-- it's kind of a helical-- but

we won't worry too much about that.

What we drew so far, at least in the last video, you had

these little myosin.

You can view them as feet or head or whatever that keep

attaching to it and then based on where they are in that ATP

cycle, they can keep getting cranked back up or

sprinr-loaded and go to the next one and push back.

Now, on top of this actin, you actually have

this tropomyosin protein.

And this tropomyosin protein, it coils around the actin.

So this is our actin right here.

This is one of the two heads of the myosin II.

And then we have our tropomyosin.

Tropomyosin is coiled around.

It's a very rough sketch, but you can imagine it's coiled

around and it goes back behind it, then it goes like that,

and then it goes back behind it, then it goes like that.

So it's coiled around it and the important thing about it

is, if there's-- let me take a step back.

It's coiled around and it's attached to the actin by

another protein called troponin.

Let's say it's attached there and-- this isn't exact, but

let's say it's attached there, and there, and there, and

there, and there by the troponin.

So let me write this down.

So you can imagine, the troponin is kind of like the

nails into the actin.

So it dictates where the tropomyosin is.

So when a muscle is not contracting, it turns out that

the tropomyosin is blocking the myosin from being able

to-- and I've read a bunch of accounts on this and I think

this is still an area of research.

It's not 100% clear one way or the other.

Tropomyosin is-- or maybe both-- blocking the myosin

from being able to attach to the actin where it normally

attaches so it won't be able to crawl up the actin-- or

sometimes the myosin is attached to the actin, but it

keeps it from releasing and sliding up the actin to keep

that walking procedure.

So the bottom line is that this tropomyosin kind of

blocks the myosin head-- this is the myosin head right

there-- from crawling up the actin, either by physically

blocking its actual binding site or if it's already bound,

keeping it from being able to keep sliding up the actin.

Either way, it's blocking it and the only way to make it

unblocked is for the troponins to actually change their

confirmation, for them to change their shape.

And the only way for them to change their shape is if we

have a high calcium ion concentration.

So if you have a bunch of calcium ions, if you have a

high enough concentration, these calcium ions are going

to bond to the troponin and then that changes the

confirmation of the troponin enough to move the

configuration of the tropomyosin.

So let me write this down.

So normally, tropomyosin blocks, but then when you have

a high calcium ion concentration, they bind to

troponin and then the troponin, they change their

confirmation so it moves the tropomyosin out of the way.

So when it moves out of the way, you have a high calcium

concentration, bonds troponin, moves tropomyosin out of the

way, then all of a sudden what we talked about in the last

video-- these guys can start walking up the actin or

pushing the actin to the right, however you

want to view it.

But then if the calcium concentration goes low, then

the calciums get released from the troponin.

You need to have enough to always hang around here.

If the concentration becomes really low here, these guys

will start to leave. So then the troponin goes back to, I

guess, standard confirmation.

That makes the tropomyosin block the myosin again.

So it's actually-- I mean, I can't say

anything here is simple.

This was only discovered maybe 50 or 60 years ago and you can

imagine to actually observe these things or to create

experiments to definitively know what's happening--

nothing is simple, but the idea is simple.

Without calcium, the tropomyosin is blocking the

ability of the myosin to attach where it needs to

attach or slide up the actin so it can keep pushing on it.

But if the calcium concentration is high enough,

they will bond to the troponin-- which essentially

nails down the tropomyosin that's wound around the actin

and when they change their confirmation with the calcium

ions, it moves the tropomyosin out of the way so that the

myosin can do what it does.

So you can imagine already, we're building up a way for--

one, for muscles to contract, but even better, for us to

control muscles to contract.

So if we have a high calcium concentration within the cell,

the muscle will contract.

If we have a low calcium concentration again, then all

of a sudden, these will release.

They'll be blocked, and then the muscle will relax again.