Understanding Muscle Contraction: The Sliding Filament Model Explained

when it comes to our muscles we don't spend a lot of time thinking about what happens at the

chemical level uh whenever we do movements but it turns out it's a somewhat complicated and i think

fascinating process we call it the sliding filament model because the filaments are pulling or

sliding across each other of course every time we move our muscles it starts in the brain and the signal

down to the muscles to get them to contract we're going to start with an action

potential from the brain and work our way down to the sliding filament model where the actual proteins are pulling on

each other to contract the muscle let's jump to the white board and get started all right so before we get into the

sliding filament steps themselves we have to take a look at something called excitation contraction coupling which is

this idea that for a muscle to contract we first need a signal or an excitation

that's going to come from the brain the brain is going to send a signal down through the spinal cord

out through a nerve to whatever muscle it is that we're trying to move and then the muscle can contract as a

response to that so in our diagram here we have those two cells we have a neuron and this is the axon terminal of a

neuron we have a synapse the connection between this neuron and this muscle cell now

because it's a neuron and a muscle cell sometimes we also call this a neuromuscular junction but it's just a

synapse and that axon terminal of course has some vesicles with neurotransmitters

there's some receptors on the postsynaptic cell which is our muscle fiber

and our muscle fiber is going to have several organelles that we're going to label here

this first one the transverse tubule that goes down into the muscle cell as well as the sarcoplasmic reticulum

which is going to store calcium to be released as part of this process now i didn't draw all of the

sarcoplasmic reticulum i sort of cut it off here so i have room to draw and write other things in the diagram

and underneath the sarcoplasmic reticulum are the myofibrils which contain

the myofilaments so these are the myofilaments here and we have two of those of course the actin and the myosin

and that's kind of our end goal is to get them to pull on each other and shorten the length of the sarcomere

which is the space between this z line and this other z line and as that happens

our muscles contract that's our end goal here but it all starts with the brain sending action potentials through

neurons which is where we're going to start right now so first we have this action potential

it's come down the axon now it's made it to the axon terminal that's going to cause the vesicles to release the

neurotransmitters which are going to bind with the receptors on the postsynaptic side down here

that's going to cause sodium to rush into the postsynaptic cell through a process called synaptic

transmission and if you need to check out my videos on action potentials and on synapses which

i'll link to the description down below now once this muscle cell depolarizes right here because of the sodium rushing

in that's going to cause more sodium channels to keep opening down the length

of the neuron which causes that new action potential to travel down the sarcolemma the cell

membrane of the muscle eventually that signal is going to make it to a t tubule now i just have one t

or transverse tubule drawn in here but there's really lots of them throughout the muscle cell and they're all going to

be conducting the signal down into the muscle cell so the signal travels down into the transverse tubule

down to the sarcoplasmic reticulum which like i said is going to be filled with calcium ions that as soon as the signal

gets there these calcium ions are going to get released out into

the myofibrils and that all started with the action potential that through this chain of events now has caused the

sarcoplasmic reticulum to leak calcium ions into the myofibrils to interact with the myofilaments

and in the presence of calcium the myofilaments will start grabbing onto each other

and contracting and then we get something like this see how they're much closer together

the length of the actin and the length of the myosin didn't change but as they pulled on each other the

length between the z lines from there to there has decreased and now we have a contracted

muscle and as long as there's signals being sent from a neuron and therefore calcium being released

from the sarcoplasmic reticulum we're going to have contracted muscles so just a quick recap

of all that so far it all starts with the brain the motor cortex in the brain sending a signal

down through the spinal cord out through a nerve and eventually that's going to make it to the end of a

neuron that connects to a muscle cell so we start with an action potential that's going to cause synaptic

transmission in the neuromuscular junction that signal is going to travel down

along the sarcolemma down into the transverse tubules where it's going to interact with the sarcoplasmic reticulum

causing it to release calcium ions in the presence of calcium ions the actinomycin

will contract like that relax contract relax contract relax contract sorry we don't

have a huge animation budget we just do two frame animations here okay now let's take a deeper dive

into the sliding filament part of this which is where the filaments grab onto each other and how is that all regulated

by calcium so let's zoom in to this section right in here

all right so here we have our two filaments we have myosin in pink that's our thick filament and we have

actin in purple here that's our thin filament if you look closely you'll see green dots you see green dots on the

myosin heads as well as green dots on all these circles which are the actin molecules

those are binding sites this is where the myosin head can latch onto or bind with the actin

molecule throughout this video i'm going to use the non-scientific term of grab like the

myosin head grabs onto the actin but a better term to use really here is bind it's going to chemically bind

with the actin molecule whenever it pulls on it but we have a problem the binding sites are actually covered

up by a molecule called tropomyosin you'll see it here i just drew it in brown and notice how it's hard to find

the green dots on there you have to look really closely now that's because this molecule that i drew in brown

here this tropomyosin has roped off or blocked off the binding sites and i think about that

like it's roped off like if you go somewhere and the seats are roped off reserved for somebody else tropomyosin

is roping off and has rope in there right trope ropomyosin it's blocking the binding

sites meaning our muscle can't contract right now we have one more little molecule on there

and i do that in yellow and that's called troponin troponin's kind of connected to the tropomyosin and they're

going to be interacting with each other in a second so again we have myosin

we have the myosin heads we have the actin molecules in purple the green binding sites which is where

the myosin heads can grab onto the actin those of course are blocked by the tropomyosin that's that kind of brown

looking rope right there and we have troponin which i have here in yellow and we'll

see what that does in a second so at this stage we're sort of in the off setting right the muscle's not

contracted yet we're about to contract the muscle and i'm going to call that stage one here in

just a moment okay now let's assume that the sarcoplasmic reticulum has released calcium ions right we've

had a neuron send an action potential from the brain down to this muscle cell the

sarcoplasmic reticulum releases its calcium and here's what's going to happen some

of that calcium is going to bind with the troponin so these orange calcium ions are going to bond with the yellow

troponin right there and that's going to cause the tropomyosin to actually peel

back out of the way and if you look closely at our diagram now you'll see there's still tropomyosin

there but notice how it's not blocking the binding sites you can see those little green dots on

there pretty clearly now because the tropomyosin has been pulled back and that happened whenever the

calcium ions binded with the troponin which pulls back the tropomyosin

so the binding sites are exposed and now the myosin heads are going to be able to form a connection to them this is sort

of the on state now our muscle's about to contract but that's going to take a few stages

for it to happen notice in our diagram now that the myosin heads are physically connected

to the actin molecules their green binding sites are lined up with each other

right there and so this is latched on we call this forming a cross bridge i think of this as the grab stage the

myosin grabs onto the actin really it binds with it and we say that the myosin head forms a

cross bridge a bridge connects two things together and so we call this sort of a bridge a

cross bridge the myosin head has formed a cross bridge between the myosin molecule and the

actin filament once we've formed a cross bridge that myosin head now is going to pull

and we call the pole this is the actual scientific name for it we call the pull the power stroke so

turn on grab pull i think these stages are a lot easier to remember if you think of them

like that turn on grab pull we'll see what happens next all right we've pulled now and we

need to reset so that we can pull again or just to relax the muscle if we're not going to contract it anymore

so this stage has two parts to it one is to release and the second is to reset atp is used

in this process so contracting our muscles is pretty atp intensive

our brain along with our muscles use more energy than any part of the body and this is one reason why

every time we have a grab and pull and release cycle here we have to use up an atp

molecule you need that energy from it for this to happen so the atp does two things it's

going to break the cross bridge and then it uses up its energy to reset the myosin head back to where it was

i like to think of this like a mouse trap the mouse trap you have to set the mouse trap right you

have to put energy in to pull the mouse trap back but once it's set

it doesn't take any more energy from you it just takes this is kind of morbid i guess

it just takes the mouse to crawl up onto it and then it snaps but the energy's already there you

already give it the energy whenever you back the bar or the myosin head here it has the

energy now it's just ready to release it same thing here with this

the atp is what's going to be used to one break the cross bridge and then two to reset or pull back to

the high energy state the myosin head and i think a mousetrap is a good metaphor for it because

this is where the energy from the atp is consumed not the power stroke which is where you would think like

oh we use the energy we really use the energy to reset the myosin head so a little bit later it can

do the power stroke so we've reset one of two things can happen here if there's calcium still

present it's just gonna keep doing this over and over and over again repeating steps two three and four that myosin

head is gonna grab onto the actin it's going to pull and then it will release and reset in the presence of atp

grab pull release and reset grab pull release and reset unless there's no more calcium

then we're done as long as there's calcium present in other words as long as the brain keeps sending

signals to the sr to keep releasing its calcium this muscle cell is going to keep

contracting and contracting and contracting as long as it can this would be like sustaining a muscle contraction

over a long period of time and you're not like letting go but we don't want all of our muscles to stay

contracted all the time that would be really bad so we have to be able to relax muscles as well and what happens

there well calcium is just going to get re-pumped back into the sarcoplasmic reticulum so

that it can't interact with this process anymore and that's going to turn off the muscle cell

because remember if there's no calcium present the tropomyosin will block off the

binding sites and the muscle can't contract anymore the muscle then will relax

and it'll be ready to contract again whenever we have more calcium released from the sr but until then

we're back to our relaxed state ah so to recap all of that we've got our myosin and our actin filaments

when the presence of calcium that's going to turn this on basically the sr releases the calcium it bonds with the

troponin the troponin pulls back the tropomyosin revealing the binding sites

the myosin heads are going to grab on to the binding sites we call that forming a cross bridge

they're going to pull on the actin molecule we call it the power stroke and then the myosin head and the

presence of atp is going to release and reset back to its excited state that process

of grabbing pulling releasing resetting grabbing pulling releasing resetting that's going to keep happening

as long as there is calcium present from the sr whenever we're ready to relax the muscle

the sr is going to pump the calcium back into the sarcoplasmic reticulum and once that calcium is gone

then the tropomyosin covers up the binding sites and we're back to our relaxed

state and of course this whole process like i said has to start with an action potential

synaptic transmission conduction down the sarcolemma into the t tubule causing the sr to release calcium

and then it goes through all of that stuff that we just did to have a nice contracted muscle now if

you want to test your understanding take a moment and pause the video describe what happens in each of the five steps

of the sliding filament model without any of the text on here as a guide if you can explain what

happens in each of those five steps then you have a pretty good understanding of the sliding filament model

of muscle contraction now here's that text back again if you want to check and see how you did

all right can you find the a in filament where's the a can you point to the a yeah good job

good job yeah what's over that can you point can you find can you find an m can you

find an m good job have we even learned that one yet how do you know where an m is