Sliding Filament Theory

eilatan

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Can anyone explain the Siliding Filament Theory to me!? I've had a look on Wikipedia and a couple of other websites and it baffles me! But I have to write an essay on it. Anyone who can explain it to me in simplified terms would be very useful!!

Thanks, Natalie

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iceman_jondoe

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its quite interesting actually..i dont have my original textbook to hand but..
Muscle fibres are composed of both actin and myosin filaments. i suggest your intro should be something on the structure of muscle fibres because you need to understand the structure in order to understand how the sliding filament theory works. You could also include some background info on who came up with the throy if you wish

.
Now Within muscles you have something called a sarcoplasmic reticulum which is basically a "netwrok" of fibres running throughout the sarcoplasm (cytoplasm of muscles). When a muscle is stimulated by the nerves calcium ions are released from vesicles in the sarcoplasmic reticulum. Now after you have looked at the structure of a sarcomere in muscles you will know what i mean here when i say the actin filaments have receptor sites which are blocked my a protein called troponin which along with tropomyosin, inhibits actin–myosin binding. This causes the myosin binding sites on the actin to be “switched on.”
Myosin cross bridges, fueled by ATP hydrolysis, bind to the actin and rotate, causing the power stroke that allows the actin to “slide” across the myosin molecule.
The myosin head detaches from the actin and recombines with an ATP molecule.
The above process happens continuously hundreds of times per second causing the length of the sarcomere, seen microscopically as the distance between the z-bands, to decrease. Thousands of sarcomeres are contained on each fiber, and as these shorten in length, so do the individual muscle fibers making up a muscle.
This contraction of the muscle continues until neural stimulation ceases and the Ca2+ is pumped back across the membrane of the sarcoplasmic reticulum. Myosin–actin binding is again inhibited, and the sarcomeres return to their usual length.

This website may help you further also http://www.blackwellpublishing.com/matthews/myosin.html

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Revenged

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(Original post by eilatan)
Can anyone explain the Siliding Filament Theory to me!? I've had a look on Wikipedia and a couple of other websites and it baffles me! But I have to write an essay on it. Anyone who can explain it to me in simplified terms would be very useful!!

Thanks, Natalie

Firstly, you need to to consider a neuromuscular junction... This is where a nerve meets a muscle...

At the neuromuscular junction, calcium ions (Ca2+) move into the end of the nerve...

This causes acetylcholine (ACh) to be released by the nerve...

ACh diffuses across the neuromuscular junction and is detected by receptors on the sacrolemma (mebmrane of the musclular fibre)...

When it binds to the receptor, it causes sodium ions to rush into the muscle...

This causes an action potential (nerve impulse) to be generated in the sacrolemma (membrane of the muscular fibres)

This action potential then travels along the t-tubules (another part of the membrane of muscular fibres)

The action potential causes sacroplasmic recticulum (a special type of smooth endoplasmic recticulum) to release Ca2+ ions...

Before I go on, muscle fibres are formed by actin filaments (thin fibres)and myosin (thick fibres with "heads")... Normally they are blocked from joining each other by a protein called tropomyosin...

In the diagram below, the red blobs are myosin and the blue fibres are actin...

Now back to the process, the Ca2+ ions that were realeased, join to a special calcium detecting protein on the actin... This protein is called troponin...

Binding of calcium to the actin filament causes a change of shape, which causes the tropomyosin to no longer block the actin sites for myosin...

This allows the myosin head to bind to the actin filament... At this time myosin has ATP bound to it...

When the myosin head binds to the actin... Myosin pulls along the actin and converts ATP into ADP... This is known as the 'power stroke'...

ATP then binds to myosin again and causes the myosin to become detached from the myosin...

Providing that calcium and ATP are present, the myosin will again bind to actin... Again the power stroke will take place and again will convert ATP to ADP... This ATP will again bind to myosin causing the myosin to become detached from actin...

Eventually, this will cause contraction and microscopically the muscular fibre will look like this (compare this to the diagram above):

The distance between the Z bands (the vertical black diagonal lines in the diagram) decrease... In biology, the distance between these Z bands is called the "sacromere"...

To ensure that this process stops, calcium is actively pumped back into the sacroplamic recticulum...

When there is no more calcium present, no calcium binds to troponin... This would cause tropomyosin to again bind to actin preventing myosin binding to the actin... This means that there is no more sliding of the myosin along the actin and no more muscle contraction...

There you go, I've tried to explain what happens as clearly as I can... If you have any questions feel free to ask... You may find this website useful:

http://images.google.co.uk/imgres?im...lr%3D%26sa%3DX

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Revenged

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(Original post by iceman_jondoe)
Myosin cross bridges, fueled by ATP hydrolysis, bind to the actin and rotate, causing the power stroke that allows the actin to “slide” across the myosin molecule.

I'm not really clear on how cross bridges formed by ATP hydrolysis cause a power stroke...

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Comp_Genius

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after ATP binds, it is partially hydrolysed into ADP and Pi but they are not released - the most stable conformation is 90 degrees, but when ATP is fully hydrolysed and ADP + Pi are released, the angle becomes 45 degrees, thereby eliciting a power stroke.

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nikk

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Yep - the old fact that anything binding to a protein can potentially change its conformation, through interaction with, addition of, or rearrangement of its intramolecular interactions (hydrophobic / hydrogen bonds / etc).

One point of interest to add - ATP binding to the myosin globular heads is required following the power stroke, to release the myosin head from the actin. In death, once intracellular ATP reserves are quickly used up, the myosin and actin filaments remain locked together, the muscle is contraced, producing the effects of rigor mortis.

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Revenged

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(Original post by Photios)
after ATP binds, it is partially hydrolysed into ADP and Pi but they are not released - the most stable conformation is 90 degrees, but when ATP is fully hydrolysed and ADP + Pi are released, the angle becomes 45 degrees, thereby eliciting a power stroke.

Cheers... I got a lecture + practical about this next week so it should be useful...

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nikk

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I was going to rep Revenged for the essay, but I have to 'spread it around more', or something. I really need to use the rep system more often.

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Revenged

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(Original post by Revenged)
I'm not really clear on how cross bridges formed by ATP hydrolysis cause a power stroke...

What I said before about the power stroke wasn't quite right...

I didn't properly understand the theory myslef when i wrote the post...

My simplified (and correct) version below should make more sense...

- Nerve impulse in muscle

- Causes Ca2+ release from sacroplasmic recticulum

- Troponin and tropomyosin bound to actin no longer inhibit binding of myosin

- Myosin binds to actin strongly

- ATP binds to myosin head

- Conformational change - myosin no longer binds to actin

- ATP hydrolysis (ATP + H2O -> ADP + Pi) occurs

- Myosin binds again futher along the actin molecule

- ADP is released causing the "power stroke" where the myosin head pulls on the actin molecule

(So does the movement of the detachment and subsequent reattachment of myosin to actin cause contraction and the "power stroke" causes force of contraction?... I'm just wondering because you have muscular contraction for movements - not to generate force - e.g. abduction of the shoulder joint - whereas you have muscular contraction with no movement e.g. lifting a heavy weight off the ground... any thoughts anyone?)

Btw, just like to say thanks to nikk... The first question on my work sheet is about rigor mortis... so i can do that one fine...

But I have some other questions i can't do:

- What are the three sequential steps in forming F-actin from G-actin?

- Name three biochemical differences between straiated and smooth muscle?

(I know the basic differences: voluntary and involuntary, multi-nucleic and single nucleus, but i doubt that these would be classified as 'biochemical differences')

- What is the difference between ATP binding to actin and to myosin?

Cheers

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nikk

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(Original post by Revenged)
- What are the three sequential steps in forming F-actin from G-actin?

I don't know of three specific steps, but this is what I do know (or rather, what I just read from last year's notes because I have forgotten already lol). I've used your own style, as for this topic, it is much easier :p:

Polymerisation of G-actin monomers into F-actin is a reversible process, dependent on the monomer concentration.

When a steady state is reached (no growth) there is a critical concentration (Cc) of monomers.

If [monomer] > Cc : G-actin will polymerise. Kon high.
If [monomer] < Cc : F-actin will depolymerise. Kon low.

(Kon refers to the rate of association)

G-actin are polarised molecules, and polymerisation occurs faster at the +end than at the –end.
• G-actin binds ATP and its hydrolysis to ADP is required to become F-actin.
• The +end ATP-actin and the –end is ADP F-actin.
• ADP F-actin depolymerises much more easily than ATP F-actin (therefore a higher Cc needed to replace F-actin at -end than at +end).

When Cc –end > [G actin] > Cc +end actin subunits can flow through the filament by attaching preferentially to the +end and dissociating from the –end. A process of ‘treadmilling’ occurs which is important for control of actin movement (important for cell growth).

I have just pasted that from my notes - it probably explains it better than I could if I tried to retype it. I would be interested to hear what these three sequential steps are? Perhaps...?
1) [monomer] > Cc +end
2) Hydrolysis of ATP by G-actin.
3) G-actin binds to +end of F-actin, now also bound to ADP.

(Original post by Revenged)
- Name three biochemical differences between straiated and smooth muscle?

Maybe:
1) Actin filaments in smooth muscle lack the regulatory protein troponin.
2) The source of the Ca2+ influx following excitation of smooth muscle is extracellular, rather than from the sarcoplasmic recticulum.
3) Ca2+ does not interact with troponin which is absent in smooth muscle, but instead activates a signalling cascade (through calmodulin) that results in myosin phosphorylation.

(Original post by Revenged)
- What is the difference between ATP binding to actin and to myosin?

ATP binding to G-actin is required for polymerisation, and once bound is quickly hydrolysed to ADP. Actin can then still depolymerise even though ADP is bound. With myosin, it cannot release from actin and resume the power stroke conformation until ADP is removed and replaced with ATP.

I am just guessing on this one!

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Revenged

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1) Actin filaments in smooth muscle lack the regulatory protein troponin.
2) The source of the Ca2+ influx following excitation of smooth muscle is extracellular, rather than from the sarcoplasmic recticulum.
3) Ca2+ does not interact with troponin which is absent in smooth muscle, but instead activates a signalling cascade (through calmodulin) that results in myosin phosphorylation.

Interesting...

I'm guessing the difference between Ca2+ influx in smooth and skeletal muscle would be because one is involuntarily controlled and one is voluntarily controlled...

ATP binding to G-actin is required for polymerisation, and once bound is quickly hydrolysed to ADP. Actin can then still depolymerise even though ADP is bound. With myosin, it cannot release from actin and resume the power stroke conformation until ADP is removed and replaced with ATP.

I am just guessing on this one!

Sounds good... I was a bit confused why actin had had ATP binding sites but now i know they are needed for the polymerisation process...

Thanks for your help nikk but i'm not going to be learning this biochem into such great depth... it is sort of skimmed over in the lecture, which makes me think that i don't have to know any of it into such depth...

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dush_2

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(Original post by Revenged)

What I said before about the power stroke wasn't quite right...

I didn't properly understand the theory myslef when i wrote the post...

My simplified (and correct) version below should make more sense...

- Nerve impulse in muscle

- Causes Ca2+ release from sacroplasmic recticulum

- Troponin and tropomyosin bound to actin no longer inhibit binding of myosin

- Myosin binds to actin strongly

- ATP binds to myosin head

- Conformational change - myosin no longer binds to actin

- ATP hydrolysis (ATP + H2O -> ADP + Pi) occurs

- Myosin binds again futher along the actin molecule

- ADP is released causing the "power stroke" where the myosin head pulls on the actin molecule

[/B]

is this the shortest note for Sliding Filament Theory

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i.am.lost

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By the way, for anyone interested... Just this month we have finally observed the myosin as it 'walks' along actin filaments. This is one step towards proving the sliding filament theory.
http://www.nature.com/nature/journal...re09450-s2.mov

(http://www.nature.com/nature/journal...l/468043a.html)

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GodspeedGehenna

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(Original post by dush_2)
is this the shortest note for Sliding Filament Theory

Well done. You bumped a thread from 2006.

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thank very much this really helped

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