Molecular basis of muscle contraction

So I'm a bit confused about the details of muscle contraction.

I know that Ca2+ binds to troponin C, causing troponin I to move and expose the actin-myosin binding site, but then I don't understand the next steps, I'm confused about where ATP is relevant and what binds where etc.

If anyone could clearly explain it that would be really helpful!

Thanks

So I'm a bit confused about the details of muscle contraction.

I know that Ca2+ binds to troponin C, causing troponin I to move and expose the actin-myosin binding site, but then I don't understand the next steps, I'm confused about where ATP is relevant and what binds where etc.

If anyone could clearly explain it that would be really helpful!

Thanks

Okay, I'll take you through this step by step:

1) Myosin heads bind to actin when the myosin binding sites are exposed by calcium binding to troponin.
2) An ADP molecule that was bound to the myosin head detaches, causing the myosin head to change its shape (its "conformation" and thus pull the actin with it. This is the "power stroke" and it pulls the Z-lines of the sarcomere together.
3) Once the myosin head has changed shape, ATP binds to the head and causes it to detach.
4) ATP hydrolyses and reverts the myosin head back to its original position.
5) You're left with a myosin head with an ADP attached, so the cycle continues until the muscles are no longer being stimulated.

So, ATP is actually used to unbind the myosin heads from the actin, that's why when you die, you get rigor mortis, because you don't have any ATP left to stimulate the myosin heads to detach from the actin, so your muscles stay contracted.

Hi there,

I'm also revising this topic and I've written it out into nice little stages to help me remember.

Step 1: An action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-Tubules to the sarcoplasmic reticulum.

Step 2: This causes the sarcoplasmic reticulum to release stored Ca²⁺ into the sarcoplasm

Step 3: Calcium ions bind to troponin C, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.

Step 4: This exposes the binding site, which allows the myosin head to bind.

Step 5: The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge.

Step 6: Calcium ions activate the enzyme ATPase, which breaks down ATP (into ADP + P_i ) to provide the energy needed for muscle contraction

Step 7: The energy released from ATP moves the myosin head, which pulls the actin filament along in a kind of rowing action.

Step 8: ATP also provides the energy to break the action-myosin cross bridge, so the myosin head detaches from the actin filament after it's moved.

Step 9: The myosin head then reattached to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated.

Step 10: Many actin-myosin cross bridges form and break very rapdily, pulling the actin filament along, which shortens the sarcomere, causing the muscle to contract.

It's a long process but once you read it over a few times it makes sense

Hi there,

I'm also revising this topic and I've written it out into nice little stages to help me remember.

Step 1: An action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-Tubules to the sarcoplasmic reticulum.

Step 2: This causes the sarcoplasmic reticulum to release stored Ca²⁺ into the sarcoplasm

Step 3: Calcium ions bind to troponin C, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.

Step 4: This exposes the binding site, which allows the myosin head to bind.

Step 5: The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge.

Step 6: Calcium ions activate the enzyme ATPase, which breaks down ATP (into ADP + P_i ) to provide the energy needed for muscle contraction

Step 7: The energy released from ATP moves the myosin head, which pulls the actin filament along in a kind of rowing action.

Step 8: ATP also provides the energy to break the action-myosin cross bridge, so the myosin head detaches from the actin filament after it's moved.

Step 9: The myosin head then reattached to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated.

Step 10: Many actin-myosin cross bridges form and break very rapdily, pulling the actin filament along, which shortens the sarcomere, causing the muscle to contract.

It's a long process but once you read it over a few times it makes sense

Thank you! So the ATP is only used for energy to revert the myosin back to its 'resting' state? and not for the power stroke? Also, what about the inorganic phosphate produced by hydrolysis of ATP?

That was nicely written! Except the ATP doesn't provide energy for the power stroke, it provides energy for the recovery stroke. The power stroke is a passive process, because the myosin head is already in a high energy state, it is simply reverting to a lower energy state by carrying out the power stroke. So in a way, ATP does provide the energy for the power stroke, but not in the way you describe d in steps 7, 8 and 9