Questions: - Mechanism of muscle contraction-show the steps of muscle contraction, label all of the following Actin Myosin Cross-bridges ATP ADP + P Tropomyosin Troponin Calcium Sarcoplasmic reticulum

Okay, I will describe the steps of muscle contraction, labeling the requested components.

Steps of Muscle Contraction

Muscle contraction is a complex process that occurs at the level of the sarcomere, the basic contractile unit of muscle fibers. Here's a breakdown of the steps:

1. Nerve Impulse and Calcium Release:

   • A motor neuron sends an action potential (nerve impulse) to the neuromuscular junction.
   • This triggers the release of acetylcholine, which binds to receptors on the muscle fiber membrane (sarcolemma).
   • The binding of acetylcholine initiates an action potential that travels along the sarcolemma and down the T-tubules.
   • The action potential reaching the sarcoplasmic reticulum triggers the release of calcium ions (\(Ca^{2+}\)) into the sarcoplasm (the cytoplasm of the muscle cell).

2. Calcium Binding to Troponin:

   • Calcium ions (\(Ca^{2+}\)) bind to troponin molecules located on the thin filaments (actin).

3. Tropomyosin Shift:

   • The binding of calcium to troponin causes a conformational change in troponin.
   • This change causes tropomyosin (which was blocking the myosin-binding sites on actin) to shift its position, exposing the active sites on the actin filament.

4. Cross-Bridge Formation:

   • Myosin heads (which are bound to ADP + P) now can bind to the exposed active sites on the actin filament, forming cross-bridges.

5. The Power Stroke:

   • Once the cross-bridge is formed, the ADP and P are released from the myosin head.
   • This release triggers a conformational change in the myosin head, causing it to pivot and pull the actin filament towards the center of the sarcomere. This is known as the "power stroke". The actin and myosin filaments slide past each other.

6. ATP Binding and Cross-Bridge Detachment:

   • A new molecule of ATP binds to the myosin head.
   • This binding of ATP causes the myosin head to detach from the actin filament, breaking the cross-bridge.

7. Myosin Reactivation:

   • ATP is hydrolyzed into ADP + P by myosin ATPase. This hydrolysis provides the energy to "recock" the myosin head, returning it to its high-energy conformation, ready to form another cross-bridge if the binding sites on actin are still exposed.

8. Cycle Repeats:

   • As long as calcium is present and the binding sites on actin are exposed, the cycle of cross-bridge formation, power stroke, detachment, and reactivation will continue, resulting in muscle contraction.

9. Muscle Relaxation:

   • When the nerve impulse stops, the sarcoplasmic reticulum actively transports calcium ions (\(Ca^{2+}\)) back into the sarcoplasmic reticulum, removing them from the sarcoplasm.
   • The decrease in calcium concentration causes calcium to detach from troponin.
   • Tropomyosin then shifts back to its blocking position, covering the myosin-binding sites on actin.
   • Cross-bridges can no longer form, and the muscle relaxes. Actin and myosin slide back to their original position.

Summary:

Muscle contraction is initiated by a nerve impulse, which leads to the release of calcium. Calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin. Myosin heads then bind to actin, forming cross-bridges and initiating the power stroke, fueled by ATP hydrolysis. The cycle repeats as long as calcium and ATP are present, resulting in muscle contraction. Relaxation occurs when calcium is removed, and tropomyosin blocks the myosin-binding sites on actin.