“Rank the sequence of cross bridge cycling, starting with the myosin-binding sites being exposed and ending with relaxation due to cross bridge cycling ending. Do not overlap any events.”
Sequence:
a. Calcium ions pumped into the sarcoplasmic reticulum
b. Myosin head forms cross bridge with actin
c. Myosin binding sites covered
d. Power stroke moves thin filament
e. Cross bridges detach from actin
f. Myosin head is re-energized
g. Calcium ion concentration decreases below the threshold for binding to troponin
h. ATP attaches to myosin head
The Correct Answer and Explanation is:
To correctly rank the sequence of cross-bridge cycling in muscle contraction, starting from the exposure of myosin-binding sites and ending with relaxation, we can use the following order:
- b. Myosin head forms cross bridge with actin
- d. Power stroke moves thin filament
- e. Cross bridges detach from actin
- h. ATP attaches to myosin head
- f. Myosin head is re-energized
- g. Calcium ion concentration decreases below the threshold for binding to troponin
- a. Calcium ions pumped into the sarcoplasmic reticulum
- c. Myosin binding sites covered
Explanation:
- Myosin Head Forms Cross Bridge with Actin (b): The process begins when calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. This exposes the binding sites, allowing the energized myosin heads to form cross-bridges with actin.
- Power Stroke Moves Thin Filament (d): Once the cross-bridge is formed, the myosin head pivots and pulls the actin filament toward the center of the sarcomere in a movement called the power stroke. This results in muscle contraction.
- Cross Bridges Detach from Actin (e): After the power stroke, the myosin heads release from the actin. This detachment is crucial for the cycling process to continue.
- ATP Attaches to Myosin Head (h): The binding of ATP to the myosin head is essential for the release of the cross-bridge from actin. ATP must be present for the myosin head to detach and reset.
- Myosin Head is Re-energized (f): Once ATP is hydrolyzed into ADP and inorganic phosphate, the myosin head is re-energized and primed for another cycle.
- Calcium Ion Concentration Decreases Below the Threshold for Binding to Troponin (g): When the stimulation ceases, calcium ions are actively transported back into the sarcoplasmic reticulum, decreasing their concentration in the cytoplasm. This leads to the re-covering of the binding sites.
- Calcium Ions Pumped into the Sarcoplasmic Reticulum (a): The active transport of calcium ions into the sarcoplasmic reticulum signifies the end of muscle contraction.
- Myosin Binding Sites Covered (c): Finally, as calcium ions are removed, tropomyosin moves back to cover the myosin-binding sites, preventing further cross-bridge formation and leading to muscle relaxation.
This sequence illustrates the dynamic process of muscle contraction and relaxation through cross-bridge cycling, highlighting the importance of ATP and calcium ions in muscle function.