Question

when does cross bridge cycling end?
cross bridge cycling ends when atp binds to the myosin head.
cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.
cross bridge cycling ends when calcium ions are passively transported back into the sarcoplasmic reticulum.
cross bridge cycling ends when calcium release channels in the sarcoplasmic reticulum open.

Explanation:

Cross - bridge cycling is part of muscle contraction. Calcium binding to troponin initiates it. When sufficient calcium is actively transported back into the sarcoplasmic reticulum, it unbinds from troponin, causing tropomyosin to block myosin - binding sites on actin, thus ending cross - bridge cycling. ATP binding to myosin head is part of the cycling process but doesn't end it. Calcium ions are actively, not passively, transported back into the sarcoplasmic reticulum to end cycling. Opening of calcium release channels in the sarcoplasmic reticulum initiates, not ends, cross - bridge cycling.

Answer:

Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.