Question

the cross bridge cycle starts when ________. select the best answer. acetylcholine diffuses away from the synaptic cleft atp binds to troponin and is hydrolyzed to adp and pᵢ ca²⁺ from the sarcoplasmic reticulum binds to tropomyosin ca²⁺ from the sarcoplasmic reticulum binds to troponin ca²⁺ is actively transported into the sarcoplasmic reticulum

Explanation:

The cross - bridge cycle in muscle contraction begins when calcium ions ($Ca^{2+}$) released from the sarcoplasmic reticulum bind to troponin. This binding causes a conformational change in the troponin - tropomyosin complex, exposing the myosin - binding sites on actin. Acetylcholine diffusion away from the synaptic cleft is related to ending neural signaling to the muscle, not starting the cross - bridge cycle. ATP binds to myosin, not troponin, in the cross - bridge cycle. $Ca^{2+}$ binding to tropomyosin is incorrect as it binds to troponin. Actively transporting $Ca^{2+}$ into the sarcoplasmic reticulum is part of muscle relaxation, not the start of the cross - bridge cycle.

Answer:

$Ca^{2+}$ from the sarcoplasmic reticulum binds to troponin