Cross bridges are formed during muscle contraction when binds to .

Cross bridges are formed during muscle contraction when actin binds to myosin.

Explanation:

Contraction of muscle allows shortening of muscles fibers.

The process of contraction begins when an action potential arrives at neurotranmitter junction (neuron binds to muscle fiber). Acetylcholine (ACh) is released and the receptors recieved the signal which allows to open the sodium ion channel and results in action potentialin sarcolema. This triggers the release of calcium ion from sarcoplasmic reticulum (SR).

Muscle fiber are consiste of several fiber protein actin, myosin, tyrosin and troponin. Actin fiber binds to myosin and form a cross-bridge that allows the binding of calcium ion at myosin and generate ATP which causes contarction of muscle fiber.

Hence, when actin binds to myosin a crossbridge forms the ATP gneeration and causes contraction.