Final answer:
C) Ca²⁺ binds troponin. The interaction between actin and myosin is mediated by calcium ions binding to troponin, which moves tropomyosin away, allowing myosin heads to bind the now exposed binding sites on actin and initiate muscle contraction.
Step-by-step explanation:
The event that allows actin and myosin to interact in muscle contraction is when Ca²⁴ binds to troponin. In the relaxed state of a muscle, the binding sites on actin are covered by a protein called tropomyosin. Troponin is a complex of three proteins attached to tropomyosin. When calcium ions are released from the sarcoplasmic reticulum (SR) during muscle excitation, they bind to one of the troponin molecules. This binding causes a conformational change in the troponin complex, which in turn moves the tropomyosin away from the myosin-binding sites on actin filaments. With the myosin-binding sites exposed, the myosin heads can attach to them, forming cross-bridges. This is the essential step that initiates the process of muscle contraction. The cycle of contraction continues as long as calcium ions remain bound to troponin and ATP is available to provide energy.
It's important to note that the interaction between Ca²⁴ and troponin is specific, and troponin does not bind directly to myosin or actin. Similarly, calcium ions do not bind directly to actin but influence the interaction indirectly through troponin and tropomyosin.
To answer the original question, 'Which event allows actin and myosin to interact?', the correct answer is C) Ca²⁴ binds troponin.