Select the single word in each sentence to make it correct.

1. Excitation-contraction coupling refers to the events that link the action potentials on the sarcolemma to activation of the (Click to select). It begins with the arrival of the (Click to select) at the neuromuscular junction.

2. Next, a wave of action potentials spreads from the end plate in all directions, reaching the T tubules, which carry the excitation wave into the (Click to select).

3. In the (Click to select), action potentials open voltage-gated ion channels, whose physical connections cause the release of calcium ions from the terminal cisterns of the sarcoplasmic reticulum into the cytosol.

4. Calcium will bind to the (Click to select) of the thin filaments.

5. On the thin filament, active sites are open for the binding of (Click to select) heads, which allows the contraction to begin.