Muscle contractions are initiated by the binding of __________ to __________.

Muscle contractions are initiated by the binding of calcium ions to troponin.

Here are the steps:

1. An action potential (electrical signal) travels along a motor neuron to its end, where it causes the release of a neurotransmitter, acetylcholine.

2. Acetylcholine diffuses across the synaptic cleft (gap between the neuron and muscle cell) and binds to receptors on the muscle cell membrane.

3. This causes an action potential to be generated in the muscle cell, which travels along the cell and into the cell through the T-tubules.

4. This action potential causes the sarcoplasmic reticulum (a specialized type of endoplasmic reticulum in muscle cells) to release calcium ions.

5. The calcium ions bind to a protein called troponin, which is part of the thin filament in muscle fibers.

6. The binding of calcium to troponin causes a conformational change in another protein, tropomyosin, which exposes binding sites for myosin (a motor protein) on the actin filament.

7. Myosin binds to these sites and uses ATP to pull the actin filament towards the center of the sarcomere, causing the muscle to contract.

8. Once the action potential ends, calcium is pumped back into the sarcoplasmic reticulum, troponin returns to its original shape, and the muscle relaxes.