Answer
When a nerve signal reaches the neuromuscular junction, this cases the voltage-gated channels to open. Ca++ ions enter the synaptic vesicles which release acetylcholine (ACH). The released ACH now cross the synaptic cleft and bind to nicotinic receptors on the sarcoplasm of the postsynaptic muscle fiber.
When the ACH binds to ACHRs they open ligand-gated ion channels. This causes Na+ ions to flow/diffuse into the myofiber and K+ ions to flow/diffuse out. The result is that the myofibril becomes depolarized and an EPP is generated
Work Step by Step
When the EPP reaches threshold it triggers an action potential(AP) which spreads from one area in all directions over the sarcolemma . On reaching the T-tubules the AP the P travels down these into the interior of the cell. The AP opens voltage gated channels in the T-tubules that are linked to Ca++ channels in the terminal cisternae of the SR. When the these calcium channel s also open Ca++ ions diffuse out of the SRvand pour into the sarcoplasm (cytosol). The Ca++ ions bind to troponin in the tropomyosin-troponin complex and causes it to change conformation(shape) . This exposes the active site of the actin filaments making them available for binding by myosin heads.
The role of ATP is to provide the energy--after hydrolysis-- for the power stroke of the sliding of the filaments
