Of these events, the first to occur when a motor neuron stops sending an impulse to a muscle is . Of these events, the first to occur when a motor neuron stops sending an impulse to a muscle is . the pumping of calcium ions out of the cytoplasm and back into the sarcoplasmic reticulum the release of myosin heads from the thin filament thin filaments slide back to their relaxed positions proteins on the thin filaments block actin's myosin-binding sites all of these events occur simultaneously