what occurs during the first event in muscle fiber contraction? select one: a. the muscle fiber membrane is stimulated and a muscle impulse travels deep into the fiber through transverse tubules. b. calcium ions diffuse from sarcoplasmic reticulum into the sarcoplasm and bind to troponin molecules. c. acetylcholine diffuses across a gap at a neuromuscular junction. d. acetylcholine is released from the end of the motor neuron.