A single contraction of skeletal muscle is most likely to be terminated by which of the following actions?: (A) Closure of the postsynaptic nicotinic acetylcholine receptor, (B) Removal of acetylcholine from the neuromuscular junction, (C) Removal of Ca++ from the terminal of the motor neuron, (D) Removal of sarcoplasmic Ca++
Answer is D. Skeletal muscle contraction is tightly regulated by the concentration of Ca++ in the sarcoplasm. Ca2+ influx causes the exocytosis of the synaptic vesicles and the release of neurotransmitter occurs. Neurotransmitter binds to ligand gated ion channels and opens it resulting in graded potentials. As long as sarcoplasmic Ca++ is sufficiently high, none of the remaining events as given in options -- would have any effect on the contractile state of the muscle.