How do ions and electrical charges play a role in communication with the muscle?

The electrical charge that travels along the motor neuron is caused by depolarization. Depolarization is caused by the inflow of Na+ ions into the neuron. At the neuromuscular junction, depolarization causes Ca2+ ions to enter the post-synaptic neuron. Binding of Ca2+ to sensor protein in the neuron causes the neuron to release neurotransmitter acetylcholine into the synaptic cleft. The neurotransmitter binds to ionotropic receptors on the sarcolemma of the muscle fiber causing contraction of the muscle fiber.

correct answer is a. and d.