In addition to neurons, muscle cells undergo depolarization, although smaller and slower than in the neuron, as

Question:

In addition to neurons, muscle cells undergo depolarization, although smaller and slower than in the neuron, as a result of the activity of the acetylcholine receptor.
(a) The acetylcholine receptor is also a gated ion channel. What triggers the gate to open?
(b) The acetylcholine receptor/ion channel is specific for Na+ ions. Would Na+ ions flow in or out? Why?
(c) How would the Na+ flow through the ion channel change the membrane potential?