The first thing that occurs when the axon terminal releases ACh is .
A. calcium ions return to the terminal cisternae of the SR
B. the troponin blocks the tropomyosin
C. calcium diffuses into the axon terminal of the motor neuron
D. diffusion of ACh across the synaptic cleft

It diffuses across the synaptic cleft  to bind  with synaptic receptors on the post-synaptic neuron . This  cause influx of sodium ions  though the ligand gated sodium channels into the synaptic cleft for depolarization. If this   is upto  to the threshold levels  it generate Excitatory  Post Synaptic Potential(EPSP) .  However if at   neuromuscular junction the acetycholine  will diffuse  across the cleft and bind with receptors  (Nicotinic receptors) on the motor end plate  to cause sodium influx. This  generate action potential  if up to the threshold and therefore muscle contraction.

Explanation:

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