Rank the sequence of events at the nmj that initiate an action potential in the muscle fiber, from first to last.- action potential arrives at the axon terminal.- acetylcholine binds to its receptors on the junctional folds.- calcium ions enter the axon terminal.- synaptic vesicles fuse to membrane of axon terminal.- junctional folds become depolarized.- action potential is initiated on the sarcolemma.- acetylcholine is released into the synaptic cleft.

Action potential arrives at the axon terminal.

Calcium ions enter the axon terminal.

Synaptic vesicles fuse to membrane of axon terminal.

Acetylcholine is released into the synaptic cleft.

Acetylcholine binds to its receptors on the junctional folds.  

Junctional folds become depolarized.  

Action potential is initiated on the sarcolemma.

Explanation:

When the electrical signal (action potential) reaches the axon terminal it opens Ca channels that release Ca ions from the outside inside the axon terminal. Ca ions trigger the fusion of vesicle with membrane of the presynaptic cell (axon terminal). As a consequence, neurotransmitter-acetylcholine is released into the synaptic cleft. It diffuses to the postsynaptic cell and binds to its receptor (nicotinic acetylcholine receptor). The binding of acetylcholine leads to the depolarization of  the sarcolemma.