What causes sodium channels to open? What triggers the opening of sodium channels in a neuronal membrane? Is it acetylcholine that activates sodium channels in the postsynaptic membrane? Are sodium channels like receptors that have to bind to something (like a protein or an ion) to open? If so, what is the substance that forces Na+ channels to open?

Sodium channels are primarily voltage-gated \- these are the channels responsible for action potentials.

Many other receptors are ligand-gated, and these are typically the signal that causes the initial voltage change that opens the voltage-gated sodium channels; however, these channels are less selective cation channels and are permeable to ions like potassium as well as sodium. Still, their permeability to sodium is quite important and so they may also be thought of as sodium channels sometimes.

These include neurotransmitter gated channels like nicotinic acetylcholine receptors and AMPA (glutamate) receptors, and transient receptor potential channels like the TRPV1 receptor that is sensitive to painful heat and the chemical capsaicin which makes chili peppers "hot."