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I know about the opening of voltage gated sodium channels and then how the membrane becomes depolarized when the rise of potential greater than threshold occurs.

But what happens initially after the stimulus that causes a change in potential and makes it reach up to the level of threshold? Like what that light, touch or chemical did?

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Ligand-gated ion channels, mainly. Ligands can be neurotransmitters like glutamate or acetylcholine, or extraneous chemicals. TRP channels can also be sensitive to things like temperature. Mechanoreceptors and hair cells in the cochlea used in audition are gated mechanically. Gating can be direct or via a second messenger; photoreceptor cells use a chemical conformation change triggered by photons to detect light, but the actual voltage change comes by a channel gated by cyclic GMP.

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