Regarding chemicals' sensitivity to light, heat, sound, it is worth contemplating that there must be a chemical pathway that translates mechanical pressure however delicate to nerve impulses or we wouldn't have a sense of touch. Our sense of hearing is likewise dependent on precise indirect mechanical sensation of vibration in a range of frequencies.
The Rube Goldberg chain reaction that turns minute variations in pressure upon skin into an interpretable sensation of texture is mind-blowingly difficult to imagine.
I read on here somewhere that someone suggested sodium caused tinnitus and 4-6 weeks on a low or no sodium diet got rid of or seriously reduced their tinnitus, so when is it not just some pressure in a contained membrane? I'll check out the Rube Goldberg reaction it sounds intriguing.
Sorry, Rube Goldberg was a cartoonist who drew ridiculously over elaborate machines. Some biochemical pathways strike me the same way (though this is mostly due to contingency and opportunistic reuse, and definitely not gratuitous complication per-se).
Edited to add: and I AM aware that much of the "over complication" affords opportunities for positive and negative feedback loops that subject a pathway to external control of various sorts and variable expression in different conditions, cell types, and tissues.
The Rube Goldberg chain reaction that turns minute variations in pressure upon skin into an interpretable sensation of texture is mind-blowingly difficult to imagine.