Drinking soda triggers tingling on a tiny spot on my upper back... but how?
The skin is a very richly innervated organ, and minor strange sensory phenomena are not exceedingly uncommon.

The thing about sensory nerves is that they can misfire and you seem to feel something when there's nothing there. This actually happens all over our bodies all the time (mostly our brain blocks them out, like it blocks out background noises after a while): an itch for no reason whatsoever (no bug, no bite, no reason) is the most common manifestation of this phenomenon.

Sensory nerves start their journey out of the central nervous system as relatively large bundles coming off the spinal cord, and keep on branching becoming finer and finer as they reach the periphery.



since there's a lot of muscle under the skin, most of these sensory nerves have to come up through muscle.



Sometimes an inflamed bundle of muscle fibers (aka a "knot") will entrap a small branch of a sensory nerve, and the pressure on the nerve will cause it to "misfire", causing a sensory illusion, if you will. If this is the case (if it's in the exact same spot every time) and a slender nerve branch is being squeezed by a muscle knot in your back, you can do a little test on yourself: have someone run a fine sharp object (say the end of a toothpick) softly over an area starting away from the spot going through it. If the sensation changes right on that spot - going from sharp to dull, for example - and does so consistently, that's what's going on. If you have the person map the change of sensation with a pen, the spot will be about the size of a quarter or smaller.

If, however, the sensation is only associated with swallowing, and there's no change to light touch, I have no idea what's going on.

Anyone reading this should know, though, that strange neurological symptoms can be an early sign of MS.