from an anatomy student
As some of you may know, I am in medical school.
It just so happens we begin our dissection of the lower limb this week - and let me just say that studying anatomy of common tickle spots is weird for me. Hey, it at least helps me study.
Anyway, without inundating y'all with jargon - let me say that the sole of the foot can be divided into 2 parts - each covered by branches from different nerves (if I begin to lose people, please indulge me - this is really cool for a ticklephile in anatomy).
Basically, tickling the arch of the foot is kinda like hitting 2 nerve switches. In short, there are a lot of nerve endings there (I know, thanks for the obvious).
I think the arch of the foot is the most ticklish part, since that is the part of the foot that remains off the ground and not in general contact with anything. The same thing with the armpit. Areas like that that are free from physical contact have "fresh" nerves that are still sensitized.
I could go on, but you get my drift. See you for next week's lecture, which will hopefully include a hands-on demonstration (models can still sign-up).
Hound