Subject: How does potassium nitrate in Sensodyne toothpaste reduce tooth sensitivity

My dentist gave me some Sensodyne-type toothpaste to help reduce tooth
sensitivity.  It contains 5% potassium nitrate (KNO3), which is also called
saltpeter.  The toothpaste's web site doesn't give a straight answer as to how
it actually works.  The two explanations I've found on the web seem to be:
1. it decreases nerve activity (presumably by acting on the neurons themselves)
2. it forms a physically protective barrier to prevent fluid movement that
would otherwise activate pain nerves.
So my questions are:
A.  What's the true explanation?
B.  Why is it okay to put this on my teeth, when saltpeter has some fairly nasty
other applications (gunpowder, etc.)
C.  Various web pages mention hazards of ingesting saltpeter (gastroenteritis,
kidney disease, etc)--at what dose should I be concerned?  5% of 1 gram per use,
even though one tries to spit it out, might add up.
Thanks,
Andrew

Re: How does potassium nitrate in Sensodyne toothpaste reduce tooth sensitivity