My reading is that under most circumstances the life tenant pays interest on a mortgage, while the remaindermen pay that portion of the mortgage which goes toward the principal.

That can change, with the life tenant being responsible for both interest and principal, if the life tenant is the mortgagor. However, if that is the case, there are still circumstances where the life tenant may have recourse to recover payments toward principal - such when the life tenant only undertook the mortgage obligation as an accommodation to the deceased.

Clear as mud?