  Current theoretical approaches to manganites mainly stem from magnetic
framework, in which the electronic transport is thought to be spin-dependent
and the double exchange mechanism plays a core role. However, quite a number of
experimental observations can yet not be reasonably explained. For example,
multiplicate insulator-metal transitions and resistivity reduction induced by
perturbations other than magnetic field, such as electric current, are not well
understood. A comprehensive analysis on earlier extensive studies is performed
and two types of origins for resistivity change are highlighted. Besides the
insulated-to-metallic transition induced by external field such as magnetic
field, the insulated-to-insulated transition induced extrinsically is even a
more important source for the colossal resistivity change. We propose an
extended framework for the electronic transport of manganites, in which the
contribution of charge degree of freedom is given a special priority.
