John W. Cain, University of Richmond


Modern toilets and cardiac cells have been famously likened to one another because of what they have in common: excitability. In the absence of electrical stimulus currents, the voltage v across a cell membrane equilibrates to an asymptotically stable resting potential vrest, much as the toilet remains in a resting state if its handle is not pressed. If either system is stimulated too weakly (sub-threshold electrical current or pressing the handle too lightly), the system relaxes rapidly to its rest state. Super-threshold stimulation causes a dramatic response known as action potential for cardiac cells and flushing in the toilet. Both systems eventually return to rest, but require time to recover excitability – briefly pressing the toilet handle (no matter how hard) during a flush does not elicit a noticeable response. Understanding