The propagation of the electrical signal in the heart can be modelled by the monodomain equation, which is a simplification of the more accurate bidomain equations. This simplification is usually made for computational purposes. The monodomain equation is a reaction-diffusion partial differential equation.
A derivation of the monodomain equation can be found on http://www.uio.no/studier/emner/matnat/ifi/INF5610/h07/undervisningsmateriale/review_part6.pdf . I have had a quick look and it seems this derivation is clear.
Reaction-diffusion equation have been applied in many areas of specialization for example, in developmental biology (Ref Alan Turing Work), neuroscience (Ref Hodgkin-Huxley work on Action Potential Propagation), Calcium dynamics (Ref James Sneyd)