  The bidomain system of degenerate reaction-diffusion equations is a
well-established spatial model of electrical activity in cardiac tissue, with
"reaction" linked to the cellular action potential and "diffusion" representing
current flow between cells. The purpose of this paper is to introduce a
"stochastically forced" version of the bidomain model that accounts for various
random effects. We establish the existence of martingale (probabilistic weak)
solutions to the stochastic bidomain model. The result is proved by means of an
auxiliary nondegenerate system and the Faedo-Galerkin method. To prove
convergence of the approximate solutions, we use the stochastic compactness
method and Skorokhod-Jakubowski a.s.~representations. Finally, via a pathwise
uniqueness result, we conclude that the martingale solutions are pathwise
(i.e., probabilistic strong) solutions.
