To study propagation at the level of cardiac myocytes, the microscopic model explicitely represents individual cells. The cardiac tissue is then viewed as two separate domains: the intra-cellular and extra-cellular domains, respectively $\Omega_i$ and $\Omega_e$ separated by cellular membranes $\Gamma$.The microscopic model consists in a set of Poisson equations, one for each sub-domain $\Omega_i$ and $\Omega_e$, coupled on interfaces $\Gamma$ with nonlinear transmission conditions involving a system of ODEs. We discretize this problem in space using finite element methods, that may be conformal or non-conformal on the interface $\Gamma$. We propose and compare various time-stepping methods for solving the microscopic model.Error estimates are obtained, proving the order of the methods in time and space. This is joint work with Pr. Yves Bourgault.