  The reaction-diffusion system of Fitzhugh Nagumo is considered. The initial-
boundary problems with Neumann and Dirichlet conditions are analyzed. By means
of an equivalent semilinear integrodifferential equation which characterizes
several dissipative models of viscoelasticity, biology, and superconductivity,
some results on existence, uniqueness and a priori estimates are deduced both
in the linear case and in the non linear one.
