We consider a class of excitable system whose dynamics is de- scribed by Fitzhugh-Nagumo (FN) equations. We provide a description for rigidly rotating spirals based on the fact that one of the unknowns develops abrupt jumps in some regions of the space. The core of the spiral is delim- ited by these regions. The description of the spiral is made using a mixture of asymptotic and rigorous arguments. Several open problems whose rigorous solution would provide insight in the problem are formulated. The authors were supported by the Catalan and Spanish Government grants 2009SGR345, MTM2008-06349-C03-01, MTM2011-27739-C04-03, MTM2008-03255 and MTM2010-16467