Fifty years ago, Fitzugh introduced a phase portrait that became famous for a
twofold reason: it captured in a physiological way the qualitative behavior of
Hodgkin-Huxley model and it revealed the power of simple dynamical models to
unfold complex firing patterns. To date, in spite of the enormous progresses in
qualitative and quantitative neural modeling, this phase portrait has remained
the core picture of neuronal excitability. Yet, a major difference between the
neurophysiology of 1961 and of 2011 is the recognition of the prominent role of
calcium channels in firing mechanisms. We show that including this extra
current in Hodgkin-Huxley dynamics leads to a revision of Fitzugh-Nagumo phase
portrait that affects in a fundamental way the reduced modeling of neural
excitability. The revisited model considerably enlarges the modeling power of
the original one. In particular, it captures essential electrophysiological
signatures that otherwise require non-physiological alteration or considerable
complexication of the classical model. As a basic illustration, the new model
is shown to highlight a core dynamical mechanism by which the calcium
conductance controls the two distinct firing modes of thalamocortical neurons