The sinus node as a nonlinear dynamic system.

The Sinus Node (SN) of the heart is the natural pacemaker driving electrical impulses into the atria, an ordinary excitable medium. In order to avoid wave interference, the SN operation must be of such a nature that no reflections from the atria should occur. It is shown that such a behavior is achieved when the SN operates in a nonlinear dynamical regime situated away from relaxation.