The quasipotential, computed using the OUM, is shown in Fig. \ref{fig:type1a_qp}. As there are two phases to an excitable event, there are two regions of the quasipotential, separated by the threshold. Around the stable fixed point is a potential well from which the trajectory must escape during the initiation phase. Around the unstable fixed point, to the right of the threshold, the quasipotential forms a horseshoe-canyon-like shape (see Fig. \ref{fig:type1a_qp_2}), the bottom of which lies flat along the unstable manifold (see green curve in Fig. \ref{fig:type1a_pp2}). Hence, the most probable MPP for the excitation phase is the deterministic unstable manifold.

While the unstable manifold wraps around the unstable fixed point only once, a stochastic trajectory can rotate around the unstable fixed point during the excitation phase, prolonging the action potential. The situation can be more pronounced if there is a stable limit cycle surrounding the unstable fixed point as shown in the next section.