Introduction

The Fitzhugh-Nagumo Model is a a two dimensional model that describes the changes in the membrane potential and ion conductances at a point in an axon. The Fitzhugh-Nagumo equations are
x' = c*(y+x-1/3*x^3)
y' = -(x-a+b*y)/c
where x is the membrane potential and y is the recovery variable. The coefficients, a, b, and c are all positive. In our analysis of this model, we investigated the fixed points, and their stability. The model itself is discussed in the background section, what we did is covered in the analysis, and our findings are presented in the conclusion.

We found four different behaviors of solutions that occur when the parameters a, b, and c are varied. The first type is a periodic solution which represents a deficiency of calcium ions. The second is that solutions for any initial conditions approach a fixed resting potential. The third is two stable fixed points and one unstable saddle point in between them. It represents too great of a shock being applied to the nerve which causes the resting potential to rise to an excited state. This is unphysical. The last type is two spiraling fixed points which represents a second impulse occuring before the original impulse has been fully transferred. The second impulse disturbs the original one and alters its approach to its equilibrium state. If the second impulse is high enough, the behavior is similar to that in the third type, wherein the resting potential rises to an excited state. This is a possible physical solution, unlike the third type that we found.


Background
Analysis
Conclusions

Andrea Sereny
Karen J. Hirst