Before revision: The Fitzhugh-Nagumo system is a prototypical model for simulating different modes of neuronal spiking activity. We introduce a minimal model for synaptic integration  of action potentials and explore the basic properties of postsynaptic neurons thus defined.
After revision: The Fitzhugh-Nagumo system is a prototypical model for simulating different modes of neuronal spiking behaviour. We study the responses of FitzHugh-Nagumo neurons to rectangular current pulses and obtain the corresponding threshold curves for excitation. We then show how the geometry of threshold curves allows us to devise a simple model for synaptic integration of action potentials and explore the basic properties of postsynaptic neurons thus defined.