We present a class of neuronal models that emits 
a spike (fires) if a scalar quantity, the membrane
potential, satisfies a threshold condition. 
Equations use two generic functions $\epsilon$ and 
$\eta$ that describe the subthreshold behavior and 
the spike process, respectively. 
 Special cases include the leaky integrate-and-fire 
neuron and the piecewise linear FitzHugh-Nagumo model.