English: The nonlinear relation F(V) between membranevoltage and input current of neurons can be extracted from experimental data by injecting a time-dependent input current I(t) and measuring the voltage and the voltage changes, following L. Badel et al., J. Neurophysiology 2008. Effective nonlinearity (blue curve) and data points (black squares) are schematically redrawn. Below and around the resting potential, the curve F(V) is linear, whereas close to the spike initation voltage it is exponential.