Dynamical Systems in Neuroscience:

Introduction 3Figure 1.2: What makes a neuron fire?giant axons, which we study in the next chapter. The irony is that the Hodgkin-Huxleymodel does not have a well-defined threshold, it does not fire all-or-none spikes, andit is not an integrator, but a resonator, i.e., it prefers inputs having certain frequenciesthat resonate with the frequency of subthreshold oscillations of the neuron. Weconsider these and other properties in detail in this book.1.1.2 Where is the threshold?Much effort has been spent trying to determine experimentally the firing thresholdsof neurons. Here, we challenge the classical view of a threshold. Let us consider twotypical experiments, depicted in Fig. 1.4, that are designed to measure the threshold.On the left, we shock a cortical neuron, i.e., we inject brief but strong pulses of currentof various amplitudes to depolarize the membrane potential to various values. Is therea clear-cut voltage value, as in Fig. 1.3, above which the neuron fires but below whichno spikes occur? If you find one, let the author know! In Fig. 1.4b we inject long butweak pulses of current of various amplitudes, which result in slow depolarization anda spike. The firing threshold, if it exists, must be somewhere in the shaded region, butwhere? Where does the slow depolarization end and the spike start? Is it meaningfulto talk about firing thresholds at all?all-or-nonespikesthresholdrestingno spikeFigure 1.3: The concept of a firing threshold.