Dynamical Systems in Neuroscience:

230 Excitability50 ms 20 mV 1 ms1000 pA-60 mV0 pA 500 pAFigure 7.4: Class 3 excitability of a mesV neuron of rat brainstem (contrast withFig. 7.3).20 mV100 ms-20mV50 ms 3 mVsubthreshold oscillations700 pA100 pAFigure 7.5: Class 3 excitability of a layer 5 pyramidal neuron of rat visual cortex. Theinset shows subthreshold oscillations of membrane potential.the strength of the applied dc-current or the strength of the incoming synaptic bombardment,into the frequency of their spiking output. Class 2 neurons cannot do that.Instead, they can act as threshold elements reporting when the strength of input isabove a certain value. Both properties are important in neural computations.Hodgkin also observed that axons left in oil or sea water for long periods exhibited• Class 3 neural excitability. A single action potential is generated in responseto a pulse of current. Repetitive (tonic) spiking can be generated only forextremely strong injected currents or not at all.Two examples of Class 3 excitable systems are depicted in Fig. 7.4 and Fig. 7.5. ThemesV neuron in the figure fires a phasic spike at the onset of the pulse of current, andthen remains quiescent. Even injecting pulses as high as 1000 pA, which result in spike