Inactivation mode of sodium channels defines the different maximal firing rates of conventional versus atypical midbrain dopamine neurons
Fig 7
Explanation of Depolarization Block Mechanisms Using Square Pulses. A.
Atypical model A1. Voltage traces (top) of a spontaneously pacing model neuron in response to a 2 sec, 75 pA square current pulse, with an additional 200 ms 50 pA step (bottom) applied after the model enters depolarization block. Depolarization block is entered via a gradual decrease in spike amplitude, with a maximum firing rate of 28 Hz. An additional current step during depolarization block does not evoke additional spikes. A2. Time course of available sodium channels (purple) (C1+C2+O1). Time course of occupancy in I1 (blue), I2 (green), and their sum (red). Inset: Low available pool prevents large oscillations following entry into depolarization block. A3. dV/dt for voltage trace in A1. Oscillations rapidly fall below the spike threshold (5 V/s), and do not pass above that point in response to added current. B. Conventional cell model. As in the previous figure, the conventional model has twice the surface area as the atypical. Voltage traces (top) of a spontaneously pacing model neuron in response to a 75 pA step current with an additional 200 ms, 50 pA step following entry into depolarization block. Depolarization block occurs abruptly at 10 Hz with a large amplitude spike, but an additional spike can be evoked with additional current. B2. Time course of available sodium channels (C1+C2+O1) (purple). Occupancy in I1 (blue), I2 (green), and their sum (red) for voltage trace in B1. Inset: relative to B2, larger fractions of sodium channel remain available for spikes evoked by additional current. B3. dV/dt for voltage trace in B1. The action potential evoked by additional current has peak slope well above spike threshold.