Figure 2. SCA3 Purkinje neurons fail to sustain spontaneous repetitive firing due to depolarization block. A, Summary of the proportion of wild-type SCA3 tg/− and SCA3 tg/tg neurons exhibiting each pattern of spontaneous firing. Numbers above the bars represent the number of neurons exhibiting each pattern of firing. B, Wild-type Purkinje neurons exhibit tonic repetitive firing (average frequency, 35.8 ± 3.1 Hz; n = 21). C–E, In contrast, SCA3 tg/− neurons exhibit tonic repetitive firing (average firing frequency, 46.3 ± 4.7 Hz; n = 20; p = 0.065; C), membrane depolarization with a loss of repetitive firing (membrane potential, −48.5 ± 1.4 mV; n = 19; D), or burst firing (E). F, The passive membrane properties of SCA3 tg/− Purkinje neurons are similar to wild-type neurons. SCA3 tg/tg neurons exhibit a lower capacitance, indicating a reduction in cell size and/or dendritic arborization (*p < 0.01). G, H, Wild-type neurons (G) and depolarized SCA3 tg/−neurons (H) can sustain similar high rates of repetitive firing in response to depolarizing current injection from a negative holding potential. I, Injection of depolarizing current evokes more spikes in SCA3 tg/− neurons than in wild-type neurons. SCA3 tg/− neurons that have retained repetitive firing at rest also display more spikes in response to similar amounts of injected current compared to wild-type neurons. J, A smaller amount of injected depolarizing current in SCA3 tg/− neurons is needed to induce depolarization block (*p < 0.001).