By demonstrating that a reduction in SCA1-KI Purkinje cell hyperexcitability is sufficient to partially rescue cell size phenotypes, these results suggest that SCA1-associated disruptions of membrane excitability may be an important driver of morphological changes in Purkinje cells; that is, hyperexcitability may be driving atrophy in the SCA1 cerebellum. Here, ATXN1 is linked to Atrophy.