The causes of most neurodegenerative diseases, including sporadic Alzheimer's disease (AD), remain enigmatic. There is, however, increasing evidence implicating mitochondrial dysfunction resulting from deafferentiation of disconnected neural circuits in the pathogenesis of energy deficit in AD. The patterns of reduced expression of both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) encoded genes is consistent with a physiological down-regulation of the mitochondrial respiratory chain in response to reduced neuronal activity. On the other hand, the role(s) of somatic cell or maternally inherited mtDNA mutations in the pathogenesis of mitochondrial dysfunction in AD are still controversial.