Complex sentence: Other brain components which have been proposed to play a role are the mitochondria, and a sodium ATPase pump, causing cyclical periods of poor neuron firing (depression) and hypersensitive neuron firing (mania).
Split into simpler sentences: Other brain components which have been proposed to play a role are the mitochondria and a sodium ATPase pump. Alterations to these components are believed to cause cyclical periods of poor neuron firing (depression) and hypersensitive neuron firing (mania).