This month's episode features Mark Humphries, author of "The Spike: An Epic Journey Through the Brain in 2.1 Seconds." We explore how the brain uses electrical signaling in surprising ways, beginning with a look at how an action potential (aka. "the spike") is produced. Then we consider what Humphries calls "the dark neurons," which are neurons that rarely generate spikes. We explore their role and also how it relates to the brain's spontaneous activity.