One singular feature of the brain that has only become apparent over the past two decades is the astounding heterogeneity of neurons. The approximately 100,000 neurons packed below each square millimeter of cortex, an area about the size of the letter “o” on this page, are highly heterogeneous. They can be distinguished based on their location, the shape and morphology of their dendrites, the architecture of their synapses, their genetic makeup, their electrophysiologic character, and the places to which they send their axons.
