The shape of the tree, and how many trees, tells us a lot about what that neuron is trying to do. Indeed, historically, it was often how we could tell neurons apart. Our trip from the retina is about to land us out in the compact, starburst tree of the first neuron in the cortex.2 Below us, the poster-child neuron of the cortex, the pyramidal cell of layer five with its two types of dendritic trees—one sticking out of the top, a single long slender stem stretching up almost to the cortical surface, the rest sticking out below the body, fat and squat (figure 3.1). Above us, in layers two and
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