The Spike: An Epic Journey Through the Brain in 2.1 Seconds

by Mark Humphries

Our spike bursts into the first vision area of the cortex, V1, the first of the many areas dedicated to seeing that make up one-third of your entire cortex.1 Its message—about one small pixel of crumbly chocolate temptation—needs passing on upward through all these areas, combining with all other messages carried by the millions of other spikes to create the perception of “cookie.” First we have to make landfall. Your cortex is a delicately layered cake, six layers in all, five layers crammed with juicy neurons, the first, top layer bereft of them. We’re about to hit axon’s end in the fourth layer of this V1 area. Above us the packed neurons of layers two and three; above them layer one, a smattering of rare small neurons, but mostly filled with axons going elsewhere and brain cells that aren’t neurons, the glia cells, who are the scaffolding, the moppers-up, the below-stairs workforce. Below us, we can make out the larger, hulking neurons of layers five and six. Their bodies may be packed into layers, but the bulk of the neurons are not. Around us it’s a forest. A vast tree sprouts from the body of every neuron in sight. The tree’s branches are thin but divide, ramify, contort. They fill a volume that dwarfs the neuron’s body. This tree—the dendrite—is how a neuron collects its inputs from other neurons and funnels them all toward its body. Axon terminals from seemingly countless other neurons pepper the gnarled trees of neurons all about us.