How to Build a Human Brain, in 7 Easy Steps
STEP 1: UNDERSTAND THE BUILDING BLOCKS
Before you get started, pause for a second to appreciate the complex architecture of just one of them. Neurobiologist Bernd Knoll at the University of Tubingen in Germany and his collaborators used electron microscopy to picture this neuron’s cobweb-like cytoskeleton (its interior scaffolding).
The cytoskeleton is made of strings of proteins that constantly stretch and shrink as the neuron sends out projections toward other neurons, making and breaking connections. This neuron is from mouse hippocampus, a part of the brain important in memory, but the ones in the human brain are constructed much the same way.
STEP 2: PAIR THEM UP
Neurons use their long arms to reach out and almost touch other neurons. Those arms nestle extremely close together—just 20 nanometers apart—so make sure your hands are steady before trying to put the pieces of your brain together.
Across the tiny spaces between the cells, known as synapses, molecules called neurotransmitters ferry messages back and forth. Here, a rat neuron from the movement-coordinating cerebellum was dyed green and caught in the act of communicating with another neuron (shown in red). Each cell has one axon (the green tail snaking from the left side of the image), which transmits impulses to the dendrites (the candelabra-like branches) of another.
STEP 3: NETWORK THEM TOGETHER
Now the instructions for building your brain get more complicated, since you will be rigging up the wiring that permits complex conversations involving billions of brain cells.
This image shows one network of neurons from the cerebellum of a mouse. The brilliant white blobs are Purkinje cells, large neurons that allow the animal to coordinate complex movements. The cells’ dendrites form the feathery outer fringe, and the axons gathered in the middle dive into the depths of the cerebellum to send signals within.
STEP 4: INSTALL THE PLUMBING
To carry blood throughout the brain, you will need pipes of all diameters and lengths. In this 1-millimeter-square view of the cerebral cortex of a living rat, large blood vessels along the surface lead to capillaries that extend deep into the brain.
To create this image, a fluorescent sugar molecule was injected into the rat’s bloodstream; here the blood-filled vessels appear white. Neuroscientist Andy Shih of the University of California at San Diego uses this imaging technique to measure vessel diameters and track blood flow rates, which change constantly depending on the needs of the local neurons.
STEP 5: BRING IN SUPPORTS
You have connected all your neurons, but you still need billions of “brain glue” cells—the neuroglia, which outnumber neurons by around 10 to 1. In recent years scientists have begun to recognize the importance of these cells, especially the enigmatic ones called astrocytes.
By inducing cells that line the capillaries to keep a tight seal, astrocytes maintain the blood-brain barrier, which protects the brain from many circulating molecules.
Astrocytes also form their own long-distance communication networks by “talking” via waves of calcium ions, and, like neurons, they can receive and release neurotransmitters.
STEP 6: FINE-TUNE THE CIRCUITS
This is what your brain looks like when you have all the neurons in the right places. Here, cells in different layers of the visual cortex show up as brilliant pink, yellow, and blue, depending on how deep they are in the brain (the colors are artificial).
But don’t get too attached to this arrangement. In order to grow and learn, this brain is going to have to change. To be the marvelous organ of adaptability that it is, the brain must constantly remodel itself, storing new memories and mastering new lessons.
STEP 7: ADD NEW PARTS
Once your brain is up and running, you won’t be stuck with the same old neurons. Your brain will keep generating some shiny new ones. Even in adulthood, brains keep churning out new neurons, either to replace old cells or to add additional firepower.
Two parts of the brain are especially fecund: the dentate gyrus (a region involved in spatial memory) and the olfactory bulb (which sits right above the nasal cavities). A cross-section of the olfactory bulb of a mouse is shown here; relatively youthful cells, born during the animal’s adulthood, glow green.