One of the best ways to understand a machine is to build it yourself. What if you wanted to get to know your brain that way, from the bottom up? It sounds like a tall order, yet the real brain does this automatically.
If you really want to know how to build a brain, take a lesson from the master: Just sit back and watch how nature herself does it.
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.
This article is a sample from DISCOVER's special issue on the brain. The issue will be on sale through December 28, only on newsstands.
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.
Michael Hausser and Beverley Clark at University College London acquired this image as part of their research on how cells transform signals from other neurons into plans for sending on more signals.
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.
Early in postnatal development, before this bouquet of cells makes connections with other types of neurons, the cells engage in cross-talk among themselves, Michael Hausser has found. The cells send synchronized waves of electrical impulses back and forth, almost like a dress rehearsal to help them learn how to hook up with other parts of the brain.
Without nature's innate processes to guide you, your wiring job would be very challenging--even if you were an experienced electrician.
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.
He has found that when blood flow is compromised--as by a stroke--nearby vessels pick up the slack, helping brain tissue recover.
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.
Astrocytes in the human cerebral cortex are so big and complex, compared with equivalent cells in other mammals, that some researchers think they may hold a key to our cleverness.
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.
This image allows researchers at the Picower Institute for Learning and Memory at MIT to observe that fine-tuning process in action. Every few days they take another look at the same patch of visual cortex of a living mouse (0.5 millimeter square), observing which neurons remain in close contact and noticing where a cell has pulled back from its old neighbors to make new contacts.
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.
Without fresh cells, the brain runs down. Ryoichiro Kageyama, Itaru Imayoshi, and their colleagues at Kyoto University in Japan found that if they destroyed a mouse's new neurons, the animal had problems retaining certain types of memories--for example, how to navigate a maze.
Your DIY brain doesn't work quite right? Maybe you have a wiring problem. For troubleshooting help, check out this image, which shows that the brain is a lot more complicated than anatomical diagrams with neat labels.
In this image--created through high-angular resolution diffusion imaging (HARDI)--the front of the brain is on the right. Bundles of fibers appear as long cables. Connections between the eyes (toward the right edge of the slice) and the visual cortex at the back (left edge) of the brain show up prominently in green.
Paul Thompson of the University of California at Los Angeles, whose group created this image, expects that imaging techniques like this will be useful for discovering abnormal connectivity associated with neurological disorders.
In the meantime, you can use his picture as a circuit diagram for debugging your new brain. Good luck; you'll need it.
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