Read Online Connectome by Sebastian Seung - Free Book Online Page B

models of neurotransmitters: glutamate
(left)
and GABA
(right)
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    On the left is the most common one, glutamate. This is best known to the public in the form of monosodium glutamate (MSG), which is used as a flavor enhancer in Chinese and other Asian cuisines. Few realize that glutamate also plays a crucial role in brain function. Shown on the right is the second most common, gamma-aminobutyric acid, or GABA for short.
    More than one hundred neurotransmitters have been discovered so far. The list sounds long. Do you ever feel overwhelmed in the liquor store, when you see the shelves stocked with so many brands of beer and wine? If you’re a creature of habit, you might buy the same one or two brands every time and serve them to your friends at every party you give. That’s what neurons do. With few exceptions, a neuron secretes the same small set of neurotransmitters—often only a single neurotransmitter—at all of its synapses. (The synapses in question are those made by a neuron onto others, not those received by a neuron.)
    Now let’s consider receptor molecules, which are much larger and more complex than neurotransmitters. Part of each molecule sticks out from the surface of the neuron, like the head and arms of a kid using an inner tube to float on water. This protrusion is the part of the receptor that senses neurotransmitter.
    A glutamate receptor senses glutamate but ignores GABA and other neurotransmitters. Likewise, a GABA receptor senses GABA but ignores other molecules. Where does this specificity come from? Think of a receptor as a lock and the neurotransmitter as a key. As we saw above, each type of neurotransmitter has a distinctive molecular shape, which is like the pattern of bumps and grooves on a key. Every type of receptor has a location called the binding site, which has a characteristic shape like the innards of a hole in a lock. If the shape of the neurotransmitter matches that of the binding site, it activates the receptor, much as the right key in the right lock opens a door.
    Once you know that the brain uses chemical signals, it’s no longer surprising that drugs can alter the mind. A drug is a molecule too, and can be shaped like a neurotransmitter. If the mimicry is faithful enough, the drug will activate receptors, much as a copy of a key can open the same lock as the original. Nicotine, the addictive chemical in cigarettes, activates receptors for the neurotransmitter called acetylcholine. Other drugs inactivate receptors, much as an inaccurate copy of a key might turn partially and jam the lock. Phencyclidine or PCP, known on the street as “angel dust” in honor of its recreational use for hallucinogenic effects, inactivates glutamate receptors.
    It’s worth pausing to consider how we usually perceive secretions. Spit. Sweat. Urine. We suppress the urge to expectorate in polite company, plug glands with antiperspirants, and flush toilets in quiet privacy. We are embarrassed by secretions, reminders of our flesh and blood. Surely they live in a world apart from entities as ethereal and refined as our thoughts. But the truth is more shocking: The mind depends on an untold number of microscopic emissions. The brain secretes thoughts!
    It may seem strange that neurons communicate with chemicals, but we humans do it too. Granted, we rely much more on language or facial expressions. But occasionally we signal each other with smells. While the message of aftershave or perfume is open to interpretation, something along the lines of “I’m sexy” or “Come hither” is a safe guess. Other animals don’t have to purchase smell in a bottle. A female dog in heat naturally secretes a chemical signal called a pheromone, which wafts through the neighborhood to bring droves of male dogs by their noses.
    Such chemical messages express desire more primitively than Shakespeare’s love sonnets. Then again, so do poems that start with “Roses