It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind by David A. Rosenbaum Read Free Book Online Page A

higher if the axon is coated with myelin (a fatty material) than if it is not.
    Myelin takes time to form over the course of development. Not until adolescence is its formation complete in human beings, but by the time of adolescence, myelination isn’t the only neural process that gets completed. Unnecessary connections between neurons are pruned away. Pruning begins in infancy and is completed in the teen years. Pruning can be seen as straightforward example of natural selection. 3
Synapses
    Until the late 1800s, it was not known whether the nervous system is a kind of diffuse cotton ball or a honeycomb of distinct cells with gaps between them. Thanks to the work of the late nineteenth- and early-twentieth-century Spanish physiologist, Santiago Ramón y Cajal, we now know that neurons are separated by tiny spaces called
synapses
, a term coined by a British physiologist, Charles Sherrington, who, like Ramón y Cajal, won a Nobel Prize for his work on neurophysiology. 4
    Synapses are important because of what goes on in and around them. At synaptic junctions, chemicals are emitted by presynaptic neurons and are picked up by postsynaptic neurons or muscles. The chemicals that are released into the synapse—
neurotransmitters
—are molecules that can affect the postsynaptic membranes. Postsynaptic neurons that have suitable receptors for a neurotransmitter may become more or less excited when they take in the neurotransmitter coming to them. Muscles may likewise contract if they are in a state of readiness to do so and if the neurotransmitter knocking on their doors is one the muscles accept.
    Neurons that take up neurotransmitters can respond in either of two ways—by becoming
more
likely to fire or
less
likely to fire. If the neuron is
more
likely to fire upon receipt of a neurotransmitter, the neurotransmitter is said to have an
excitatory
effect on the neuron. If the neuron is
less
likely to fire upon receipt of a neurotransmitter, the neurotransmitter is said to have an
inhibitory
effect on the neuron. 5
    Neurotransmitters produced by any given neuron have different effects depending on the neurons to which they project. Which neurons produce which neurotransmitters, which neurons accept which neurotransmitters, and whether a neuron gets more or less excited when it takes in a neurotransmitter are matters are of no concern here. More relevant are the effects of neurons being activated or deactivated.
    Active neurons suck up more oxygen and glucose than do less active neurons. Active neurons are also more likely than less active neurons to form liaisons with other neurons. The more often two nearby neurons fire in close temporal proximity, the tighter the link between them. Neuroscientists express this principle with a catchy phrase: “Neurons that fire together wire together.” 6
How to Survive If You’re a Neuron
    Because neurons that fire together wire together, it’s good for neurons to team up with other neurons. Teaming up is important because the neuralecosystem has a limited supply of oxygen, glucose, and other needed materials. Individual neurons benefit from joining up with other “like-minded” neurons—that is, neurons tuned to similar functional events—just as we humans tend to do better if we join with others than if we go it alone. 7
    I have just emphasized cooperation among neurons. In response, you might say, “Now wait a minute. If it’s a jungle in there, shouldn’t neurons just
inhibit
their neighbors as much as possible? Shouldn’t neurons be as aggressive as possible all the time, suppressing their neural neighbors as fiercely as they can? Why excite your neural neighbors if it’s a dog-eat-dog world in there?”
    Neurons need to be on good terms with other neurons with whom they have connections. If you’re a neuron, you need input to get activated. It won’t serve you well over the long run to deflate all the neurons with whom you have ties because, through their activity, they