| 2. Inactive synapses become silent synapses |
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KOBAYASHI: Being a pediatrician, I would like to relate these insights to the issue of human development. The number of a baby's synapses sharply increases after birth, and then declines shortly afterwards. The baby is engaged in a "scrap-and-build operation", disposing of synapses. Discharged synapses are bound to disappear, aren't they? MOCHIDA: The early developmental phenomenon called "synaptic pruning", where synapses first overgrow, followed by extensive retraction is well known. Recently an interesting phenomenon that synapses become silent is also found. We regard them as "silent synapses". Some of them still retain their intact physical form, but they are no longer in use. If they are used again repeatedly, however, as we have discovered, they can sometimes be activated again. I suppose, synapses themselves do not disappear, but they are simply out of work, or out of function. Here again, the key role is held by neurotransmitters. When no neurotransmitters are released, synaptic connections deteriorate. KOBAYASHI: Do you mean that synapses simply cease to work; neither disappear nor trim off their branches, as previously assumed? MOCHIDA: One hypothesis is that they become silent. There is a cellular receptor for neurotransmitter signals called "spines" in the dendrites. Some scientists assume that the number of spines is in constant fluctuation. Another hypothesis still is that the synapses themselves disappear entirely. This may be the case during early development, but is not as prevalent in the adult brain. When you see how quickly a child acquires new words or learns to play a musical instrument and improves in skill, you cannot help wondering what is happening in the child's brain. My guess is that synaptic connections are sharply increasing in efficacy and number. KOBAYASHI: Let's take the synapses in the nervous system which are related to language: what kind of neurotransmitter is mainly in use? MOCHIDA: For 70% of the cerebral cortex, it's glutamate. KOBAYASHI: Why should glutamate, such a banal ingredient of ajinomoto, ever have come into use at all? That doesn't mean, though, that the more glutamate you eat, the more intelligent you will become (laughing). MOCHIDA: You are absolutely right (laughing). After all, glutamate is useless unless you can mobilize and activate more and more synaptic circuits to respond to stimulation. KOBAYASHI: Talking about education, I often point out that we must design spaces for education that will excite children with a joy for play and learning. Your story about neurotransmitters really convinces me that, when you are full of joy, you are driving the whole network of neurons into motion, thus preventing synapses from becoming silent. MOCHIDA: I quite agree with you in that they must be given stimulation constantly. When I say stimulation, though, I don't mean sitting at a computer to play a game. I mean a variety of stimulation. You must activate a wide variety of neurons. Stimulation doesn't work if only a small part of one's neurons are highly developed. |
