Striking electron microscope
pictures from inside the brains of mice suggest what happens in our own brain
every day: Our synapses – the junctions between nerve cells – grow strong and
large during the stimulation of daytime, then shrink by nearly 20 percent while
we sleep, creating room for more growth and learning the next day. When a
synapse is repeatedly activated during waking, it grows in strength, and this
growth is believed to be important for learning and memory. According to “synaptic
homeostasis hypothesis” (SHY), however, this growth needs to be balanced to
avoid the saturation of synapses and the obliteration of neural signaling and
memories. Sleep is believed to be the best time for this process of
renormalization, since when asleep we pay much less attention to the external
world and are free from the here and now. When synapses get stronger and more
effective they also become bigger, and conversely they shrink when they weaken.
Thus, researchers reasoned that a direct test of SHY was to determine whether
the size of synapses changes between sleep and wake. To do so, they used a
method with extremely high spatial resolution called serial scanning 3D
electron microscopy.
The research itself was a massive
undertaking, with many research specialists working for four years to
photograph, reconstruct, and analyze two areas of cerebral cortex in the mouse
brain. They were able to reconstruct 6,920 synapses and measure their size. The
team deliberately did not know whether they were analyzing the brain cells of a
well-rested mouse or one that had been awake. When they finally broke the code
and correlated the measurements with the amount of sleep the mice had during
the six to eight hours before the image was taken, they found that a few hours
of sleep led on average to an 18 percent decrease in the size of the synapses.
These changes occurred in both areas of the cerebral cortex and were
proportional to the size of the synapses. The scaling occurred in about 80
percent of the synapses but spared the largest ones, which may be associated
with the most stable memory traces. This shows, in unequivocal ultrastructural
terms, that the balance of synaptic size and strength is upset by wake and
restored by sleep. It is remarkable that the vast majority of synapses in the
cortex undergo such a large change in size over just a few hours of wake and
sleep.
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