Brain complexity: how plasticity works (Introduction)

by David Turell , Thursday, June 21, 2018, 21:12 (2134 days ago) @ David Turell

It is obvious that if the brain increases its complexity by simply adding synapses it might get into an overload situation in which the neurons could not handle all the new connections. What happens is that as new synapses start or become stronger others become more quiet:

https://medicalxpress.com/news/2018-06-scientists-fundamental-brain-plasticity.html

"In a new study in Science, researchers at the Picower Institute for Learning and Memory at MIT demonstrate for the first time how this balance is struck: when one connection, called a synapse, strengthens, immediately neighboring synapses weaken based on the action of a crucial protein called Arc.

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"When one synapse goes up, within 50 micrometers there is a decrease in the strength of other synapses using a well-defined molecular mechanism."

"This finding, he said, provides an explanation of how synaptic strengthening and weakening combine in neurons to produce plasticity.

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"'I think it's quite amazing that we are able to reprogram single neurons in the intact brain and witness in the living tissue the diversity of molecular mechanisms that allows these cells to integrate new functions through synaptic plasticity," El-Boustani said.

"As the synapse for the new receptive field grew, the researchers could see under the two-photon microscope that nearby synapses also shrank.

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"Having seen the new rule in effect, the researchers were still eager to understand how neurons obey it. They used a chemical tag to watch how key "AMPA" receptors changed in the synapses and saw that synaptic enlargement and strengthening correlated with more AMPA receptor expression while shrinking and weakening correlated with less AMPA receptor expression.

"The protein Arc regulates AMPA receptor expression, so the team realized they had to track Arc to fully understand what was going on. The problem, Sur said, is that no one had ever done that before in the brain of a live, behaving animal. So the team reached out to co-authors at the Kyoto University Graduate School of Medicine and the University of Tokyo, who invented a chemical tag that could do so.

"Using the tag, the team could see that the strengthening synapses were surrounded with weakened synapses that had enriched Arc expression. Synapses with reduced amount of Arc were able to express more AMPA receptors whereas increased Arc in neighboring spines caused those synapses to express less AMPA receptors.

"'We think Arc maintains a balance of synaptic resources," Ip said. "If something goes up, something must go down. That's the major role of Arc."

"Sur said the study therefore solves a mystery of Arc: No one before had understood why Arc seemed to be upregulated in dendrites undergoing synaptic plasticity, even though it acts to weaken synapses, but now the answer was clear. Strengthening synapses increase Arc to weaken their neighbors.

"Sur added that the rule helps explain how learning and memory might work at the individual neuron level because it shows how a neuron adjusts to the repeated simulation of another."

Comment: New brain plasticity is thus controlled from an area becoming overloaded. As all biological systems must exert total control, this system must also have feedback loops to adjust the activity levels within proper functional limits. Such a system must be designed.