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Why We Remember Important Things And Forget Trivia: Neuron's Synapses Remodel
Only when the transmission terminals (on the red cells) and the receiver
stations (on the green cells) are in the right proportion to each other can
communication actually take place in the brain. (Credit: Max Planck Institute
of Neurobiology / Nägerl)
ScienceDaily (Dec. 3, 2008) — Where would we be without our ability to remember
important information or, for that matter, to forget irrelevant details? Thanks
to the flexibility of the nerve cell's communication units, called synapses, we
are good at both. Up to now, only the receiving side of a synapse was believed
to play an active role in this reorganization of the brain, which is thought to
underlie our ability to learn but also to forget.
An incorrect assumption, as scientists at the Max Planck Institute of
Neurobiology in Martinsried could now show. In the scientific journal Neuron,
they report that the neurotransmitter-releasing part of a synapse dramatically
remodels itself in response to electrical stimulation. It may thus make a
decisive contribution to the adaptability of the brain to ever-changing
Communication is the be-all and end-all of the brain. Every one of the hundred
billion nerve cells that comprise our brain is a master of data exchange, with
contacts to thousands of neighbouring cells. At these points of contact, known
as synapses, the neuronal information flows along a one-way channel; from the
upstream cell to the downstream cell. The brain can deal with its complicated
tasks only when the nerve cells manage to exchange information at the right
time and place via their synapses.
It therefore comes as no surprise that one of the most outstanding attributes
of the brain is its great adaptability. This is due to the versatility of the
synapses, which, depending on whether they are required or not, can proliferate
or are pruned accordingly. Most scientists are of the opinion that this
flexible exchange of information is what makes learning and memory possible in
the first place.
The two sides of information transmission
The receiver side of the points of contact, the spines, plays an active role in
the assembly and break-down of new synapses. The more information to be
processed, the more receiver stations the nerve cell will set up. New spines
grow towards neighbouring cells to form new synapses. If the flow of
information weakens, the synapses disappear and the spines can regress. By
comparison, the other side of the synapse, the transmitter unit, also known as
bouton, was believed to play only a passive role in the formation of synapses.
However, this presumption turned out to be false, as scientists at the Max
Planck Institute of Neurobiology have now shown. They are the first to
successfully observe both the receiver side and the transmitter terminal of a
synapse over an extended period of time. This involved tagging a number of
nerve cells with a red fluorescent dye and labelling the connected cells in
green. Using a high-resolution two-photon microscope, changes on both sides
could be observed in time-lapse sequences.
It soon became clear that the transmitter unit of a synapse played a
considerably more active role in the assembly and disintegration of the synapse
than hitherto assumed. Once the flow of information to be passed on by a cell
is reduced, many of the meanwhile superfluous transmitter stations are broken
down. Furthermore, since this novel experimental approach enabled them to watch
the contacts between boutons and spines breaking down directly under the
microscope, the scientists were able to verify that the reduction in the number
of spines does, in fact, result in the loss of synapses.
The brain's reorganization is unexpectedly complex
"What is particularly exciting is that, all in all, the number of transmitter
terminals remains constant", project leader Valentin Nägerl explains. While the
number of synapses is reduced when the flow of information weakens, new
transmitter terminals emerge elsewhere in a seemingly balanced fashion. Since
only those cells that originally communicated with each other were tagged, the
scientists do not know whether the new transmitters pass the information on to
nerve cells that were hitherto not involved in the communication. "Perhaps the
cells form new synapses to inhibitory nerve cells, which would reduce the
transmission of synaptic information even more", Nadine Becker speculates on
her results. The scientists now aim to investigate precisely this possibility
by also visualizing synapses formed with inhibitory neurons. One thing is for
certain: The processing of information is not exclusive to the receiver cell.
The transmitter cell reacts actively
to the situation at hand and therefore plays an important role in our ability
to learn and remember things.
1. Nadine Becker, Corette Wierenga, Rosalina Fonseca, Tobias Bonhoeffer, U.
Valentin Nägerl. LTD induction causes morphological changes in presynaptic
boutons and reduces their contacts with spines. Neuron, November 26, 2008
Adapted from materials provided by Max-Planck-Gesellschaft.
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Max-Planck-Gesellschaft (2008, December 3). Why We Remember Important Things
And Forget Trivia: Neuron's Synapses Remodel Themselves. ScienceDaily.
Retrieved December 3, 2008, from http://www.sciencedaily.com
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