Nature: Challenge traditional cognition! The characteristic selectivity of neurons comes from the total number of activated synapses

19, 2020 // --- A common metaphor used to describe the brain is that it consists of tiny interconnected computers.
Each of these computers, or neurons, processes and transposes activity from thousands of other neurons, creating complex networks that enable us to perceive our surroundings, make decisions, and guide our actions.
communication between neurons is carried out through tiny connections called synapses, each of which integrates the activity of these synapses to form a single output signal.
, however, not all synapses are the same.
synapses that converge on a single neuron are different in size, but the size is related to strength: larger synapses are stronger than smaller synapses and have a greater impact on the output of neurons.
but why are some synapses stronger than others, and how does this affect the processing of incoming signals by individual neurons? A famous theory suggests an answer.
Hebbian model of neurotranscing development suggests that the intensity of synapses between two neurons is determined by the similarity of their activity.
synapses between neurons that are highly co-active are stronger than those that do not active together very often.
this relationship provides a clear prediction of the diversity of synhap sizes present in mature neurons.
large synapses are produced in neurons with very similar reaction characteristics, which play a leading role in determining the output of neurons.
, small synapses are produced in neurons with less similar reaction characteristics and have less effect on neurons.
although there is some evidence to support this model, direct validation requires measuring the activity, size, and output signals of individual synapses and their neurons, which has been difficult to achieve with current technology.
now, researchers from the Max Planck Florida Institute of Neuroscience have reported for the first time the results of a new method that allows them to make these measurements.
their study challenged the predictions of the Hebbian model, confirming that the size of the synapses was independent of the similarity of the reactions and showing that the neuroreactive characteristics reflected the total number of active synapses (weak synapses and strong synapses).
results were published online December 16, 2020 in the journal Nature under the title "Cortical response selectivity derives from strength in numbers of synapses".
image from Nature, 2020, doi:10.1038/s41586-020-03044-3.
Benjamin Scholl, co-author of the paper and a postdoctoral fellow at david Fitzpatrick's Lab at the Max Planck Florida Institute of Neuroscience, was inspired to explore the problem in the visual cortical layer.
in the visual cortical layer, individual neurons are highly selective in their responses to different features in the visual scene, such as the direction of the edges or the direction of moving objects.
phenomenon, known as feature selectivity, is caused by the integration of thousands of synapses that transmit different signals, but it is not clear exactly how this happened.
Sholl explains, "Our goal is to test the hypothesis that the reaction of strong synapses closely matches the characteristic selectivity of neurons, while weak synapses do not."
to test this hypothesis, the researchers used optical microscope technology to visualize the activity of synth groups on individual neurons in real time.
, however, has a serious limitation in itself--- synapses can only be observed, not their intensity.
to measure synaptic strength, Scholl and his team worked with Dr. Naomi Kamasawa of the Max Planck Florida Institute of Neuroscience's electron microscope core facility.
, co-author of the paper and an expert on electron microscopes, explains, "Electron microscopes capture incredible synth detail images at the nanoscale, which allows us to accurately measure their structure.
structural measurements tell us how strong each synapse is.
by first examining synth activity with an optical microscope and then measuring the strength of these same synapses with an electron microscope, we know we can answer that question.
" combination of these technologies, correlated optical and electron microscopes (Correlated Light and Electron Microscopy, CLEM), allows these researchers to measure the function (feature selectivity) and structure (strength) of synactic groups from several neurons.
In the initial study, the researchers found nothing unexpected: some synapses share a common characteristic selectivity with neurons, while others are different;
when the data were put together, they were surprised to find that both strong and weak synapses exhibited a variety of functional characteristics: there was no theoretically strict relationship.
, however, by examining the activity of the entire synapse community, they realized that they had discovered the potential synapse basis for feature selectivity.
" scholl describes, "We observed a phenomenon of 'power in numbers'.
our data show that neuron characteristic selectivity comes from the total number of active synapses, including strong synapses and weak synapses.
, we found more weak synhap-driven feature selectivity, suggesting that they may have a dominant effect.
to support this, they observed that adjacent weak synapses were more often co-active, which could even enhance their effects on neurons.
these findings challenge the mainstream Hebbian model --- strong synapses to exist only between neurons with similar reaction characteristics and play a leading role in determining the characteristic selectivity of neurons---
, feature selectivity seems to come from the total number of synapses, suggesting that synaptic democracy, similar to popular voting, is the most important.
still doesn't solve the question of why some synapses are stronger than others, and the researchers are using these techniques to explore other aspects of synapses that might be important.
with neuron structural and functional measurements, we can get a more complete view of how our brains calculate information," thomas said.
CLEM technology is technology that gives us a better understanding of the basic functioning of the brain. The
study highlights the creative synergies that bring together curiosity-oriented scientists from different disciplines, providing unexpected new insights into neural circuit tissue, laying the groundwork for future progress in understanding brain dysfunction.
(Bioon.com) :1. Benjamin Scholl et al. Cortical response selectivity derives from strength in numbers of synapses. Nature, 2020, doi:10.1038/s41586-020-03044-3.2.New approach reveals structure and function of individual synapses。