Silence for Thought: The Private Inner Networks of the Human Brain

Summary: Human cortical networks have developed a new neural network that relies on abundant connections between inhibitory interneurons.

source: Max Planck Institute

Analysis of the human brain is a major goal of neuroscience. However, for methodological reasons, research has largely focused on model organisms, particularly mice.

Now, neuroscientists have gained new insights into human neural circuits using tissue obtained from neurosurgical interventions. 3D electron microscopy data revealed a new expanded network of endogenous neurons in humans compared to the mouse.

The discovery of this prominent retinal component in the human cerebral cortex encourages more detailed analysis of its function in health and disease.

At first glance, mouse and human brains are surprisingly similar: the neurons that make up our brains have very similar shapes and properties, the molecular mechanisms of electrical excitation are highly conserved, and many biophysical phenomena found in other species also seem to apply to human brains.

asks Moritz Helmsteder, director at the Max Planck Institute for Brain Research (Frankfurt) who led the new study published June 23 in the journal. Sciences.

By analyzing neural networks in mice, monkeys and humans and mapping their complete structures in biopsies of brain tissue, so-called neural networks, Helmsteder and his team discovered that human cortical networks have developed a new type of neural network that is essentially absent from mice. This neural network relies on abundant connections between inhibitory interneurons.

Using biopsies from neurosurgical interventions, conducted by Hanno neurosurgeon Sebastian Meyer and his team at the University of Munich, the researchers applied a 3D electron microscope to map nearly one million synapses in human brain samples.

Their data, in humans, revealed an unexpected bias for the interneurons (enriched in humans) connecting to each other, while the innervation (synaptic connections) of the main neurons remained largely similar.

“This points us to nearly ten times the expansion of a network of interneurons to the interneuron network,” says Sahil Lumba, one of the lead authors of the studies.

“Inner neurons make about a quarter to a third of cortical neurons that behave in a very strange way: they are so active, however, that they don’t activate the other neurons, instead silencing them. Just like the kindergartners, or the keepers in the museum: Their very strenuous and energy-intensive activity is keeping others peaceful and calm,” explains Helmsteder.

Networks of human neurons, mapped from different parts of the cerebral cortex. A concomic comparison with the mouse revealed significantly expanded intrinsic neural networks in humans. Credit: Loomba, Helmstaedter, MPI Brain Research; Loomba et al., Science

“Now imagine a room full of museum guards, all silencing each other. This is what the human mind has developed!”

But what could this mean? Theoretical work suggested that such networks of silencers could extend the time during which recent events could be retained in a neural network: expanding working memory.

“In fact, it is very plausible that longer working memory can help you deal with more complex tasks, and increase your ability to think,” Helmstedter says.

The new discovery marks the first clear retinal innovation in humans that deserves more intensive study.

He adds: “It can also be the site of pathological changes, and should be studied in the context of neuropsychiatric disorders. And last but not least: none of the major AI methods today use intranet neural networks.”

About this research in Neuroscience News

author: Irina Epstein
source: Max Planck Institute
Contact: Irina Epstein – Max Planck Institute
picture: Image credited to Loomba, Helmstaedter, MPI for Brain Research; Loomba et al., Science

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This is a schematic diagram of the study

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Concomic comparison of mouse and human coretex” by Moritz Helmstaedter et al. Sciences


Concomic comparison of mouse and human cortex

The human cerebral cortex contains 1,000 times more neurons than the mouse cerebral cortex, but possible differences in synaptic circuits between these species are still poorly understood.

We used 3D electron microscopy of rat, monkey macaque and human cortex samples to study cell type composition and synaptic circuit structure.

The 2.5-fold increase in intrinsic neurons in humans compared to mouse was compensated for by altered axonal connectivity probabilities, and thus did not result in a commensurate increase in the balance of inhibitory versus excitatory inputs on human pyramidal cells.

Instead, increased inhibition created an expanded interneuron-to-interneuron network, driven by an expansion of interneuron-targeting types and an increase in interneuron selectivity.

These constitute major neural network changes in the human cortex.