What distinguishes our brain from that of animals? To answer this question, researchers compared the networks of nerve cells in the brains of mice, humans and monkeys. Accordingly, the human cerebral cortex has a surprising peculiarity: it is criss-crossed by a network of inhibitory nerve cells that is ten times more pronounced than in mice. The exact functioning of this network is still unclear. The researchers suspect, however, that it could be precisely this built-in brake that enables us to hold and track thoughts longer.
With its approximately 86 billion nerve cells, the human brain forms a complex network that has only been understood to a limited extent. For ethical and practical reasons, scientists often turn to the brains of model organisms, especially mice, to learn more about how the brain works. At the molecular level, the brains of mice and humans are largely similar. They have the same types of ion channels and conduct electrical stimuli by the same mechanisms. But does this also apply to the networks between the nerve cells?
Differences between human and mouse brains
A team led by Sahil Loomba from the Max Planck Institute for Brain Research in Frankfurt has now dealt with this question. “Until now, it was simply assumed that the networks between the nerve cells in humans are structured similarly to those in mice,” the researchers explain. “In order to check this, however, comparative studies of the nerve cell connections with a resolution at the synapse level are required.”
Loomba and his team have now carried out such a study. To do this, they use biopsies from the human cerebral cortex that were taken during medically necessary neurosurgical interventions in brain tumor patients. The samples examined comprised only healthy brain tissue. With the help of three-dimensional electron microscopy, the researchers mapped around a million synapses and their function in the tissue samples. For comparison, they examined samples from the cerebral cortex of mice and macaques in the same way.
Calmers calm each other down
The result: Compared to mice, the connections between so-called inhibitory interneurons are much more pronounced in humans and macaques. “Inhibitory interneurons make up around a quarter to a third of the nerve cells in the human cortex, and they have an amazing effect: they are highly electrically active themselves, but they don’t stimulate other nerve cells, but rather inhibit their activity,” explains Loomba’s colleague Moritz Helmstaedter.
He compares the activity of these nerve cells with the tasks of educators in kindergarten or stewards in football stadiums or museums: “Their very strenuous and energy-consuming work is aimed at calming others down.” calm. The network analyzes show that the connections to the large pyramidal cells, the main excitatory neurons of the cerebral cortex, are only slightly stronger in humans than in mice. On the other hand, the inhibitory interneurons are ten times more interconnected. “Imagine a room full of museum attendants, a stadium full of football stewards, all calming each other down,” describes Helmstaedter. “The human brain developed this type of network.”
© MPI for Brain Research/ Helmstaedter Lab
meaning not yet clarified
The exact function and importance of these inhibitory networks is not yet clear. “There is theoretical evidence that they lead to longer retention of sensory impressions or ‘thoughts’, i.e. they can lengthen the working memory,” says Helmstaedter. “This allows far-reaching speculation: Are these novel networks the basis of extravagant thinking? A thought, an idea to be able to keep longer in order to use it as an object of further thinking?”
Further studies are needed to clarify such questions. Their results could have important implications for mental illness, but also advance the development of artificial intelligence. “The complete lack of such networks in today’s ‘AI’ may be an incentive to examine this invention of evolution for its value for new types of artificial intelligence,” says Helmstaedter.
Source: Sahil Loomba (Max Planck Institute for Brain Research, Frankfurt) et al., Science, doi: 10.1126/science.abo0924