Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Max Planck Institute for Brain Research in Frankfurt am Main and Helene Schmidt at the Bernstein-Center of Humboldt-University in Berlin have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.
The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in this region of the brain are sorted very precisely along the electrical cables of the nerve cells. The nerve cells establish an unexpectedly precise circuit motive, in which first so-called inhibitory nerve cells are contacted before in the next step the actual (excitatory) activation of the next nerve cell can be executed. This motif of nerve cell “trios” can be considered a core connectivity motif in this type of cortex. Scientists speculate that such a highly precise circuit motive could be used for computing hypotheses about the next step in space.
For a couple of years, novel electron microscopes and improved data analysis methods have enabled scientists to map the neuronal networks in the brain with remarkable accuracy. This rather novel research area called “Connectomics” has its own department at the Max Planck Institute for Brain Research (Frankfurt am Main), Germany. Scientists in this department have now used their repertoire of measuring and analysis techniques to study the part of the cerebral cortex in which so-called grid cells provide a very particular representation of the space around the individual animal or human. These grid cells are known to be active when the animal or human is located at highly ordered grid-like locations in a room or a large space. Previously, scientists around Michael Brecht of Humboldt University of Berlin, who is a co-author of this study, had already found a special arrangement of nerve cells in this region of the brain, and had speculated at the time that within these special cell assemblies particular nerve cell circuits could exist.
Thanks to the novel methods in Connectomics, scientists can in fact now have a much more closer look. Already the first glance into the medial entorhinal cortex provided surprising results and further studies will follow.
Helene Schmidt, Anjali Gour, Jakob Straehle, Kevin M. Boergens, Michael Brecht, Moritz Helmstaedter, Axonal synapse sorting in medial entorhinal cortex. Nature (2017). DOI: 10.1038/nature24005