Neurons in the brain change their size

21.01.2021 08:56

The size of nerve connections determines the strength of the signal

Kurt Bodenmüller communication
University of Zurich

Nerve cells communicate with each other via synapses. Their performance is likely to be much higher than previously assumed, as neuroscientists from the University of Zurich and ETH Zurich show. The larger a synapse, the stronger the signal transmission. These findings make it possible to better understand how the brain works and how neurological diseases arise.

In the nerve cells of the cerebral cortex, the neocortex, humans process sensory impressions, store memories, give commands to the muscles and plan for the future. These computing processes are possible because each nerve cell is a highly complex minicomputer, which in turn is in contact with around 10,000 other neurons. Communication takes place via special contact points: the synapses.

The larger the synapse, the stronger the signal

Researchers in Kevan Martin's team from the Institute for Neuroinformatics at the University of Zurich (UZH) and ETH Zurich have now shown for the first time that the size of the synapses determines the strength of their information transmission. «Larger synapses lead to stronger electrical impulses. With this knowledge, we are closing a central knowledge gap in neurobiology, ”says Martin. “This knowledge is also crucial in order to understand how information flows through the circuit diagrams of the brain and thus how our brain functions”.

Nerve connections in the cerebral cortex reconstructed

First, the neuroscientists characterized how strong the synapse currents are between two connected nerve cells. To do this, they made wafer-thin sections of a mouse brain and inserted fine glass electrodes into two neighboring nerve cells in the cerebral cortex under the microscope. This enabled them to artificially activate one of the two nerve cells and at the same time measure the strength of the resulting synapse current in the other cell. They also injected a dye into both neurons in order to reconstruct their branched cell extensions in three dimensions under the light microscope.

Synapse size correlates with signal strength

Since synapses are tiny, the researchers used the high resolution of an electron microscope to reliably identify and precisely measure the neural contact points. In the light microscopic reconstructions, they first marked all points of contact between the cell extension of the activated cell, which transmits the signal, and the cell extension of the cell, which receives the synapse current. They then identified all synapses between the two nerve cells under the electron microscope. They then correlated the size of the synapses with the previously measured synapse currents. “We discovered that the strength of the synapse current is directly related to the size and shape of a synapse,” says lead author Gregor Schuhknecht, a former doctoral student in Kevan Martin's team.

Understand the circuits of the cerebrum in greater depth

This relationship can now be used to estimate how strong the information transfer is based on the measured synapse size. “In future, this will enable the circuits in the cerebral cortex to be precisely mapped using electron microscopy and the flow of information to be simulated and interpreted on the computer,” explains Schuhknecht. This work enables a better understanding of how the brain normally functions and how “wiring defects” can lead to neurodevelopmental disorders.

More computing power and storage space than expected

The team was also able to clarify another central question of neurobiology. According to previous doctrine, synapses in the cerebral cortex only release a single vesicle with messenger substances per activation process. With the help of mathematical analyzes, the researchers were able to show that synapses can release several vesicles at different points at the same time. “Synapses are therefore more complex and can regulate their signal strength more dynamically than previously thought. The computing power and storage capacity of the entire cerebral cortex is most likely much greater than previously thought, ”says Kevan Martin.

Scientific contact:

Dr. Gregor Schuhknecht
Department of Molecular and Cellular Biology
Harvard University
Tel. +1 617 384 9773
Email: [email protected]

Prof. Dr. Kevan A.C. Martin
Institute for Neuroinformatics
University of Zurich, ETH Zurich
Tel. +41 44 635 30 51
Email: [email protected]

Original publication:

Simone Holler, German Köstinger, Kevan A. C. Martin, Gregor F. P. Schuhknecht, Ken J. Stratford. Structure and function of a neocortical synapse. Nature. 13 January 2021. DOI: 10.1038 / s41586-020-03134-2

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