Equilibrium in the brain

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Nerve cells are "information addicts". To process and store new information or to optimize already existing ways of processing it, minute appendages emerge continually from their surface and grow towards neighboring cells. At the end of these appendages, a synapse can develop via which the two nerve cells can then exchange information. Scientists at the Max Planck Institute of Neurobiology in Martinsried and the Ruhr University of Bochum were already able to show how quickly such nerve cells can reorganize themselves even in the adult brain, so that they are constantly able to process information: After a small retinal lesion, the nerve cells responsible for processing information from this area were "out of work". However, during the weeks to follow, the neurobiologists observed that these nerve cells increased the number of appendages sent towards their neighbouring cells. The cells that had been temporarily redundant were thus reconnecting themselves and could take on new tasks within the processing network.

However, optimal processing in the brain depends not only on the circulation of information but also on the direct inhibition of the flow of information at given points. What actually happens to these so-called inhibitory synapses when conditions change in the brain?  Since this area has hardly received any detailed scientific attention, the team of scientists set out to examine the fate of these synapses in the nerve cells that receive no information on account of the small retinal lesion.

"One possible outcome was that inhibitory synapses remained, maybe to inhibit these cells which would otherwise pass on no, or only meaningless, information", explains Tara Keck, whose study has just been published in the scientific journal Neuron. However, the neurobiologists discovered that precisely the opposite was the case. They showed that those cells which had been rendered redundant reduced the number of their inhibitory synapses by about one third within one day. Such was the extent of this downsizing that the imbalance in the flow of information, brought about by the loss of the excitatory signals from the retina, was quashed. "The exciting thing about this result is the insight that the brain appears to be constantly seeking to maintain the balance between excitation and inhibition", Keck relates.

The scientists already have a theory as to the importance of this lower level of the established balance. "The decimation of the inhibitory synapses may act as a signal to neighbouring cells by advertising: Nerve cells seeking work. Please get in touch", reflects Mark Hübener, the head of the study. The scientists now hope to establish whether this is indeed the case and whether more inhibitory synapses are produced to regain the original balance once the rewiring with other cells is complete.

the vertebrate cerebrum() is formed by two cerebral hemispheres that are separated by a groove, the . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of , the , that is supported by an inner layer of . In  (placental) mammals, the hemispheres are linked by the , a very large bundle of . Smaller commissures, including the , the  and the , also join the hemispheres and these are also present in other vertebrates. These commissures transfer information between the two hemispheres to coordinate localized functions.

There are three known poles of the cerebral hemispheres: the , the , and the .

The  is a prominent fissure which separates the  from the  and the  from the .

the hemispheres are roughly mirror images of each other, with only subtle differences, such as the  seen in the , which is a slight warping of the right side, bringing it just forward of the left side. On a microscopic level, the  of the cerebral cortex, shows the functions of cells, quantities of  levels and  subtypes to be markedly asymmetrical between the hemispheres. However, while some of these hemispheric distribution differences are consistent across human beings, or even across some species, many observable distribution differences vary from individual to individual within a given species.

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