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uni'wissen 02-2012 ENG

The precise way in which the nerve cells in our brains present, process, and pass on infor- mation is a secret that neuroscientists are par- ticularly eager to unravel. “We know that the neurons communicate with one another in their own distinct way,” says Prof. Dr. Ulrich Egert from the Laboratory for Biomicrotechnology at the Faculty of Engineering of the University of Freiburg, “but unfortunately we hardly know any- thing at all about what they talk to each other about.” Egert is co-director of the new Cluster of Excellence BrainLinks–BrainTools. Scientists at the University of Freiburg’s center for neurotech- nology from the fields of biology, medicine, mi- crosystems engineering, and computer science are investigating the function of the human brain and developing systems to communicate with it directly: interfaces that enable patients to control technical devices over their nervous system and implants that produce their own energy, identify changes in brain activity, and step in to counter- act them. A Host of Forces Acting in Concert What the close to 100 million nerve cells of the human brain have in common are the dynam- ics of their individual activity. These dynamics, most of which have already been described in detail, are based on changes in voltage above the cell membrane, which creates a nerve im- pulse that is also referred to as an action poten- tial or spike. The difficulty, explains Egert, lies in proceeding from the relatively well-understood biophysics of the individual cell to networks at different spatial levels: “We can measure the spikes of individual neurons using electrodes, but we don’t see what the networks make out of them.” A mere three cells is all it would take to make up a small network, and the next larger- sized network would consist of maybe 300 nerve “We know that the neurons communicate with one another in their own distinct way, but unfortunately we hardly know anything at all about what they talk to each other about” cells. In the case of three cells, it is a straightfor- ward affair to observe the transmission of stimu- li over the so-called axons, cable-like conduction systems, to the synapses, the points of transmis- sion between individual neurons. Unfortunately for neuroscientists, however, the behavior of a small network reveals only little about that of a much larger network. The brain can contain up to 10,000 synapses for every pyramidal cell, the most common type of nerve cell in the cerebral cortex. This means that the influence of one nerve cell on the next is much different in a large network than it is in a network in which each cell only has one connec- tion to two other individual cells. “The individual cell exhibits a completely different type of behav- ior in this large network,” says Egert. “The larger the network becomes, the less I can assume that all of the cells in it behave in the same way.” Dif- ferent types of cells enter the picture: Some have an inhibiting effect, others have a stimulat- ing effect, and still others belong to one of many subtypes. The system is complicated further by incoming impulses from outside, from the cortex, and from other parts of the brain as well as by local feedback. To make things even more con- fusing, the next cell translates the spike into a current signal that looks completely different from the action potential of the source cell. Hence, there is hardly any knowledge to be gained by studying the form of the spike. “The important thing then is the temporal sequence of the spikes,” says Egert. “In other words, I need a device that tells me which cell fired when.” Scientists are responding to this challenge by first breaking the system down into smaller units. One approach involves observing the electro- physiological activity, i.e., the activity of the ac- tion potential, of cell cultures with approximately 5