Scientists from Freiburg have used new methods to analyse signal processing in the brain. They found that neurones have a much greater ability to precisely transmit signals than previously assumed.
The brain is a highly complex information processing system. When we see, hear, or remember things, information is passed from neurone to neurone in the form of electrical signals – this enables us to recognise pictures and understand speech. But how is information stored in neuronal stimuli? Does the transduction of information depend on the number of stimuli or on their exact timing? A team of scientists led by Dr. Clemens Boucsein from the Bernstein Centre for Computational Neuroscience and the University of Freiburg, in cooperation with scientists from Martin Nawrot’s team at the Bernstein Centre for Computational Neuroscience in Berlin focused on this key neuroscientific issue. The scientists used new methods to investigate the neocortex in greater detail. They found that neurones have a much greater ability to precisely transmit signals than previously assumed. Their research has recently been published in the scientific journal “Frontiers in Neural Circuits”.
All neurones transmit information as a sequence of neuronal impulses. But there are huge differences in the way this information is encoded and how the signals are read by the postsynaptic cells. Some sensory and nerve cells that stimulate muscles use a "rate code": the more impulses per time unit, the brighter the light perceived, the louder a tone heard or the stronger the muscle contractions. Other cells use a "time code" which, rather than depending on the number of impulses, depends on the exact timing of the impulses, i.e. whether one particular cell sends an impulse a few milliseconds before or after another cell. Dr. Boucsein and his colleagues examined the strategy used by neocortical cells.
The neocortical cells receive many signals from other, presynaptic cells. If neocortical cells used a "temporal" code, they might also be able to react to incoming signals with high temporal precision. In order to test this hypothesis, Dr. Boucsein and his team developed a new method which measures the electrical activity of a cell in a tissue slice, while the presynaptic cell is activated in a precisely defined temporal sequence. The researchers used a chemical that is released upon the influence of light, thus stimulating the nerve cells. They used a laser and a mirror system to repeatedly switch on the presynaptic cells following the same temporal sequence. "We were surprised to see how reproducible and temporally exact the postsynaptic cell reaction was to the sequence of incoming signals," said Dr. Boucsein. These findings were all the more important because such a process is far from simple. Every signal emitted by the presynaptic cell has to run along long cellular projections before being transmitted to the postsynaptic neurone from where it travels along the projections to the cell body. All these processes might - at least in theory - be associated with temporal inaccuracies. The fact that the cells react very accurately clearly shows that neurones are perfectly suited to a code that depends on a precisely timed signal transmission. This research has shown that if the neurones of the cerebral cortex used a rate code, they would not be able to transmit information as reliably.
Original publication: Nawrot MP, Schnepel P, Aertsen A, Boucsein C. Precisely timed signal transmission in neocortical networks with reliable intermediate-range projections. Front Neural Circuits. 2009;3:1. Epub 2009 Feb 10doi:10.3389/neuro.04.001.2009
Further informaiton:Dr. Clemens Boucsein Department of Neurobiology & Biophysics Institute of Biology III University of Freiburg Tel.: +49 (0)761/203-2862 E-mail: boucsein(at)biologie.uni-freiburg.de