Billions of nerve cells, billions of connections – how does the brain manage to reproduce the world in the right order and react accordingly? It is assumed that the different areas of the complex organ contain ordered processing units that make such processes possible. The neuroanatomist Prof. Dr. Jochen Staiger from the University of Freiburg is investigating the so-called barrels in the somatosensory cortex of rodents, which represent a body map with which the “tactile environment” can be perceived. Prof. Staiger is looking for the basic circuit in this highly ordered and structured part of the brain which enables the connection between perception and behaviour.
"I am a neuroanatomist by training and as such I'm very fond of structures," said Prof. Dr. Jochen Staiger, Professor for Cell Biology at the Centre for Neuroscience at the University of Freiburg. "As a modern neuroanatomist, I always look at the structures with regard to their function. "The brain, which can roughly be divided into cerebellum, thalamus and cerebral cortex, also reveals a highly structured composition when examined in detail with the proper methods. Staiger, who was born in 1964 in Karlsruhe, chose to focus on the somatosensory cortex, which he worked on during his habilitation in 2000. The somatosensory cortex is the area of the cerebrum where information from the whiskers of mice or rats is processed. Using specific staining methods, the researchers can visualise barrel-shaped structures with a diameter of about 400 micrometers. Individual barrels are arranged in columns in the cortex, which become visible in a cross section and span several layers of the cortex. The columns are arranged next to each other. The staining of the columns leads to the striped appearance of the cortex. The stripes separate neuron assemblies from each other, and also form units that belong together in functional terms.
Mice or rats actively explore their environment by sweeping their whiskers around the area to be investigated. Through several intermediary stations in the brain, each individual whisker preferentially sends its information to a specific column of the somatosensory cortex. The barrels are the first to receive the information from the whiskers. If the rodents sweep their whiskers across the surface of a walnut for example, the different hairs on its snout touch different areas of the shell. Each hair projects its “findings” into its related barrel. The spatial correlations remain, and the scientists refer to this as the map-like representation of the tactile environment in the somatosensory cortex. Perception processes such as the differentiation between tactile (personal space) and non-tactile (extrapersonal space), and between rough and smooth need to be processed by circuits that not only integrate the information obtained from the whiskers. They also have to take into account higher abstraction stages. In addition, they must also be able to trigger behaviour such as grasping or biting. “What is the smallest number of nerve cells needed to enable these perception processes to turn the information into specific behaviour?” asks Staiger. The smallest number of nerve cells is the basic module that Staiger is looking for and whose function he is seeking to clarify.
In order to achieve this goal, Staiger and his group of researchers have developed a method that enables the extremely targeted intervention of the circuits in- and outside of certain columns in the somatosensory cortex. By focusing light flashes on a very small area of the brain slices, the researchers are able to release chemically inactivated glutamate. Glutamate is a neurotransmitter that is released upon the trigger of nerve impulses from pre-synaptic cells and which activate glutamate receptors on the post-synaptic cell, thereby activating specific nerve cells. This provides the researchers with a switch which they can use to activate specific nerve cells. This is a very accurate method. Using an electrode, the researchers can measure which neurons the cell transmits its excitation to in more distant areas of the brain section. In addition, they can also find out whether a particular nerve cell has an inhibitory or excitatory effect on its communication partners. They are able, step by step, to determine the path covered by information within one column and between columns.
“Many of the connections in the somatosensory cortex predicted using this method, could later be confirmed with pair-recordings using two electrodes,” said Staiger. The most important finding made by Staiger and his team was the discovery that there are cells inside the barrels of the somatosensory cortex which only exchange information within the same barrel, and that there are others that communicate with neighbouring regions. Thus, the researchers were able to show that information is already processed at a relatively low level, and not just at higher levels as had previously been assumed. It would appear that the information originating from the whiskers is not analysed in the barrels in a locally restricted manner. The findings suggest that the parallel integration of information with neighbouring regions is also possible. The information obtained in a certain part of the area scanned with the whiskers seems to be integrated right from the beginning with the content of information originating from a neighbouring area.“When one touches an object, both local and global aspects are important,” said Staiger. For example, when reading Braille, the number and spatial orientation of the raised dots is important. In the long term, Staiger also plans to provide evidence for the existence of the principle of parallel information processing in the barrels of living animals. Therefore, the neuroanatomist is looking for rodent mutants whose somatosensory cortex does not have columns. Is organisation in columns necessary for the animals to be able to differentiate between rough and smooth, or sharp and blunt? “I actually see myself as a systems neurobiologist,” said Staiger. “I am always interested in the behaviour or processes such as perception or plasticity and memory.”
Initially, Staiger hopes to use optogenetics methods to be able to better assess the contribution of individual cell types to the processing of information. Using these completely new molecular biology methods, the scientists are able to interfere with the neuronal circuits and inhibit or overactivate individual cells or smaller cell assemblies. The method enables the researchers to introduce specific ion channels into defined cells and activate them in the living animal using light pulses. These ion channels therefore represent switches with which the researchers are able to switch cells on and off as required. For the time being, the group still lacks the know-how to be able to do this. Staiger has just accepted a position at the University of Göttingen where he will head up the Department of Neuroanatomy from 2010 onwards. He is already planning to work with a group at the University of Göttingen who are specialists in optogenetics.mn