How the brain communicates

An important mechanism by which the human brain hemispheres communicate with each other has been discovered by a team of researchers from Berlin and the University of Bern. The findings, which appear in the current issue of the journal Science, provide new insights into nerve cell communication in the brain that could also play a role in stroke.

 On the way to the brain, nerve pathways in the human body cross each other. As a result, stimuli are processed in the opposite hemisphere of the brain. For example, if someone touches our right hand, the stimulus is received in the left half of the brain. However, both halves of the brain have to coordinate their activities. Since some functions, such as language, are strongly pronounced in only one half of the brain, their signals always have to be communicated to the other half. This is even more obvious in daily activities such coordinating the hands or feet, which requires very precise communication between both brain hemispheres. The signals that reach the brain hemispheres are sent via a massive nerve pathway called the corpus callosum from one half of the cerebral cortex to the other.

 The research group of Matthew Larkum of the Cluster of Excellence NeuroCure at the Charité – Universitätsmedizin Berlin and Humboldt-Universität zu Berlin investigates the mechanisms in the brain controlling neuron activity in the cerebral cortex. In their current study in cooperation with the University of Bern, the researchers focused on the processing of tactile sensations. To do this Larkum and his team used a range of methods such as intracellular measurements of single nerve cells in the intact brain and various imaging techniques during the sensory stimulation of the hind paw of a rat.

The scientists discovered that stimulating the right and left paws of the rat has a relatively slow, nearly half-second-long sustained inhibitory effect on nerve cell activity. „That is very slow“, notes Larkum. „Normally, signal transmission happens much faster. For that reason, we wanted to find out which circuit of nerves underlies this mechanism and identify the cellular communication pathways,“ he further explains.

The researchers were able to do this with the help of a new technology called optogenetics, which makes it possible to stimulate specific nerves with light. The researchers could show that nerve fibers coming out of the opposite hemisphere activate a special group of local inhibitory nerve cells. These nerve cells in turn activate slow-acting receptors that lead to lower activity in the other nerve cells of the same brain hemisphere.

For stroke research in particular, these findings could be an additional building block in the development of new therapies, as this mechanism plays an important role in the disease. However, communication between the brain hemispheres in the cerebral cortex is crucial not only in stroke damage but also for a range of cognitive abilities, which is why the results of this study could have far-reaching impact. 

Selected publications:

Palmer LM, Schulz JM, Murphy SC, Ledergerber D, Murayama M, Larkum ME (2012) The cellular basis of GABAB-mediated interhemispheric inhibition. Science In press.