Cortical neurons are tightly linked over long distances. It is estimated that the entire length of the linkages between the neurons within a single cubic millimetre of grey matter would cover about three kilometres. Every cell thus receives numerous thousands of input signals and transmits just as many output signals to, in part, distant neurons. This leads to a rapid wave-like distribution of excitations.
What, however, happens if some of the normal input signals are suddenly no longer present due to the lesion of a sensory organ? Dr. Dirk Jancke (Institute for Neuroinformatics) explained that the neuronal networks are formed and stabilized during the early phases of development.
For many years, it had thus been assumed that the adult brain is not capable of compensating such lesions. During the past decades, scientists have however been able to demonstrate that the adult brain is also capable of changes in plasticity, even if only to a limited degree. Old, no-longer used contacts between cells are weakened or become detached, and new contacts are formed. Dr. Jancke explained that cortical neurons suddenly no longer have a direct entrance after a retinal lesion, but that their connection to still functional distant neighbours at least enables them to look around the corner again. This process is already experimentally discernible within only a few weeks after the lesion, the activity waves from the intact surroundings into the affected areas being strengthened.
Optical measurement with voltage-sensitive dyes
Older measuring procedures that evaluate the electric activity on the neurons can only present total signals. The new method is more sensitive, latent input signals also lighting up. Dr, Jancke explained that, for the first time, the scientists have now been able to demonstrate the long assumed continuous dispersion and strengthening of initially subthreshold waves of activity within the affected area using this newly developed imaging process. Gradual changes in the synaptic potential that develop during neuronal activity are registered as changes in the intensity of the fluorescent light. To this end, a dye is incorporated in the cell membrane and releases photons proportional to the voltage via the cell membranes.
A high-resolution camera system detects these light signals, which can be visualized by subsequent computing. The dyes and the basic measuring technology were developed in the laboratory of Prof. Amiram Grinvald, Weizmann Institute of Science, Israel. During his two years of work in Israel, Dr. Jancke managed to present activity waves in the optic cortex and to supply the first description of their significance for visual perception processes. Dr. Jancke has established this newly developed imaging method at RUB within the frameworks of his Assistant Professorship “Cognitive Neurobiology, Faculty for Biology and Biotechnology.”
Interfaculty cooperation at RUB
The current study is a continuation of work performed by RUB’s Medical Professor, Dr. Ulf Eysel, one of the pioneers and leading scientists in the field of neuronal plasticity, who has worked in close cooperation with the Max-Planck Institute for Neurobiology in Munich. The results showed that three times as many nerve fibres (so-called “spines”) develop in the cerebral cortex of mice after small spot lesions in the retina. The newly established cell contacts often broke before stable networks were finally established. It thus seemed appropriate to investigate the relationship between this extensive reorganization of the neuronal structure and the changes in the activity dynamics in large cell aggregates.
Ganna Palagina, scholarship holder of the International Graduate School (IGSN) at the Ruhr University, selected this subject for her doctoral thesis. During the two years in which she carried out the current study on mice, she commuted between the laboratory in the Dept. of Neurophysiology (Prof. Eysel), where the retinal lesions could be accurately made, and the Optical Imaging Laboratory in the Dept. of Biology (Dr. Jancke) for optical measurement using voltage-sensitive dyes.
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