Control of the excitability of neuronal tissue by weak external forces.

The spreading depression (SD) is a pronounced example of excitation-depression waves in excitable media, to which neuronal tissue according to its structure and functions belongs. SD waves can especially easily be observed in the vertebrate retina which is neuronal tissue and a true part of the central nervous system (CNS). According to the high intrinsic optical signal (IOS) concomitant with the retinal spreading depression (rSD), it can be monitored with standard video imaging techniques, thus the retina has been used in our studies as a suitable model system for neuronal tissue in general. In particular, the control of wave set-up and propagation in excitable media by weak external forces is of high interest. Accordingly, the interaction of rSD waves with DC and AC electromagnetic fields of low amplitude and frequency and with gravity has been investigated in this study. The dependence of rSD-wave propagation velocity on the given parameters as one important indication of excitability control has been investigated in detail. Our results with rSD waves are partially compared to another well known excitable medium, the Belousov-Zhabotinsky reaction, where some data about the effects of electrical fields and gravity have already been published.