Excitation waves in central grey matter: The retinal spreading depression

This review deals with a phenomenon which, although it has been known for five decades, has not been recognized until recently as a suitable tool for investigations of neuronal-glial interactions and the interesting consequences of the non-linearities arising from these cooperative effects, such as the transition from quiescent states to the propagation of excitation. For neurophysiologists interested in the excitability control of neuronal populations, the retinal spreading depression wave can also be very useful, especially as a basic model for epileptic activity triggering and propagation mechanisms. In the isolated eye cup, spreading depression waves (RSDs) can be observed with ease. In fact, the RSD wave can be seen with the naked eye, which gives the observer a complete and non-invasive two-dimensional view of propagation. In the intrinsic optical signal of the RSD, several components can be separated, either by their spectral preferences or by their temporal and spatial characteristics and experimental manipulations. Glial membrane channel activity and physical changes in the composition of the extracellular medium appear to the major contributions for the optical changes during RSDs. Different components are also found in the electrophysiological concomitants of the waves, some with neuronal (synaptic terminals) and some with glial predominance. Besides the clinical and physiological aspects, the RSDs are part of a general class of self-organizing spatiotemporal structures that arise in systems far from equilibrium known as dissipative structures. In the retina, low dimensional dissipative structures self-organized, self-sustained and spatially structured wave activity, can be maintained for hours either as a soliton 'circling wave' or as a complex sequence of interacting spirals. Therefore. the RSD can serve as a powerful tool for cross-disciplinary research in the field of non-linear dynamics. (C) 1997 Elsevier Science Ltd.