Leão’s Spreading Depression, an Example of Diffusion-Mediated Propagation of Excitation in the Central Nervous System

Long before the concept of autowaves was formulated and generally recognized, a phenomenon of this class had been described in the vertebrate brain [l,2], Leao’s [2] spreading EEG depression (SD) is characterized by a decrease of electrical activity which propagates from the point of stimulation as a concentric wave with the velocity of 3 mm/min over the cerebral surface. The front of the SD wave is accompanied by a negative slow potential (20–30 mV) and by an increase of extracellular potassium concentration [K+]e, reaching 50 to 70 mM and lasting 30 to 60 sec [3]. SD can be elicited not only in the cerebral cortex but also in other neural structures. A particularly important preparation for SD studies is the isolated retina of fish, frogs, and birds, where its propagation can be directly observed as a change of light scattering of the retinal tissue [4,5], SD wave appears as a milky or dark spot spreading in all directions across the retinal surface while the central region of the circular area gradually returns to the pre-depression state. The optical concomitants of SD make it possible to delineate the whole advancing front of SD with an accuracy and completeness which could never be achieved with complex arrays of hundreds of electrodes.