Observations of non-stationarities in extracellular potassium dynamics within the gray matter neuropil during self-sustained SDs.

The chicken retina is an easily accessible piece of grey matter with a very prominent but simplified neuropil, the inner plexiform layer, which is formed mainly by synaptic terminals and Müller cell processes. In this preparation a self-sustained form of "spreading depression" waves, the circling wave, can be recorded for many hours. We have found that in some circling wave experiments extracellular potassium could display non-stationarities both, in the extracellular potassium wave dynamics during SDs, as well as in the intervals between successive waves. The implications of non-stationary behaviour for the understanding of the genesis of excitable states in the CNS are discussed in the paper.