According to the conventional approach neural ensembles are modeled with fixed ionic concentrations in the extracellular environment. However, recent studies on the patho-physiology of tissue from epileptic human patients revealed aberrations in potassium regulation. These findings can be interpreted as neuronal interaction via specific chemical pathways, that influence strongly the behavior of single neurons and large ensembles. Released chemical agents, which diffuse in the extracellular medium is able to lower thresholds of individual excitable units. We address this problem by studying a modified stochastic FitzHugh-Nagumo dynamics. In our model the neurons interact only chemically via the released and diffusing potassium in the surrounding non-active medium. The dynamics of a single neuron as well as neurons embedded in a heterogeneous medium is investigated. We find coexistent excitable and oscillatory states for the local neuron and patterns ranging from spirals and traveling waves to oscillons and wandering spots in the spatially extended situation. Also a poisoned state appears in which a high level potassium concentration leads to a frozen depolarized state corresponding to cell death.