  According to the conventional approach to model neural ensembles the
extracellular environment has fixed ionic concentrations. However, in many
cases the extracellular concentration of potassium ions can not be regarded as
constant. That represents specific chemical pathway for neurons to interact and
can influence strongly the behavior of a single neuron as well as large
ensembles. The released chemical agent follows a diffusive dynamics in the
external medium, that lowers the threshold of individual excitable units. We
address this problem by studying simplified excitable units given by a modified
FitzHugh-Nagumo dynamics. In our model neurons interact only chemically via the
released and diffusing potassium in the surrounding non-active medium. We study
the dynamics of a single excitable unit embedded in the extracellular matter.
That leads to a number of noise-induced effects, like self-modulation of firing
rate in an individual neuron. In the spatially extended situation various
patterns appear ranging from spirals and traveling waves to oscillons and
inverted structures depending on the parameters of the medium.
