In this image there are two neurons and three astrocytes. These processes are pressed up against the soma of a neuron to cover every square micron of surface where there is not actually a direct synapse. The neuron is thus, in effect, physically isolated into its own little compartment. When a neuron's plasma membrane depolarizes, ions are released into the space around it.
Potassium ions flow out and sodium ions flow in.
If the local concentration of potassium in the intercellular space becomes too great, the rapid depolarization of that segment of membrane becomes more difficult. The ions are moved from place to place.