  In these companion papers, we study how the interrelated dynamics of sodium
and potassium affect the excitability of neurons, the occurrence of seizures,
and the stability of persistent states of activity. We seek to study these
dynamics with respect to the following compartments: neurons, glia, and
extracellular space. We are particularly interested in the slower time-scale
dynamics that determine overall excitability, and set the stage for transient
episodes of persistent oscillations, working memory, or seizures. In this
second of two companion papers, we present an ionic current network model
composed of populations of Hodgkin-Huxley type excitatory and inhibitory
neurons embedded within extracellular space and glia, in order to investigate
the role of micro-environmental ionic dynamics on the stability of persistent
activity. We show that these networks reproduce seizure-like activity if glial
cells fail to maintain the proper micro-environmental conditions surrounding
neurons, and produce several experimentally testable predictions. Our work
suggests that the stability of persistent states to perturbation is set by
glial activity, and that how the response to such perturbations decays or grows
may be a critical factor in a variety of disparate transient phenomena such as
working memory, burst firing in neonatal brain or spinal cord, up states,
seizures, and cortical oscillations.
