Epileptic seizures are stereotypical patterns of abnormal synchronized activity among different cellular populations. They are products of pathological activation of networks involved in controlling the temporal (initiation, maintenance, termination) or semiological evolution of these ictal events. Often seizure activity forms a reverberating circuit that can perpetuate and self-sustain, as is the case with temporal lobe seizures. Alternatively, a seizure can be paroxysmal but temporally confined, albeit with the potential to recur, such as in epileptic myoclonus or spasms. The basic elements of these networks may belong to a single or multiple systems, which are recognized based on well-identified physiological functions. As such, they are useful in localizing seizure activity and defining seizure propagation. Communications within a system or between different systems, during a seizure, can be via anatomical connections (i.e., afferent or efferent neuronal projections) or humoral (i.e., hormonal, metabolic or immune responses). A sine qua non of epilepsy is the propensity for unprovoked seizures to recur. In certain cases, this is already a predetermined feature of the original epileptic seizure network, as occurs in certain types of primary idiopathic epilepsies, that is, absence. The expression of the seizures, in such cases, appears to depend more on modifiers involved in the physiological maturation of the network, such as age or hormonal factors. In other types of epilepsy, the epileptic predisposition is not as strong initially, but dynamic changes of the network triggered by seizures or unrelated epigenetic factors promote epileptogenesis. The classical example is temporal lobe epilepsy (TLE), whereby initial precipitating events may increase the likelihood that TLE will manifest.
Epileptic networks include an initiating circuit, which, in certain cases, may be ignited by endogenous or exogenous triggers (such as stress, fever, photic stimulation, and sleep–wake cycle). Seizure activity may remain regionally contained or spread to secondary domains, via propagation pathways. Initiating networks are different for generalized or localization-related epilepsies. Identification of the networks and systems involved in seizures and epilepsy is important for their correct localization, classification, and treatment, as well as the identification of methods to predict their recurrence and their outcome. In this review, we will summarize some of the known networks and systems implicated in common types of epileptic seizures and syndromes, discuss them in the context of specific types of epileptic seizures, and review factors that can modify them.
BASIC NETWORKS IMPLICATED IN EPILEPTIC SEIZURES
Probably, the most popular focus of attention in epilepsy research has been the mesial temporal lobe structures, due to their high epileptogenicity and the high incidence of TLE. Medial temporal lobe structures are the dentate gyrus (DG), hippocampus, subiculum, pre- and parasubiculum, entorhinal, perirhinal, and parahippocampal cortices. These are normally involved in long-term memory formation, consolidating information obtained through connections with associational cortices. The basic circuit interconnects the subiculum, DG, and cornu ...