Hippocampal electrical activity IV. Abnormal electrical activity

Abstract 1. 1. Transformations in intrinsic electrical activity of the hippocampus or in evoked potential shape which precede an electrical “seizure discharge” are associated with progressive changes in the shape of extracellularly recorded action potentials. 2. 2. The action potentials show a distribution which differs from that seen in an “inactivation” process since there is an increased amplitude and broadening of the negative phase. 3. 3. At the same time that the action potentials recorded from the stratum pyramidale change shape, sharp transient potentials called “pips” appear in the stratum radiale with highest amplitude. 4. 4. As a result changes in extracellular current flow occur. 5. 5. “Pips” seem to represent both the activity of single cells and groups of synchronously firing cells. 6. 6. It is proposed that ionic changes in the small extracellular spaces lead to alteration of threshold of various parts of the neurone membrane. Because of neuro-neuronal apposition, seizure discharges become synchronized. At the same time collateral paths may also play a role. 7. 7. Changes of excitability thus induced may cause both changes in the degree of depolarization of hitherto passive parts of the dendritic membrane and alterations in the extent and behaviour of the trigger zone of the neurones.