We have been interested in those induced by tetanic stimulation of the hippocampal CA1 region. These are characterised by large population spikes, produced by the synchronous discharge of most pyramidal cells. Roger Traub developed a model of tetanically-induced gamma based on tht used for Interneuronal Network Gamma. It involved a network of mutually inhibitory interneurons, and pyramidal cells which excited the interneurons and which in turn were inhibited by them. This model predicted that synchronisation faster that the conduction delay between pairs of stimulation sites could be explained by interneurons firing spike doublets and providing an error correction mechanism.
New experiments suggest an alternative model for tetanically-evoked gamma rhythms.
We found that the slow depolarisation produced by tetanic stimulation had physiological and pharmacological properties of a GABAergic depolarisation. Neurons close to the stimulating electrode became too leaky to respond to fast synaptic inputs, but they do fire because of the depolarisation. Spatial aspects Tetanically evoked gamma rhythms extend over 600-800 microns along the CA1 pyramidal cell layer. The structure of the rhythm and the contributions of neurons depend on distance from the stimulus site and the time after the stimulus. Cells close to the stimulus are very depolarised and leaky.