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Krasnow Institute > Monday Seminars > Abstracts
A tale of two entorhinal inputs: German Barrionuevo Damage to the hippocampal formation in humans and animal subjects results in deficits to form long-term episodic memories. While the mechanisms allowing the hippocampus to support the formation of such memories are still unknown, one key assumption underlying many computational models of the hippocampus is that area CA3 is capable of robust "pattern separation". Pattern separation refers to the ability of the CA3 neuronal network to represent cortical activity while minimizing overlap of cortical representations. Computer simulations indicate that pattern separation can be achieved by coordinated activation of a weak and diffuse input to convey the memory representation from the entorhinal cortex (EC), and a strong but sparse input to select the subpopulations of CA3 pyramidal cells that establish non-overlapping memory representations via their recurrent collaterals. Indeed, one unique feature of hippocampal CA3 pyramidal cells is that they receive two converging excitatory inputs from EC. One input is conveyed monosynaptically via the perforant path (PP), and is relatively weak and diffuse. The second input from EC is conveyed disynaptically via the mossy fiber (MF) axons of dentate gyrus granule cells, and is sparse and strong. An important issue is whether the sparseness and strength of the mossy fiber synaptic input are the only features at play during pattern separation. For example, MF axons innervate more interneurons than pyramidal cells; thus, granule cell discharge exerts a strong inhibitory control over pyramidal cell firing, which could further "sharpen" the focus of the MF excitatory drive onto the CA3 neuronal network. This talk will describe our recent in vitro electrophysiological and anatomical findings on synaptic plasticity and synaptic integration of MF and PP inputs onto CA3 interneurons. Our results show that MF input to interneurons can undergo two mechanistically separate and synergistic modifications: Long term potentiation, and increased supralinearity in the temporal summation with PP input.
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