Trisynaptic loop

The trisynaptic circuit, or trisynaptic loop is a relay of synaptic transmission in the hippocampus. The circuit was initially described by the neuroanatomist Santiago Ramon y Cajal, in the early twentieth century, using the Golgi staining method. After the discovery of the trisynaptic circuit, a series of research has been conducted to determine the mechanisms driving this circuit. Today, research is focused on how this loop interacts with other parts of the brain, and how it influences human physiology and behaviour. For example, it has been shown that disruptions within the trisynaptic circuit leads to behavioural changes in rodent and feline models.

The trisynaptic circuit is a relay of synaptic transmission in the hippocampus, which is made up of three major cell groups: granule cells, CA3 ("Cornu Ammonis" area 3) pyramidal neurons, and CA1 ("Cornu Ammonis" area 1) pyramidal cells. The hippocampal relay involves 3 main regions within the hippocampus which are classified according to their cell type and projection fibers. The first projection of the hippocampus occurs between the entorhinal cortex and the dentate gyrus. The entorhinal cortex transmits its signals from the parahippocampal gyrus to the dentate gyrus via granule cell fibers known collectively as the perforant path. The dentate gyrus then synapses on pyramidal cells in CA3 via mossy cell fibers. CA3 then fires to CA1 via Schaffer collaterals which synapse in the subiculum and are carried out through the fornix. Collectively the dentate gyrus, CA1 and CA3 of the hippocampus compose the trisynaptic loop.

EC → DG via the perforant path(synapse 1), DG → CA3 via mossy fibres(synapse 2), CA3 → CA1 via schaffer collaterals(synapse 3)

The Entorhinal cortex (EC) is a structure in the brain located in the medial temporal lobe. The EC is composed of six distinct layers. The superficial (outer) layers, which includes layers I through III, are mainly input layers that receive signals from other parts of the EC. The deep (inner) layers, layers IV to VI, are output layers, and send signals to different parts of the EC and the brain. Layers II and III project to the CA3 area of the hippocampal formation (via the perforant pathway) and to the granule cells of the dentate gyrus, respectively.

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