Trisynaptic loop

The trisynaptic loop (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)

Criticism: Recent anatomical methods challenge the simple notion of a tri-synaptic circuit.

[1] Amaral DG, Witter, MP. 1995. Hippocampal formation. In: Paxinos G, editor. The rat nervous system, 2nd ed. San Diego: Academic Press.