Trisynaptic circuit

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, 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 neural circuit in the hippocampus, which is made up of three major cell groups: granule cells in the dentate gyrus, pyramidal neurons in CA3, and pyramidal neurons in CA1. 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 path) and to the granule cells of the dentate gyrus, respectively. The dentate gyrus (DG) is the innermost section of the hippocampal formation. The dentate gyrus consists of three layers: molecular, granular, and polymorphic. Granule neurons, which are the most prominent type of DG cells, are mainly found in the granular layer. These granule cells are the major source of input of the hippocampal formation, receiving most of their information from layer II of the entorhinal cortex, via the perforant pathway. Information from the DG is directed to the pyramidal cells of CA3 through mossy fibres. Neurons within the DG are famous for being one of two nervous system areas capable of neurogenesis, the growth or development of nervous tissue. The CA3 is a portion of the hippocampal formation adjacent to the dentate gyrus. Input is received from the granule cells of the dentate gyrus through the mossy fibres. The CA3 is rich in pyramidal neurons (like those found throughout the neocortex), which project mainly to the CA1 pyramidal neurons via the Schaffer collateral pathway. The CA3 pyramidal neurons have been analogized as the “pacemaker” of the trisynaptic loop in the generation of hippocampal theta rhythm. One study has found that the CA3 plays an essential role in the consolidation of memories when examining CA3 regions using the Morris water maze. The CA1 is the region within the hippocampus between the subiculum, the innermost area of the hippocampal formation, and region CA2. The CA1 is separated from the dentate gyrus by the hippocampal sulcus. Cells within the CA1 are mostly pyramidal cells, similar to those in CA3. The CA1 completes the circuit by feeding back to the deep layers, mainly layer V, of the entorhinal cortex.