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Szendrei Alex BV4XT1
The trisynaptic neuronal network of the hippocampus.
The hippocampus is a small region of the brain that forms part of the limbic system and is primarily
associated with memory and spatial navigation. The hippocampus is located in the medial temporal
lobe of the brain, underneath the cortical surface. Its structure is divided into two halves which lie in
the left and right sides of the brain. The organ is curved with a shape that resembles a seahorse, and
its name is derived from a coupling of the Greek words "hippo" for horse and "kampos" for sea.
In general, there are four gross regions of the hippocampus:
1. Dentate Gyrus (DG)
2. Hippocampus proper:
 CA1–3 (Cornu ammonis)
3. Subicular complex:
 Subiculum (Sb)
 Presubiculum (PrS)
 Parasubiculum (PaS)
4. Entorhinal cortex (EC):
 Medial (MEC)
 Lateral (LEC)
When cut in the transverse plane, the hippocampus shows three interconnected afferent pathways
known as the trisynaptic circuit. The trisynaptic circuit is made up of three major cell groups: granule
cells, CA3 pyramidal neurons, and CA1 pyramidal cells. The first projection of the hippocampus
comes from the entorhinal cortex to the dentate gyrus. The dentate gyrus then synapses to 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.
Entorhinal cortex constitutes the major gateway between the hippocampal formation and the
neocortex. The entorhinal cortex with the hippocampal formation, appears to specifically deal with
the translation of neocortical information into complex representations that, when combined with
motivational representations, will serve cognitive functions, in particular conscious memory.
Dentate Gyrus has a largely unidirectional nature of its inputs and outputs. The mossy fibers give rise
to massive synaptic endings that have as many as 40 active sites onto postsynaptic neurons.
The mossy fiber synapse is one of the largest and most powerful synapses in the brain. It plays a
precise and unique role in the hippocampal information processing that ultimately leads to the
production of declarative memories.
Cornu ammonis is subdivided into CA3, CA2 and CA1.
CA3 dendrites have “thorny excrescences” with complex spine shapes. Each granule cells contacts
only about 15 CA3 pyramidal cells. Each pyramidal cell receives input from only about 72 granule