Hippocampal Structure

The hippocampus, or hippocampal formation is a structure belonging to the forebrain and innermost fold of the temporal lobe. It includes the hippocampus proper, the dentate gyrus, the subiculum, and is a key part of the limbic system.

This complexly-shaped structure receives the majority of its inputs from the entorhinal cortex, which channels information through the perforant path into the dentate gyrus, more specifically, the granule cells. These granule cells connect with the CA3 field of Ammon's horn (another name for the hippocampus proper). This field, as well as fields CA1, CA2 and CA4, contains pyramidal cells, which are shaped, not surprisingly, somewhat like a pyramid. The pyramidal cells of the CA3 field are connected with other similar cells in field CA1 and with strucutres in the basal forebrain. CA1 cells in turn connect with the hippocampal formation in the opposite side of the brain, to the subicular complex and the fimbria, each of which then connect to the entorhinal cortex and the basal forebrain, respectively.

Pyramidal cells in the hippocampal formation are majorly glutamic cells. They hold AMPA and NMDA receptors, both of which are ionotropic receptors that control, respectively, sodium and calcium ion gradients.

The hippocampal formation receives noradrenergic input from the locus coeruleus, dopaminergic stimulation from the ventral tegmental area (VTA), acetycholinergic stimuli from the medial septum and serotonergic input from the raphe nucleus. Yet, only the connections of the hippocampus to the mammilary bodies, through the fornix, seem to be 'information' related. These connections are then relayed to the anterior thalamus and ultimately to the cingulate cortex. The former connections seem to play a regulatory role in hippocampal function while the latter circuit regards the output of information from the hippocampus to the cingulate cortex. The hippocampal formation also receives information from the amygdala regarding odors and emotions.

Hippocampal Function

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