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The hippocampus (plural: hippocampi) or historically the cornu Ammonis, is an important component of the human brain, situated in the temporal lobe. It plays a role in information processing and the reproductive cycle and is involved in Alzheimer disease.
The hippocampus lies in the hippocampal sulcus immediately below the floor of the temporal horn of the lateral ventricle, and in cross section (coronal) has appearances that are reminiscent of a seahorse. It has a head (posterior to the amygdala), a body, and a tail (which follows the upwardly curving lateral ventricle).
Although there is a lack of consensus relating to terms describing the hippocampus and the adjacent cortex, the term hippocampal formation generally applies to the dentate gyrus, fields CA1-CA3 (CA4 is frequently called the hilus and considered part of the dentate gyrus), and the subiculum (parahippocampal gyrus). The hippocampus proper is made up of CA1, CA2 and CA3 fields (Figure 1).
NB: CA is an abbreviation of cornu Ammonis
The majority of its output is via the alveus into the fimbria which breaks free to the hippocampus to form the fornix. Information flows through the hippocampus proceeds from the dentate gyrus to CA3 to CA1 to the subiculum, with additional input information at each stage and outputs at each of the two final stages. CA2 represents only a very small portion of the hippocampus and its presence is often ignored in accounts of hippocampal function, though it is notable that this small region seems unusually resistant to conditions that usually cause large amounts of cellular damage, such as epilepsy.
The perforant path, which brings information primarily from entorhinal cortex (but also perirhinal cortex, among others), is generally considered the main source of input to the hippocampus. Layer II of entorhinal cortex (EC) brings input to the dentate gyrus and field CA3, while EC layer III brings input to field CA1 and the subiculum. The main output pathways of the hippocampus are the cingulum bundle and the fimbria/fornix, which arise from field CA1 and the subiculum.
Perforant path input from EC layer II enters the dentate gyrus and is relayed to region CA3 (and to mossy cells, located in the hilus of the dentate gyrus, which then sends information to distant portions of the dentate gyrus where the cycle is repeated). Region CA3 combines this input with signals from EC layer II and sends extensive connections within the region and also sends connections to region CA1 through a set of fibers called the Schaffer collaterals. Region CA1 receives input from the CA3 subfield (these afferents selectively target the basal dendrites in the stratum oriens, and the apical dendrites in the stratum radiatum - this is the Schaffer collateral pathway), EC layer III and the nucleus reuniens of the thalamus (which project only to the terminal apical dendritic tufts in the stratum lacunosum-moleculare). In turn, CA1 projects to the subiculum as well as sending information along the aforementioned output paths of the hippocampus. The subiculum is the final stage in the pathway, combining information from the CA1 projection and EC layer III to also send information along the output pathways of the hippocampus.
It is widely accepted that each of these regions has a unique functional role in the information processing of the hippocampus, but to date, the specific contribution of each region is poorly understood. The hippocampus also plays a vital role in the reproductive cycle.
- collateral branches of the posterior cerebral artery and anterior choroidal artery 6
- forming the deep intrahippocampal arteries 6
- intrahippocampal veins, draining in the superficial hippocampal veins, forming two arches, which ultimately drain into the basilar vein 6,7
- posterior end: draining in the medial atrial vein 6,7
- anterior end: inferior ventricular vein, draining in the medial atrial vein 6,7
The hippocampus is best imaged in the coronal plane, angled perpendicular to the long axis of the hippocampal body. The three parts of the hippocampus (head, body and tail) can be identified based on morphology and by using local landmarks 3.
- amygdala-hippocampal head junction
- hippocampal head
- hippocampal body
- hippocampal tail
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