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hippocampus (hippocampal formation)

Function: - the hippocampus forms part of the limbic system - the trisynaptic circuit channels much of the activity through the hippocampal formation. Structure: - a complex, convoluted structure forming the medial margin of the temporal lobe - thus, much of the hippocampus lies in the floor of the temporal horn of the lateral ventricle [2]. - cytoarchitecturally, it its a unique form of allocortex - the hippocampus (proper) consists of two gyri, Ammon's horn & the dentate gyrus, together with their white matter, the alveus & fimbria - the subiculum & entorhinal cortex are also considered part of the hippocampal formation [2], since they are linked by prominent & largely unidirectional connections that appear to unite them as a functional entity - the individual fields of the hippocampal formation (dentate gyrus, Ammon's horn, subiculum & entorhinal cortex) can be envisioned as adjacent strips of simple cortex running rostro-caudally in the medial temporal lobe - in its midportion or body, the hippocampus assumes a C-shape on coronal sections & each field is seen only once - as the hippocampal formation reaches its rostral end (uncus), it bends sharply in a medial direction, then in a caudal direction - thus coronal sections through the uncus tend to portray the hippocampus as a complex (distributed) structure - at its caudal end, the hippocampus bends dorsally as it ascends toward & around the splenium of the corpus callosum - thus coronal sections of the caudal hippocampus are also complex - the hippocampus has 4 layers in contrast to the cerebral cortex that has 6. a) stratum oriens (ventricular surface) - basal dendrites of pyramidal cells - axon collaterals of pyramidal cells - incoming axons of the alvear pathway* b) stratum pyramidale (pyramidal layer) - perikarya of hipocampal pyramidal cells c) stratum radiatum - apical dendrites of pyramidal cells - recurrent axon collaterals of pyramidal cells d) stratum moleculare et substratum lacunosum (lacunar-molecular layer) - distal dendrites - incoming axons of the perforant pathway* * one of 2 major pathways projecting from the entorhinal cortex into the hippocampus Afferents: (major) 1) subcortical a) amygdala b) claustrum c) septal nuclei (by way of the fornix) d) nucleus basalis of Meynert e) supramammillary nucleus f) anterior nucleus of thalamus g) midline thalamus h) ventral tegmental area i) raphe nuclei j) locus ceruleus 2) cortical a) perirhinal cortex (areas 35 & 36) b) parahippocampal gyrus (areas TF & TH) c) cingulate cortex d) piriform cortex e) insular cortex f) orbitofrontal cortex g) superior temporal gyrus Efferents: 1) subcortical a) septal nuclei b) anterior nucleus of thalamus c) mammillary body d) claustrum e) amygdala f) nucleus accumbens g) striatum (caudate/putamen) h) hypothalamus 2) cortical (from subiculum & entorhinal cortex) a) perirhinal cortex (areas 35 & 36) b) parahippocampal gyrus (areas TF & TH) c) cingulate cortex d) piriform cortex e) temporal polar cortex f) superior temporal gyrus g) ventral insular cortex h) orbitofrontal cortex Pathology: - blood vessels in the hippocampus may be less able to dilate than in the visual cortex, resulting in relative hypoxia during episodes of increased neuronal activity [3] - hypoxia resulting in memory impairment as an early sign of Alzheimer's disease suggested [3] - hippocampal atrophy is linked to cognitive decline independently of amyloid-beta or microtubule-associated protein tau in older adults [4] - hippocampal atrophy may contribute to cognitive decline independent of Alzheimer pathology [4] - bilateral temporal lobectomy results in profound loss of memory - see HM of Brenda Milner

Related

hippocampal afferents hippocampal efferents lateral ventricle lesions of the hippocamus splenium of corpus callosum trisynaptic circuit uncus

General

brain structure

References

  1. Stedman's Medical Dictionary 27th ed, Williams & Wilkins, Baltimore, 1999
  2. The Human Nervous System, George Paxinos, Academic Press, San Diego CA 1990
  3. University of Sussex. Alzheimer's: Blood oxygen levels could explain why memory loss is an early symptom. ScienceDaily, 28 May 2021. http://www.sciencedaily.com/releases/2021/05/210528114028.htm - Shaw K, Bell L, Boyd K et al Neurovascular coupling and oxygenation are decreased in hippocampus compared to neocortex because of microvascular differences. Nature Communications. 2021; May 27;12(1):3190 PMID: 34045465 Free article
  4. Hanseeuw BJ, Jacobs HJ, Schultz AP et al Association of pathological and volumetric biomarker changes with cognitive decline in clinically normal adults: Harvard Aging Brain Study. Neurology. 2023. Nov 15. PMID: 37968130 https://n.neurology.org/content/early/2023/11/15/WNL.0000000000207962

Component-of

limbic system temporal lobe

Components

alveus hippocampi cornus ammonis (Ammon's horn) dentate gyrus entorhinal cortex (Brodmann's area 28) fimbria hippocampi stratum moleculare et substratum lacunosum (lacunar-molecular layer) stratum oriens (oriens layer) stratum pyramidale (pyramidal layer) stratum radiatum (radiant layer) subiculum

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