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.

• 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, 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.
  • stratum oriens (ventricular surface)
    • basal dendrites of pyramidal cells
    • axon collaterals of pyramidal cells
    • incoming axons of the alvear pathway*
  • stratum pyramidale (pyramidal layer)
    • perikarya of hipocampal pyramidal cells
  • stratum radiatum
    • apical dendrites of pyramidal cells
    • recurrent axon collaterals of pyramidal cells
  • 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)

• subcortical
  • amygdala
  • claustrum
  • septal nuclei (by way of the fornix)
  • nucleus basalis of Meynert
  • supramammillary nucleus
  • anterior nucleus of thalamus
  • midline thalamus
  • ventral tegmental area
  • raphe nuclei
  • locus ceruleus
• cortical
  • perirhinal cortex (areas 35 & 36)
  • parahippocampal gyrus (areas TF & TH)
  • cingulate cortex
  • piriform cortex
  • insular cortex
  • orbitofrontal cortex
  • superior temporal gyrus

Efferents

• subcortical
  • septal nuclei
  • anterior nucleus of thalamus
  • mammillary body
  • claustrum
  • amygdala
  • nucleus accumbens
  • striatum (caudate/putamen)
  • hypothalamus
• cortical (from subiculum & entorhinal cortex)
  • perirhinal cortex (areas 35 & 36)
  • parahippocampal gyrus (areas TF & TH)
  • cingulate cortex
  • piriform cortex
  • temporal polar cortex
  • superior temporal gyrus
  • ventral insular cortex
  • 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
  • hypoxia resulting in memory impairment as an early sign of Alzheimer's disease suggested
• hippocampal atrophy is linked to cognitive decline independently of amyloid-beta or microtubule-associated protein tau in older adults
  • hippocampal atrophy may contribute to cognitive decline independent of Alzheimer pathology
• bilateral temporal lobectomy results in profound loss of memory
  • see HM of Brenda Milner

More general terms

• brain structure

Additional terms

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

Component of

• limbic system
• temporal lobe

Components

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