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Hippocampus
Brain image adapted, with permission, from the Yakovlev-Haleem collection from the National Museum of Health and Medicine.

Hippocampus

HIPPOCAMPAL ANATOMY
Key related anatomy: tentorium cerebelli, pons, midbrain, ambient cistern (of the basal cisternal subarachnoid space), and temporal horn of the lateral ventricle, choroid fissure (which separates the temporal horn of the lateral ventricle from the subarachnoid space).
key components of the hippocampus (from inside to outside):
  • Dentate gyrus
  • Cornu ammonis (CA)
  • Subicular complex
To draw the hippocampus, first draw a double-sided S.
  • The cornu ammonis comprises the superior turn.
  • The subicular complex forms the horizontal stretch.
  • The C-shaped undefinedlies internal to cornu ammonis – it cups it.
cornu ammonis - Anatomy
The cornu ammonis, which is Latin for "horn of the ram," is also known as Ammon's horn or the hippocampus proper.
  • CA4 lies adjacent to the dentate gyrus.
  • CA3 lies along the medial vertical.
  • CA2 lies along the top of the turn.
  • CA1 lies along the lateral vertical.
CA1 forms the largest stretch of Ammon's horn and is distinctly susceptible to cerebral anoxia.
Uncal hernitation
  • Involves herniation of the uncal portion of the medial temporal lobe.
  • Anteriorly, the uncus encompasses the amygdala, and posteriorly, it forms the anterior aspect of the hippocampus.
  • During medial temporal lobe herniation, the uncus herniates over the tentorium cerebelli and compresses the lateral aspect of the midbrain. It commonly compresses:
1. The ipsilateral CN 3 as it exits the midbrain, causing an oculomotor palsy.. 2. Causes either direct compression of the ipsilateral cerebral peduncle or compresses the contralateral cerebral peduncle against the contralateral tentorium cerebelli, forming a so-called Kernohan's notch in that peduncle, affecting the corticospinal tract. Cerebral peduncle injury causes unilateral upper and lower extremity weakness. The laterality of the weakness depends on the side of cerebral peduncle that is injured.
parahippocampal gyrus
Include: the medial aspect of the fusiform gyrus.
  • The entorhinal cortex forms the medial aspect of the parahippocampal gyrus.
  • The perirhinal cortex forms the lateral aspect of the parahippocampal gyrus.
  • The rhinal sulcus separates the perirhinal and entorhinal cortices.
  • The entorhinal and perirhinal cortices form the anterior parahippocampal gyrus.
  • The posterior parahippocampal gyrus is referred to as the parahippocampal cortex (the latter corresponds to Von Economo areas TF and TH).
  • The collateral sulcus distinguishes the fusiform gyrus, laterally, from the parahippocampal gyrus, medially.
  • The hippocampal sulcus separates the subicular complex from the dentate gyrus.
  • The white matter bundle between the cornu ammonis and the lateral ventricle divides into:
    • The alveus: the region along the border of the cornu ammonis.
    • The fimbria: the superomedial region.
The subicular complex
From lateral to medial (ie, from CA1 to the ambient cistern), the subicular complex comprises:
  • The subiculum, which is the source of the postcommissural fornix fibers, which terminate in the mammillary nuclei and which form a key step in the Papez circuit.
  • The presubiculum
  • The parasubiculum
The Terminologia Anatomica lists the complete subicular complex as part of the hippocampus, whereas other sources list the subiculum, only, as part of the hippocampus and list the pre- and parasubiculum as part of the parahippocampal region.

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