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Cornu Ammonis (CA)

 

Position within the brain

The Cornu Ammonis (CA) subfield forms the area between dentate gyrus and the subiculum. It is subdivided into CA3, CA2 and CA1, according to the nomenclature of Lorente de Nó. The polymorphic layer of the dentate gyrus is sometimes referred to as CA4, also defined by Lorente de Nó. However, the existence of CA4 is disputed and we do not delineate this subdivision.

  • CA3 borders with the dentate gyrus and CA2
  • CA2 borders with CA3 and CA1. The CA3/CA2 border is marked by a thickening of the stratum lucidum of CA3.
  • CA1 borders with CA2 and the subiculum

Stratification

In this description of the stratification/lamination of the Cornu Ammonis, the most superficial layer, close to the pial surface of the brain comes first, followed by the layers below. The stratification of the Cornu Ammonis is largely the same for all subfields. The main difference is that CA3 has a layer called the stratum lucidum, whereas this layer is absent in CA2 and CA1. Below, the layers of CA3, CA2 and CA1 are summarized. 

CA3

  • Stratum lacunosum-moleculare; contains the terminations of, for example, the perforant pathway.
  • Stratum radiatum; contains CA3-CA3 associational connections and CA3-CA1 Schaffer collaterals
  • Stratum lucidum; contains the mossy fiber axons from the dentate gyrus.
  • Pyramidal cell layer or stratum pyramidale; principal cell layer
  • Stratum oriens;

CA2 / CA1

  • Stratum lacunosum-moleculare
  • Stratum radiatum
  • Pyramidal cell layer or stratum pyramidale; principal cell layer
  • Stratum oriens

Cells

Several good overviews exist, mostly tailored towards the physiology of interneurons (e.g. Freund and Buzsaki, 1996; Klausberger, 2009). In the near future, an online repository containing all known cell types of the hippocampal formation and their characteristics will become available on: www.hippocampome.org.

The animation above displays a CA3 (left) and CA1 (right) pyramidal neuron. The cells were reconstructed by Ishizuka, 1995 and downloaded from neuromorpho.org (Ascoli, 2007).

The authors of these web pages express their gratitude to the many authors that helped provide order and overview in the field of hippocampal anatomy and connectivity. A special mention of thanks for the authors of the 1st, 2nd and 3rd edition of the hippocampus chapter in the book titles The Rat Nervous System  and The Parahippocampal Region. The authors of this website welcome feedback to improve the information that is displayed. Please use the contact form for such enquiries. These pages are currently (2013) under active development and will be frequently updated and expanded.

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