Abstract
The aim of this chapter is to extend the standard simplified diagram of the connectional organization of the hippocampus found in many current textbooks, by adding details on the connectivity of area CA2 and on entorhinal intrinsic wiring. In the chapter, some of the ‘traditional wisdoms’ on hippocampal connectivity are discussed, emphasizing the need for a more inclusive framework to model the hippocampus. The chapter focusses on intrinsic connections, and many of the well-known extrinsic connections of the hippocampus will not be covered in this chapter, for two reasons. First, the information is already available at a summarized (meta) level, and a new summary would not assist those who need anatomical details to contribute to the explanation of the functional outcome of a study. Second, this chapter is meant to provide a framework of knowledge to support computational modelling of the region, and therefore only the most relevant and quantitative data on the connectivity of the hippocampus are covered.
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Notes
- 1.
The lateral and medial entorhinal cortex or Brodmann’s areas 28a and 28b, respectively, have been further subdivided by a large number of authors (for a more detailed description and comparison of different nomenclatures used in the rat and in other species, the reader is referred to a number of reviews (cf. Witter et al., 1989)). In the rat, and likewise in the mouse, a further division into dorsolateral (DLE), dorsal-intermediate (DIE), ventral-intermediate (VIE), caudal (CE) and medial (ME) subdivisions have been proposed (Insausti et al., 1997, Hippocampus 7:146; van Groen etal., 2003, Hippocampus 13: 133–149). In monkeys, humans and in other species in which the entorhinal cortex was described, such as cat, dog, guinea pig and bat (Amaral et al., 1987 J Comp Neurol 264: 326–355; Witter et al., 1989, Progr Neurobiol 33:161–254; Buhl and Dann 1991, Hippocampus 1: 131–152; Insausti et al., 1995, J Comp Neurol 355: 171–198; Uva et al., 2004 J Comp Neurol 474: 289–303; Woznicka et al. 2006, Brain Res Rev. 52: 346–367), comparable partitioning schemes have been proposed. However, in case of most species, there is a tendency to consider the entorhinal cortex as composed of two primary components, the lateral and medial entorhinal cortex, most likely reflecting functional differences (see further Witter et al. 2017a, Front Syst Neurosci 11:46).
- 2.
Note that some authors have adopted a slightly different nomenclature in which the lamina dissecans is either without number or considered to be the deep part of layer III (layer IIIb), such that layer IV is used to designate the superficial part of layer V, characterized by the presence of rather large pyramidal cells that stain strongly for Nissl substance.
- 3.
See Witter et al. 2017 Brain Behav Evol 90:15–24 for details on the complex and sometimes confusing terminology used to describe EC-HF projections.
- 4.
Note that the term temporo-ammonic tract is often used to refer to all of the entorhinal projections to the CA fields but more commonly only to all fibres that reach CA1. In the temporal portion of the hippocampus, most of the entorhinal fibres reach CA1 after perforating the subiculum (classical perforant pathway). At more septal levels, however, the number of entorhinal fibres that take the alvear temporo-ammonic pathway increases.
A third route taken by fibres from the entorhinal cortex involves the molecular layers of the entorhinal cortex, para- and presubiculum, continuing into the molecular layer of the subiculum. The latter route has not been given a specific name.
- 5.
The laminar pattern has been extensively described in the rat and available data in mice, guinea pigs and cats indicate a similar laminar terminal differentiation between the lateral and medial components of the perforant path. In contrast, in the macaque monkey the situation is different in that irrespective of the origin in EC, at all levels of the dentate gyrus, projections have been reported to distribute throughout the extent of the outer two-thirds of the molecular layer and stratum lacunosum –moleculare in CA3. It is important though that in all species information from functionally different entorhinal domains converges onto a single population of dentate and CA3 cells.
- 6.
In rodents, the layer II components from the LEC and the MEC apparently do not overlap with respect to their respective terminal zone in the molecular layer of the DG and likely the same holds true for CA3. It has not been established whether the same holds true for the respective layer III components, i.e. whether or not they have a zone of overlap in the centre part of CA1 or the Sub.
- 7.
Amygdala inputs reach only the ventral two-thirds of the CA1 and the Sub. The dorsal one-third of both fields is devoid of input from the amygdala.
Resources
http://www.temporal-lobe.com/: online searchable database of hippocampus connectivity and visualization tools
http://www.rbwb.org/: online atlas tools with detailed description of the anatomy of the hippocampal region in the rat
http://neuromorpho.org/index.jsp: index of morphologies of hippocampal neurons
Further Reading
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Acknowledgements
The revision of this chapter has been supported by the Kavli Foundation, the Centre of Excellence scheme – Centre for Neural Computation and research grant # 191929 and 227769 of the Research Council of Norway and The Egil and Pauline Braathen and Fred Kavli Centre for Cortical Microcircuits.
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Witter, M.P. (2018). Connectivity of the Hippocampus. In: Cutsuridis, V., Graham, B., Cobb, S., Vida, I. (eds) Hippocampal Microcircuits. Springer Series in Computational Neuroscience. Springer, Cham. https://doi.org/10.1007/978-3-319-99103-0_1
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