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The hippocampus proper refers to the actual structure of the hippocampus which is made up of three regions or subfields. The subfields CA1, CA2, and CA3 use the initials of
cornu Ammonis The hippocampus proper refers to the actual structure of the hippocampus which is made up of three regions or subfields. The subfields CA1, CA2, and CA3 use the initials of Hippocampus#Name, cornu Ammonis, an earlier name of the hippocampus. St ...
, an earlier name of the
hippocampus The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
.


Structure

There are four hippocampal subfields, regions in the hippocampus proper which form a neural circuit called the trisynaptic circuit.


CA1

CA1 is the first region in the hippocampal circuit, from which a major output pathway goes to layer V of the
entorhinal cortex The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in the ...
. Another significant output is to the
subiculum The subiculum (Latin for "support") is the most inferior component of the hippocampal formation. It lies between the entorhinal cortex and the CA1 subfield of the hippocampus proper. The subicular complex comprises a set of related structures ...
.


CA2

CA2 is a small region located between CA1 and CA3. It receives some input from layer II of the entorhinal cortex via the
perforant path In the brain, the perforant path or perforant pathway provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields (including CA1), and the subiculum. Though it ...
. Its pyramidal cells are more like those in CA3 than those in CA1. It is often ignored due to its small size.


CA3

CA3 receives input from the mossy fibers of the
granule cell A granule is a large particle or grain. It can refer to: * Granule (cell biology), any of several submicroscopic structures, some with explicable origins, others noted only as cell type-specific features of unknown function ** Azurophilic granul ...
s in the
dentate gyrus The dentate gyrus (DG) is part of the hippocampal formation in the temporal lobe of the brain, which also includes the hippocampus and the subiculum. The dentate gyrus is part of the hippocampal trisynaptic circuit and is thought to contribute t ...
, and also from cells in the entorhinal cortex via the perforant path. The mossy fiber pathway ends in the
stratum lucidum The stratum lucidum (Latin, 'clear layer') is a thin, clear layer of dead skin cells in the epidermis named for its translucent appearance under a microscope. It is readily visible by light microscopy only in areas of thick skin, which are found ...
. The perforant path passes through the stratum lacunosum and ends in the stratum moleculare. There are also inputs from the
medial septum The medial septal nucleus (MS) is one of the septal nuclei. Neurons in this nucleus give rise to the bulk of efferents from the septal nuclei. A major projection from the medial septal nucleus terminates in the hippocampal formation. It plays a ro ...
and from the
diagonal band of Broca The diagonal band of Broca is one of the basal forebrain structures that are derived from the ventral telencephalon during development. This structure forms the medial margin of the anterior perforated substance. This brain region was described b ...
which terminate in the stratum radiatum, along with commisural connections from the other side of the hippocampus. The pyramidal cells in CA3 send some axons back to the dentate gyrus hilus, but they mostly project to regions CA2 and CA1 via the
Schaffer collateral Schaffer collaterals are axon collaterals given off by CA3 pyramidal cells in the hippocampus. These collaterals project to area CA1 of the hippocampus and are an integral part of memory formation and the emotional network of the Papez circuit, and ...
s. There are also a significant number of recurrent connections that terminate in CA3. Both the recurrent connections and the Schaffer collaterals terminate preferentially in the septal area in a dorsal direction from the originating cells. CA3 also sends a small set of output fibers to the lateral septum. The region is conventionally divided into three divisions. CA3a is the part of the cell band that is most distant from the dentate (and closest to CA1). CA3b is the middle part of the band nearest to the fimbria and fornix connection. CA3c is nearest to the dentate, inserting into the hilus. CA3 overall, has been considered to be the “pacemaker” of the hippocampus. Much of the synchronous bursting activity associated with interictal epileptiform activity appears to be generated in CA3. Its excitatory collateral connectivity seems to be mostly responsible for this. CA3 uniquely, has pyramidal cell axon collaterals that ramify extensively with local regions and make excitatory contacts with them. CA3 has been implicated in a number of working theories on memory and hippocampal learning processes. Slow oscillatory rhythms (theta-band; 3–8 Hz) are cholinergically driven patterns that depend on coupling of interneurons and pyramidal cell axons via gap junctions, as well as glutaminergic (excitatory) and GABAergic (inhibitory) synapses. Sharp EEG waves seen here are also implicated in memory consolidation.Jerome Engel TAP, ed. Epilepsy: A Comprehensive Textbook in Three Volumes. Philadelphia, PA: Lippincott Williams & Wilkins; 2008


CA4

CA4 is a misleading term introduced by Lorente de Nó (1934). He observed that the pyramidal layer of the CA3 was continuous with polymorphic layer of the dentate gyrus and that the "modified pyramids" (later known as mossy cells (Amaral, 1978)) had Schaffer collaterals similar to CA3 pyramdidal cells. Amaral (1978) showed that the mossy cells in the CA4 of Lorente de Nó did not have schaffer collaterals and that they in contrast to pyramidal cells project to the inner molecular layer of the DG and not to CA1. The same author thus concluded that the term CA4 should be abandoned and that the zone should be regarded as the polymorphic layer of the dentate gyrus (the area dentata of Blackstad (1956)). The polymorphic layer is often called the hilus or hilar region (Amaral, 2007). The neurons in the polymorphic layer, including mossy cells and GABAergic interneurons, primarily receive inputs from the granule cells in the dentate gyrus in the form of mossy fibers and project to the inner molecular layer of the dentate gyrus via the associational/commissural projection . They also receive a small number of connections from pyramidal cells in CA3. They, in turn, project back into the dentate gyrus at distant septotemporal levels.


Additional images

File:Coronary brain section through hippocampus.jpg, High resolution micrograph of human brain,
coronal section The coronal plane (also known as the frontal plane) is an anatomical plane that divides the body into dorsal and ventral sections. It is perpendicular to the sagittal and transverse planes. Details The coronal plane is an example of a longi ...
through the hippocampus. File:HippocampalRegions.jpg, Diagram of hippocampal regions in a rat brain.


References

{{Authority control Hippocampus (brain)