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. 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. CA4 ĬA4 is a misleading term introduced by Lorente de Nó (1934). Sharp EEG waves seen here are also implicated in memory consolidation. 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. CA3 has been implicated in a number of working theories on memory and hippocampal learning processes. CA3 uniquely, has pyramidal cell axon collaterals that ramify extensively with local regions and make excitatory contacts with them.
Its excitatory collateral connectivity seems to be mostly responsible for this. Much of the synchronous bursting activity associated with interictal epileptiform activity appears to be generated in CA3. CA3 overall, has been considered to be the “pacemaker” of the hippocampus. CA3c is nearest to the dentate, inserting into the hilus. CA3b is the middle part of the band nearest to the fimbria and fornix connection.
CA3a is the part of the cell band that is most distant from the dentate (and closest to CA1). The region is conventionally divided into three divisions. CA3 also sends a small set of output fibers to the lateral septum. Both the recurrent connections and the Schaffer collaterals terminate preferentially in the septal area in a dorsal direction from the originating cells. There are also a significant number of recurrent connections that terminate in CA3. 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 collaterals. There are also inputs from the medial septum and from the diagonal band of Broca which terminate in the stratum radiatum, along with commisural connections from the other side of the hippocampus. The perforant path passes through the stratum lacunosum and ends in the stratum moleculare. The mossy fiber pathway ends in the stratum lucidum. It is often ignored due to its small size.ĬA3 receives input from the mossy fibers of the granule cells in the dentate gyrus, and also from cells in the entorhinal cortex via the perforant path.
Its pyramidal cells are more like those in CA3 than those in CA1. It receives some input from layer II of the entorhinal cortex via the perforant path.
Another significant output is to the subiculum.ĬA2 is a small region located between CA1 and CA3. There are four hippocampal subfields, regions in the hippocampus proper which form a neural circuit called the trisynaptic circuit.ĬA1 is the first region in the hippocampal circuit, from which a major output pathway goes to layer V of the entorhinal cortex.