Region III of hippocampus proper (CA3) is a portion of the hippocampal formation. CA stands for the latin cornus ammonis.
Region CA3 receives input along the mossy fibers from granule cells in the dentate gyrus (DG) and from projection cells in entorhinal cortex along the perforant path. The mossy fiber pathway terminates in stratum lucidum while the perforant path passes through stratum lacunosum and terminates in stratum moleculare. The inputs from the medial septum and diagonal band of Broca terminate in stratum radiatum, along with commisural connections from the contralateral hippocampus. The pyramidal cells in CA3 (of which there are approximately 200,000 in each hemisphere in the rat) send some axons back to the hilus, but the majority project to regions CA2 and CA1 (a pathway called the Schaffer collaterals), in addition to a significant number of connections that terminate within CA3 (called recurrent connections). Both the recurrent connections and the Schaffer collaterals terminate preferentially in the septal or 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 3 divisions. CA3a is the part of the cell band which is most distal from the dentate (and closest to CA1). CA3b is the middle part of the band nearest to the fimbria/fornix connection. CA3c is the most proximal to the dentate, inserting into the hilus. The region overall has been considered to be the “pacemaker” of the hippocampus. In CA3 much of the synchronous bursing activity associated with interictal epileptiform activity appears to be generated. The excitatory collateral interconnectivity appears to be the property of CA3 most responsible for this. CA3, unlike other regions, has pyramidal cell axon collaterals ramifying extensively with local region and making excitatory contacts with neighbors. CA3 has been implicated in a number of working theories on memory and learning hippocampal processes. Slow oscillatory rhythms (theta-band; 3-8Hz) 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.