Region III; CA3

The region III receives input along the mossy fibers from granule cells in the dentate gyrus 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 region III send some axons back to the hilus, but the majority project to regions II and regions I, in addition to a significant number of connections that terminate within regions III. Both the recurrent connections and the Schaffer collaterals terminate preferentially in the septal is dorsal direction from the originating cells. Regions III also sends a small set of output fibers to the lateral septum. The region is conventionally divided into 3 divisions. Regions III a is the part of the cell band which is most distal from the dentate. Regions III b is the middle part of the band nearest to the fimbria/fornix connection. Regions III c 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 regions III much of the synchronous bursting activity associated with interictal epileptiform activity appears to be generated. The excitatory collateral interconnectivity appears to be the property of regions III most responsible for this. Regions III, unlike other regions, has pyramidal cell axon collaterals ramifying extensively with local region and making excitatory contacts with neighbors. Regions III has been implicated in a number of working theories on memory and learning hippocampal processes. Slow oscillatory rhythms are cholinergically driven patterns that depend on coupling of interneurons and pyramidal cell axons via gap junctions as well as glutaminergic and GABAergic synapses. Sharp EEG waves seen here are also implicated in memory consolidation.