Region I; CA1

The region 1 is the first region in the hippocampal circuit that yields a significant output pathway, which goes to entorhinal cortex layer V. It also sends significant output forward to the subiculum. Like region III, it receives input from superficial entorhinal cortex along the perforant pathway. Unlike region III, however, it contains very few recurrent connections. In the rat, region I contains approximately 250,000 pyramidal cells.