Fig. 2.

Schematic drawing of the circuitry in the hippocampal CA1 area (redrawn from Paulsen and Moser 1998). Pyramidal cells in the hippocampal CA1 area project to γ-aminobutyric acid (GABA)ergic interneurons via feed-forward and feedback circuits. GABAergic interneurons whose cell bodies are in the pyramidal cell layer (SP) of the CA1 region and are activated in a feed-forward manner include axo-axonic cells (AACs), basket cells (BCs), and bi-stratified cells (BSCs). These contact the axon initial segments and the somatic and dendritic compartments of the pyramidal cells. GABAergic feedback loops include at least one short loop involving basket interneurons and at least one long loop involving the horizontal cells (HCs) of the stratum oriens-alveus (SO-A), which project to the stratum lacunosum-molecular (SL-M). The interneurons in the hippocampus CA1 areas are merely illustrated to show the principles of organization, and this is not an exhaustive list