Schematic diagrams of the trisynaptic pathway showing pyramidal neurons (triangular) and GABA cells (square) in stratum oriens (SO), stratum pyramidal (SP), and stratum radiatum (SR) of sectors CA3/2 and CA1. The perforant path projection from the entorhinal cortex project to the granule cells and these in turn send mossy fiber projections that synapse on the apical dendrites of pyramidal neurons in sector CA3/2. The latter cell sends projects axons into the SO where they travel as Schaffer collaterals that eventually form excitatory synapse with the apical dendrites of pyramidal neurons in sector CA1. Collateral branches of pyramidal neurons in CA3/2 and CA1 form synapses with GABAergic interneurons on the SO. The arrows at the bottom of each diagram show the direction of feed-forward excitation along the trisynaptic pathway and suggest that this may be increased in schizophrenics because of diminished GABAergic tone in both CA3/2 and CA1. In BDs, however, feed-forward excitation may be attenuated at the level of CA1 because of the heightened activity of GABA cells with in SO at this locus.