Crossing fiber arrays in the rat hippocampus as demonstrated by three-dimensional reconstruction.

The hippocampus is a neural substrate playing a key role in short-term memory. In order to achieve a better understanding of how the hippocampus functions in "learning and memory," we conducted an intracellular horseradish peroxidase (HRP) study of the CA3 pyramidal neurons and the granule cells of the fascia dentata. The axon of the CA3 pyramidal neurons has two components, the longitudinal association system and the Schaffer collateral system. The latter component is organized in a lamellar fashion and follows the alvear fiber stream. An electron microscopic analysis of myelinated fibers suggested that most myelinated fibers in the hippocampus are organized parallel to the alvear fibers. The mossy fibers of the granule cells, however, do not follow the alvear fiber stream. We propose a new model of the organization of the intrinsic excitatory circuitry of the rat hippocampus in which the distinct lamellar organization of the pyramidal and granule cells creates a crossing neural network.