Fiber trajectories of olfactory bulb efferents in the hamster.

Olfactory bulb efferents sweep caudally over the surface of the piriform lobe in a broad fiber sheet. The internal organization of this axon population was analysed by topologically transforming the cortical surface from its in situ cylindrical form into an unrolled (flattened) map. The distribution of degeneration elicited by restricted bulb lesions and fiber transections was then reconstructed onto this map. Most of the projection cortex of the main olfactory bulb is innervated in a widespread, non-topographic manner by axons that collect in the compact bundle of the lateral olfactory tract (LOT). LOT collateral branches bound for the prepiriform cortex veer laterally off the main trajectory of the tract at an angle of 50 degrees or less. Thus, transection of discrete fiber populations leaves only a small wedge-shaped pocket of totally denervated cortex distal to the cut. The medial half of the olfactory tubercle and the hippocampal rudiment receive their bulbar input along medially disposed fibers that do not join the LOT proper. The lateral half of the olfactory tubercle, however, receives an input from LOT fiber collaterals as well as these medial bulb efferents. Finally, much of the corticomedial amygdaloid complex receives fibers from the accessory olfactory bulb along a specialized subdivision of the LOT, the accessory olfactory tract. These observations are expressed in a schematic summary of the trajectories of olfactory bulb efferents as they appear in the unrolled map and in the more standard ventral view of the hamster brain.