Axonal ramifications of hippocampal Ca1 pyramidal cells.

Intracellular injections of Lucifer Yellow into CA1 pyramidal cells of the in vitro guinea pig hippocampal slice enabled us to examine in detail the morphology of the axons of these neurons. We also recorded the electrophysiological responses of these neurons to alvear stimulation. In our morphological examinations, we found that many axons bifurcate in the alveus, with the major branch projecting caudally toward the subiculum and the second, thinner branch projecting rostrally toward the fimbria. Either axons may bifurcate further to produce several axon branches which follow parallel paths in the alveus. These axons also have local collaterals which project into strata oriens and pyramidale. In addition, a very fine plexus of axonal processes was observed in stratum oriens located largely within the basal dendritic field of the parent cell. Our electrophysiological experiments demonstrated that neurons could be activated antidromically by stimulation of the alveus at sites both rostral ad caudal to the neuron. Weak alvear stimulation occasionally evoked small potentials which appeared similar to fast prepotentials. The local axonal ramifications may be involved in recurrent pathways mediating feedback inhibition and/or excitation. The axonal bifurcations also may provide a basis for understanding the origins of fast prepotentials elicited with antidromic stimulation.