Convergence of associational and commissural pathways on CA1 pyramidal cells of the rat hippocampus

The interaction of the commissural and associational systems to the CA1 region of the hippocampus was studied by recording extracellular field potentials and single unit activity in anesthetized rats. Associational fibers were activated by stimulating the stratum oriens of the CA1 region contralateral to recording: this stimulation activated the Schaffer collaterals by antidromically firing the pyramidal cells of the CA3 region on the side of recording. Commissural fibers were stimulated where they emerge from CA3 region. Both pathways excited both the basal and apical dendrites of the CA1 pyramidal cells. Commissural activation in stratum oriens was more efficient than associational path stimulation, while the opposite was seen in stratum radiatum. Responses elicited by associational path activation had their peak negativity 100–150 μm deeper in stratum radiatum than commissurally evoked responses. Both pathways were able to discharge pyramidal neurons. Both homonymous and heteronymous double pulse stimulation showed response facilitation. Simultaneous activation of both pathways induced a greater amplitude population spike than predicted by algebraic summation of the independent responses. Over 80% of the responsive CA1 cells could be fired by either pathway. These results show a considerable convergence of the commissural and associational pathways on CA1 pyramidal cells, although their predominant locus of excitation might be different.