Septotemporal distribution of entorhinal projections to the hippocampus in the cat: Electrophysiological evidence

The projection of the entorhinal cortex (EA) to the hippocampus in the cat has been studied by electrophysiological methods. Field potentials elicited by EA stimulation sites were measured in the hippocampus (fascia dentata). Different topographic distributions of the amplitude and of the onset latency of average evoked potentials (AEPs) were obtained depending on the place of the stimulation along a lateromedial axis in the Ea. The lateral EA elicited the largest AEPs in the septal part of the hippocampus and the medial EA evoked maximal responses in the temporal part of the hippocampus, while the intermediate part of the EA evoked the largest AEPs in the splenial (intermediate) part of the hippocampus. Unit activity elicited by hippocampal stimulation was measured in the EA. Analysis of the antidromic unit activity showed that the pathways analysed were monosynaptic. Different conduction velocities to the septal part of the hippocampus were found; the pathway from the lateral EA was the fastest and the pathway from the medial EA the slowest. Assuming that the sites of maximal AEP amplitude correspond to the location of the major synaptic inputs, it can be concluded that the active synaptic inputs arising along a latero-medial axis in the EA are distributed within the hippocampus according to a septotemporal axis, although with some overlap between the different projections. Therefore it may be concluded that the hippocampus is not homogeneous with respect to the inputs from the EA. The present observations are discussed regarding anatomical data and putative functional differences between septal and temporal hippocampus.