Antidromic conduction in a direct posterior pathway from midbrain to hippocampus

An average field response to electrical stimulation in the hippocampus has been recorded in dorsomedial rat midbrain. The fully developed response is triphasic. Latencies of the three components and their behavior during repetitive stimulation are compatible with the assumption that they are antidromic and originate, respectively, in proximal axons, cell bodies and dendrites. Conduction velocity is estimated to be 5 m/s. Reversal in polarity of the response at different recording points within a distance of 1 mm and abolition by small midbrain lesions confirm location of the active neural elements in that region. Lesions in the white matter of posterior cingulum and in the thalamocortical radiation abolish the response. Latencies of the response recorded in the thalamocortical radiation are in accord with the conclusion that the fibers pass in this location. The findings support results of an earlier study suggesting that visual information is transmitted without a synapse from dorsomedial midbrain to dentate gyrus by a pathway that reaches the cerebral hemisphere in the thalamocortical radiation and enters hippocampus from white matter in the posterior cingulum. A study of visual responses in the medial and dorsal wall of the cerebral hemisphere in lizards may help to evaluate the hypothesis that in these regions are the ancestral homologues for dentate gyrus and cornu ammonis, respectively, in mammalian hippocampus.