Cat parahippocampal unit discharge patterns during limbic stimulation

Previous anatomical studies have demonstrated various limbic projections to the parahippocampal gyrus. The influences on parahippocampal units resulting from impulses passing over these fibers are unknown. Units were tested for spike entrainment or time-locking to a restricted regional limbic excitation. More units (40 out of 114) were entrained by septal stimuli than by hippocampal, cingulum, or contralateral parahippocampal gyrus stimuli. Some units fired one time-locked spike while others fired multiple spikes for each stimulus. Time-preferred spikes, varying more in latency than time-locked spikes, were discharged by other units. Pattern changes during septal stimulation suggest multisynaptic paths were involved. During separate periods of septal and hippocampal stimulation a number of units stopped firing. These units frequently did not fire for periods lasting up to 5 sec following the last stimulus. It is concluded the septum, possibly by way of the hippocampus, exerts excitatory and strong inhibitory influences on parahippocampal units. Signals passing over the cingulum and contralateral parahippocampal gyrus may contribute to, but possibly do not block, inputs entering the hippocampus via parahippocampal projections.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32902/1/0000282.pd