The significance of the rabbit's septum as a relay station between the midbrain and the hippocampus I. The control of hippocampus arousal activity by the septum cells

Abstract

1. 1. Septum unit activity and hippocampus EEG were recorded simultaneously in 22 unanesthetized, curarized rabbits. Theta activity was induced by reticular or sensory stimulation, or by repeated administration of eserine. A vector method was used to display phase correlations between the unit firing pattern and theta waves. 2. 2. Two types of septal units were differentiated by their discharging pattern, namely A-units, which fired more or less at random, and B-units, which fired in bursts synchronous with the hippocampus theta rhythm. 3. 3. Phase angles with respect to theta waves remain fairly constant within B-units and vary between them. Phase angles within B-units, however, may change after temporary abolition of the theta rhythm, e.g. by hippocampus seizure. 4. 4. The burst activity of B-units may remain unchanged even when the theta rhythm is temporarily replaced by another type of activity, a fact which supports the assumption that the rhythm of the theta activity is the consequence rather than the cause of the burst firing pattern of septum cells. 5. 5. B-units are distributed within a median area of the septum which corresponds, anatomically, to a zone best described by Young (1936) under the name “nucleus of the diagonal band”. It is identical with the zone the stimulation of which influences the theta rhythm at the lowest threshold.