Activity in Single Hippocampal Pyramids

Abstract

AbstractEuler, C. V. and J. D. Green. Activity in single hippocampal Pyramids.Activity from single pyramidal cells of dorsal hippocampus has been studied in rabbits under light urethane anaesthesia or with encéphale isolé. Capillary microelectrodes were used for recording. Respiratory and cardiovascular movements of the brain were reduced by using a ‘closed system’. Extracellular biphasic positive‐negative spikes of large amplitude were frequently recorded. The shape of the extracellular spikes indicate that the soma spikes are usually not followed by excitation of the dendritic membrane. However, occasionally spikes with negative‐positive potential sequence indicating somatofugal excitation of the dendrites, have been recorded after a burst of positive‐negative spikes. This occurred in response to prolonged repetitive stimulation and in after‐discharges. Spikes were normally not recorded in the dendritic layer. Repetitive stimulation, however, elicited sharp spikes in the apical dendrites increasing in number and amplitude with increasing stimulus strength and duration.Action potentials and slow changes in membrane potential were commonly recorded with big attenuation but without other signs of serious injury.Polarizing hippocampal neurones with minute currents in the order of 2 × 10‐9A through the recording microelectrode stimulated or inhibited that cell from which the activity was extracellularly recorded.The hippocampus is an uncomplicated cortical structure with a relative uniform and simple lamination. These and other features make the Ammon's horn a suitable site for studies of cortical properties (see e. g. Euler, Green and Ricci 1958). This paper presents an analysis of the spontaneous activity in single hippocampal pyramids and a component analysis of spike potentials recorded extracellularly. A subsequent paper (Euler and Green 1960) deals with the activity patterns which can be elicited through afferent stimulation. A ‘positive’ wave with rapidly declining ‘positive’ spikes proved to be a pattern entity of possible significance for the ‘brain waves’ of the hippocampus. Although no successful intracellular records of this phenomenon were obtained, the experimental data presented in the present paper lend themselves to the conclusion that the spikes and the wave originate in the same neurone and that the phenomenon thus may be identified as the ‘inactivating process’ of Granit and Phillips (1956).