Organization of cerebral cortex. IV. Posttetanic potentiation of hippocampal pyramids

Abstract

The response of hippocampal pyramidal cells following single and repetitive stimulation of ascending fornical and commissural afferents has been studied in anesthetized rabbits and cats. Low intensity single pulse stimulation evokes a response of 10–15 msec. duration. The region of the apical dendrites is positive and the region of the cell body, basal dendrites and axon negative with respect to a distant electrode. Single stimuli did not produce a detectable efferent discharge in the alveus or fimbria. Following repetitive stimulation at 5–20/sec., a 3–5-mv potential with a duration of 3 msec. is superimposed upon the response seen following a single stimulus. This large spike shows both a tetanic and posttetanic potentiation characterized by an increased amplitude and a decreased latency. Minimal latency occurs 1–2 seconds following the termination of repetitive stimulation while maximum amplitude is attained during the repetitive stimulation. During potentiation the spike may decrease its latency up to 8 msec. When the high amplitude spike is present an efferent volley may be recorded from the alveus and fimbria. Isopotential contour mapping of these responses indicates that the spike which displays potentiation is a reflection of depolarization of the cell body and stem of the apical dendrites of hippocampal pyramidal cells.