Effect of hippocampal stimulation on evoked electrical activity of various brain structures

Abstract

Functional relations between the entorhinal cortex and hippocampus were stud~ ~d in acute and chronic experiments on cats. The dorsal hippocampus depresses electrical activity and responses of the ventral hippocampus evoked by stimulation of the entorhinal cortex and responses of the entorhinal cortex and ventral hippocampus to cutaneous electrical stimulation. The ventral hippocampus, on the other hand, facilitates evoked electrical activity of the dorsal hippocampus in response to stimulation of the entorhinal cortex. and also responses of the entorhinal cortex and dorsal hippocampus evoked by cutaneous electrical stimulation. The results suggest regional differences in nervous connections of the different parts of the hippocampus with the entorhinal cortex.According to the theory of Papez [23] emotion has its structural basis, and the "limbic brain," with extensive afferent and efferent connections, plays a major role in its organization. It has been postulated that emotions arise as a result of activation of interacting structures: the hippocampus, entorhinal cortex, gyrus cinguli, hypothalamus, and anterior thalamic nuclei. The whole of this system is known in the literature as the "circle of Papez."Many investigations ([3, 18, 22] etc.) have pointed to the participation of the hippocampus in the manifestation of autonomic and somatic elements of emotional responses. Morphological and electrophysiological data indicate its extensive afferent and efferent connections both with the neocortex and with many subcortical brain formations. It has been found that however different the formations of the forebrain are in their structural organization, they share one common morphological feature: They all project on the hippocampus and the hippocampal pyramids are as it were the "final common path for the varied afferentation converging on them from these formations." It is at the hippocampal level that the functional relations which lie at the basis of emotional experience and which determine its character are formed [7].One of the principal afferent inputs into the hippocampus has been described, namely, the powerful temporo-ammonic tract {perforating tract) running from the temporal region through the lateral regions of the entorhinal cortex to Ammon's horn. On reaching the latter, it penetrates into it and terminates in the layer of the alveus [14].Tracts from the temporal region to the hippocampus have been shown to contain a nonperforating alvear bundle as well as the perforating bundle. The number of axons of temporal neurons which penetrate into the hippocampus in the composition of the perforating and alvear tracts and the number of contacts which they form with neurons in each part of the hippocampus differ [2]. There is also evidence of the presence of axon collaterals of hippocampal pyramidal cells forming contacts with collaterals of neurons of the entorhinal cortex [14,20].In experiments on marsupials (Trichosurus vulpecula), to study relations of the hippocampus with the entorhinal cortex, it was assume...