Abstract
Excitatory actions of unspecific midbrain structures upon CA2–3 hippocampal neurons were studied intracellularly in curarized cats under barbiturate anesthesia. Midbrain stimulation induced EPSPs at long latencies in 62% of neurons tested. Responses contained one or, more commonly, a sequence of several, EPSPs which produced ‘oscillatory’ patterns. Individual EPSPs were characterized by a prolonged rising phase but differed in their time course after attaining the maximum. Variations observed have been interpreted as a result of interaction between different types of afferent influences and secondary hyperpolarizing processes of intrahippocampal origin. Average patterns of slow potential changes and spike discharges have been derived in an attempt to characterize the population activity induced in the CA2–3 region by mesencephalic stimulation. Observations on the modes of spike generation have led to the conclusion that output discharge of hippocampal neurons in response to activation of polysynaptic ascending pathways is defined, in a majority of cells, by electronic depolarization of the soma membrane brought about by axo-dendritic excitatory synapses. An involvement of apical dendrites in the transmission of excitatory effects was indicated by an increased discharge rate of fast prepotentials in those neurons spontaneously showing this type of activity.
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