Responses of the hippocampus to stimulation of the olfactory bulb and of various afferent nerves in five mammals

The proportion of olfactory and other afferent impulses reaching the hippocampus was found to vary between the species examined, and evidence of a multisynaptic pathway from the prepiriform cortex to the anteroventral recurved part of the hippocampus has been found in the rabbit.

Abstract

The proportion of olfactory and other afferent impulses reaching the hippocampus was found to vary between the species examined. In the rat, a large neuronal action potential was recorded extracellularly in Ammon's horn when single electric shocks were applied to the ipsilateral olfactory bulb. This action potential was invariably augmented by rapid repetition of the stimulus. In the guinea pig, only a small action potential was seen in response to single olfactory stimuli, but a large response always appeared with fast repetitive stimulation. In the rabbit, single olfactory stimuli evoked responses that were hardly discernible in the presence of spontaneous activity, but large neuronal action potentials invariably appeared after fast repetitive stimulation. In these three species, no response was recorded in Ammon's horn to stimulation of various other afferent nerves. In the cat, a small action potential was often seen in response to single olfactory stimuli, and at low rates of repetition, definite responses were seen at many hippocampal placements with olfactory stimuli and often with stimuli applied to other afferent nerves as well. The responses in the cat were only occasionally augmented by rapid repetition of the stimulus. In the monkey, a small part of Ammon's horn was found to respond to slowly repeated olfactory stimuli and to give just discernible responses to single shocks. The responses were never augmented by fast repetitive stimulation, and in this region, were not evoked by stimulation of other afferent nerves. Evidence of a multisynaptic pathway from the prepiriform cortex to the anteroventral recurved part of the hippocampus has been found in the rabbit.