Neural pathway associated with the EEG arousal response by olfactory stimulation

Abstract

In trigeminal deafferented encéphale isolé and intact cats immobilized with gallamine triethiodide, and midpontine pretrigeminal cats, an arousal effect of olfactory stimulation on the neocortical and hippocampal electrical activities was studied. The goal was to explore the desynchronizing system for the olfactory arousal and to determine the olfactory pathway to this desynchronizing system. 1. 1. Neocortical and hippocampal arousal responses were elicited either by odor or by electrical stimulation of the olfactory bulb. The latter stimulation occasionally provoked seizure discharges in the prepyriform cortex and hippocampus. Phenobarbital markedly affected the olfactory arousal response and abolished it in doses of 6–10 mg/kg. 2. 2. In a high cerveau isolé cat, in which the brain stem was transected at the border between the diencephalon and the mesencephalon, either blowing of odorized air into the nostril or electrical stimulation of the olfactory bulb failed to elicit the EEG arousal response, though electrical stimulation of the nuclei ventralis anterior and centralis medialis of the thalamus still provoked a neocortical desynchronization. This is interpreted as meaning that the mesencephalic reticular formation plays an important role in producing the olfactory arousal. 3. 3. Among anatomically proposed olfactory pathways to the mesencephalon, a lesion of the medial forebrain bundle led to a complete disappearance of the olfactory arousal, while it was not abolished by lesions of the fornix and the stria terminalis. It is concluded that the olfactory arousal response is produced by projection of olfactory impulses to the mesencephalic reticular formation through the medial forebrain bundle.