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Electrical activity of the olfactory bulb Graystone, Peter

Abstract

The electrical activity of the olfactory bulbs of many vertebrates is characterized by large amplitude regular bursts of waves. These waves, known as induced waves, appear with each inspiration of odourous air through the nostrils. The work described in this thesis establishes that the induced waves are detectable in a variety of species investigated, from amphibia to mammalia. The occurrence of the waves under similar experimental conditions leads to the conclusion that the mechanism of their genesis is similar in all animals. The group activity of the neurones in the various layers of the olfactory bulb was studied together with the amplitude of the induced wave signal. It was found that most cellular activity occurred in the external plexiform and mitral cell layers whereas the maximum of the induced wave activity was in the granular layer. A peak of cellular activity was observed in the external plexiform and mitral cell layers with both nostrils occluded. On opening the ipsilateral nostril the cellular activity was enhanced in these same layers and on opening the contralateral nostril it was decreased. With strong olfactory stimulation an increase in the cellular activity in the granule cell layer was observed. Recordings from single olfactory neurones were made and whereas these were easily observed in the external plexiform and mitral cell layers they were found only rarely in the granule cell layer. It is proposed that the granule cell layer is the site of integration of the inhibitory influence from the contralateral olfactory bulb and from higher centres of the brain. It is further postulated that the cells in the granule cell layer do not normally generate action potentials but that they are capable of doing so with strong olfactory stimulation. It is thought that these cells are the site of genesis of the induced wave activity which is probably generated at the dendrodendritic synaptic connections with the secondary neurones. Evidence in favour of these dendrodendritic synapses being the normal pathway between the mitral and granule cells is presented.

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