CENTRIFUGAL NERVOUS SYSTEM TO THE OLFACTORY BULB

Abstract

The activity of the centrifugal olfactory connections were studied in toads, frogs and bullfrogs. When an electrical pulse was applied to the anterior part of the diencephalon, an electrical wave was induced in the olfactory bulb (E-wave). The function of this wave was studied in connection with the one (A-wave) induced in the olfactory bulb by stimulation of the olfactory epithelium or nerve. The E-wave generally has a strong inhibitory effect upon the A-wave which lasts for two to five seconds. This inhibition in the olfactory bulb was blocked by picrotoxin. On the other hand, the latter wave inhibits the former one only slightly and for 600 msec at the longest. Consequently, it was concluded that the inhibition of the A-wave by the E-wave is due to a real inhibitory mechanism, while that of the E-wave by the A-wave is due to the refractoriness of neurons in the bulb. In a few cases, facilitation of the A-wave by the E-wave was found. Similar inhibition was also found at various points of the cerebrum. Recording of the E-waves at various points of the cerebrum surface disclosed that this olfactory centrifugal nervous system originates in the anterior part of the diencephalon, divides and courses as a pair of tracts along the lateral olfactory tracts bilaterally to the olfactory bulbs. In the light of the histological finding, a neural mechanism of inhibiton and facilitation was proposed in the olfactory bulb. The biological role of the centrifugal nervous system was considered.