RECIPROCAL CONNECTION BETWEEN THE OLFACTORY BULB AND THE PYRIFORM CORTEX IN THE RAT

Abstract

Reciprocal connection between the olfactory bulb and the pyriform cortex in the rat was studied with the degeneration (Nauta-Gygax, Fink-Heimer techniques) and the retrograde (Horseradish peroxidase, HRP) tracing methods. Moreover, the feedback system from the pyriform cortex to the olfactory bulb was investigated. The following conclusions were drawn from the present study.
1. Whole areas of the pyriform cortex receive fibers from the olfactory bulb via two separated pathways. (1) Passing through the lateral olfactory tract, mitral cells in the main olfactory bulb send their fibers to the superficial half of the plexiform (I A) layer of the pyriform cortex. (2) Projection fibers from the anterior olfactory nucleus, running through the deep layers of the pyriform cortex and the anterior limb of the anterior commissure, terminate in the deep half of the plexiform (I B), II and III layers of the pyriform cortex.
2. Descending fibers from the pyriform cortex pass through the deeper layer of the cortex, the anterior limb of the anterior commissure, the medial forebrain bundle and the lateral olfactory tract. The rostral and middle parts of the pyriform cortex send fibers to the anterior olfactory nucleus. And it was clearly demonstrated that the main olfactory bulb receives cortical fibers exclusively from the rostral pyriform cortex. Whole cortical regions issue fibers to the nucleus of the horizontal limb of the diagonal band (HLDB) and the lateral hypothalamic area (LHA), many neurons in these two nuclei are always labelled after HRP injection into the main olfactory bulb, and to the mediodorsal nucleus of the thalamus (MD) and the lateral habenular nucleus (LH).
3. The present study exhibited that direct feedback impulse may come from the rostral pyriform cortex to the main olfactory bulb. Activity of the bulb will be indirectly influenced by descending fibers originated in the middle and caudal cortical areas via the anterior olfactory nucleus, and the HLDB and the LHA, respectively. Moreover, the fact that the MD and the LH receive fibers from whole cortical regions suggests strongly a possible existence of other feedback mechanisms to the bulb from heigher cortical centers via the two nuclei.