The differential projections of the olfactory bulb and accessory olfactory bulb in mammals

Abstract

Three species were studied, the rabbit, opossum and rat. Lesions of the main olfactory bulb caused terminal degeneration, assayed by the Fink‐Heimer method, to occur in the ipsilateral olfactory tubercle, prepyriform cortex (including its periamygdaloid part), ventrolateral entorhinal area, and in anterior and posterolateral divisions of the cortical amygdaloid nucleus. The various parts of the ipsilateral anterior olfactory nucleus and the rostroventral end of the anterior continuation of the hippocampus (hippocampal rudiment) also received this projection. Lesions of the accessory olfactory bulb, which receives its sensory input from the vomeronasal (Jacobson's) organ, caused terminal degeneration to occur in the medial amygdaloid nucleus and in a posteromedial part of the cortical amygdaloid nucleus. This projection was conveyed by an accessory olfactory tract, which is accompanied in part of its course by a small nucleus, the bed nucleus of the accessory olfactory tract. The accessory olfactory tract is initially a part of the lateral olfactory tract but becomes increasingly individuated at more posterior levels. It parts company with the lateral olfactory tract at the rostral end of the amygdaloid region, and, in addition to distributing to the medio‐cortical amygdaloid region, it enters the stria terminalis to terminate in the bed nucleus of the stria terminalis in a small region bearing cytoarchitectonic resemblance to the medial amygdaloid nucleus. The topographic segregation of the areas of termination of the olfactory and accessory olfactory (vomeronasal) projections is suggestive of a functional dichotomy in the organization of the olfactory system. A new interpretation of the fundamental subdivisions of the classically described cortico‐medial group of amygdaloid nuclei is evident from these results and was supported by concurrent cytoarchitectonic analysis. It was observed that the medial amygdaloid nucleus (vomeronasal) and the anterior cortical nucleus (olfactory) together form an almost uniform field of small cells. The posteromedial cortical nucleus (vomeronasal) and the posterolateral cortical nucleus (olfactory) form a larger‐celled region. Thus, a quadrantic subdivision of the cortico‐medial area allows two meaningful modes of subdivision, a mediolateral division along a vomeronasal‐olfactory boundary and an antero‐posterior division according to cellular composition. In the context of this analysis, the frequently described posterior extension of the medial amygdaloid nucleus is denied. The origins of the components of the stria terminalis are reviewed in relation to the hypothesis that the segregation of vomeronasal and olfactory pathways is maintained in further projections into the diencephalon. The quadrantic composition of the cortico‐medial area of the amygdala has not been considered in the experimental studies on the origins of the stria terminalis, but resolution of the diencephalic projections of the vomeronasal‐olfactory complex depends upon analytic inspection of the projections from the four quadrants.