This study reinvestigates the efferent projections of the main and accessory olfactory bulbs and describes for the first time the centrifugal projections to the main and accessory olfactory bulbs in the snake Thamnophis sirtalis, using the intraaxonal transport of the anterograde and retrograde tracer biotinylated dextran amine and the retrograde tracer horseradish peroxidase. The olfactory projection consists of three tracts: the lateral olfactory tract, which projects bilaterally to the lateral cortex and the rostral amygdala, crossing the midline through the stria medullaris-habenular commissure system; the intermediate olfactory tract, which projects ipsilaterally to the olfactory tubercle and contributes to the contralateral projection; and the medial olfactory tract, which projects ipsilaterally to the dorsomedial retrobulbar formation. The vomeronasal projection is formed by a single tract, the accessory olfactory tract, that projects ipsilaterally to the nucleus of the accessory olfactory tract, the medial amygdala and the nucleus sphericus. The centrifugal projections to the main and accessory olfactory bulb are composed of two components: one that arises in areas that receive the olfactory or vomeronasal input (neurons in the olfactory tubercle, retrobulbar formation and lateral cortex project to the main olfactory bulb; and neurons in the nucleus of the accessory olfactory tract, the medial amygdala and the nucleus sphericus project to the accessory olfactory bulb), and another that arises in areas not directly implicated in processing the chemosensory information (the nucleus of the diagonal band of Broca and the dorsal cortex). These data allow the recognition of the general pattern of organization of the reptilian olfactory and vomeronasal systems.

Keywords:
Main olfactory bulb, Accessory olfactory bulb, Olfactory projections, Vomeronasal projections, Centrifugal projections, Snakes, Reptiles, Evolution
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