The olfactory system

Abstract

Transduction of olfactory information occurs when odorant molecules contact the dendrites of olfactory receptor neurons (ORNs). These neurons reside in the olfactory epithelium, a specialized region of the dorsal nasal cavity. ORN axons project through the lamina propria underlying the olfactory epithelium and into the glomerular layer of the main olfactory bulb. This projection forms the olfactory nerve, or cranial nerve I. Within glomeruli, ORN axons synapse onto the apical dendrites ofmitral and tufted cells, which are the output neurons of the main olfactory bulb (MOB). In turn, axons from these cells project to the primary olfactory cortex, via the lateral olfactory tract. The primary olfactory cortex comprises several brain regions, including the anterior olfactory nucleus, the piriform cortex, parts of the amygdala, and the entorhinal cortex. These areas, in turn, are interconnected with many areas of the brain, including the neocortex, hippocampus, mediodorsal thalamus, preoptic area, hypothalamus, and other parts of the limbic system. Through these connections, the olfactory system influences a wide range of behaviors and physiologic functions including reproduction, social behavior and communication (e.g., scent marking), food finding and selection, and maternal behavior in addition to regulating neuroendocrine functions. We are only beginning to understand how odors are "coded" by primary olfactory neurons in the nasal epithelium and by activity patterns in the MOB. Recent advances in genetic approaches, electrophysiology, and imaging of neural function promise to rapidly close this critical gap.