In mammals, odorants are inhaled through the nose and inside the nasal cavity they trigger olfactory sensory neurons (OSN)  that are located within the olfactory epithelium. OSN project their axons into glomerular structures of the olfactory bulb. There they synapse with dendrites of second-order neurons that project their axons to the olfactory cortex. Thus, olfaction is based on direct intera...