The olfactory system: From odorant molecules to perception

The process leading to olfactory perception begins in the nasal cavity, where odorant molecules reaching the olfactory epithelium bind to a large number of different odorant receptors located in the cilia of olfactory sensory neurons. The number of odorant receptors varies between about 300 in humans and 1200 in mice, representing about 1%-4% of proteins encoded by the entire genome. However, each olfactory sensory neuron expresses only one odorant receptor type that can bind different odorant molecules. Vice versa, each odorant molecule can bind to several odorant receptors according to a unique combinatorial code. Axons of olfactory sensory neurons send information to second-order neurons (mitral and tufted cells) in the olfactory bulb, which in turn project to several cortical areas. Sensory coding in the