[How we smell and what it means to us: basic principles of the sense of smell]

The origins of the sense of smell lie in the perception of environmental molecules and go back to unicellular organisms such as bacteria. Odors transmit a multitude of information about the chemical composition of our environment. The sense of smell helps people and animals with orientation in space, warns of potential threats, influences the choice of sexual partners, regulates food intake and influences feelings and social behavior in general. The perception of odors begins in sensory neurons residing in the olfactory epithelium that express G protein-coupled receptors, the so-called olfactory receptors. The binding of odor molecules to olfactory receptors initiates a signal transduction cascade that converts olfactory stimuli into electrical signals. These signals are then transmitted to the olfactory bulb, the first relay center in the olfactory pathway, via the axons of the sensory neurons. The olfactory information is processed in the bulb and then transferred to higher olfactory centers via axons of mitral cells, the bulbar projection neurons. This review describes the mechanisms involved in peripheral detection of odorants, outlines the further processing of olfactory information in higher olfactory centers and finally gives an overview of the overall significance of the ability to smell.