Figure 7.4: The anatomy of smell Molecules of odorants enter the nose through the nostrils, become dissolved in the mucous fluid covering the olfactory epithelium, and bind to receptor sites on the sensitive tips of olfactory sensory neurons, where they initiate action potentials. The sensory neurons send their axons through the cribriform plate (a small bone shelf) to form synapses on second-order olfactory neurons in the glomeruli of the olfactory bulb, directly above the nasal cavity. As illustrated in the right-hand diagram by the use of color, each glomerulus receives input from only one type of olfactory sensory neuron (defined by its type of receptor sites). Only two types of such neurons are shown here (depicted as yellow and blue), of the roughly 350 types that exist in the human olfactory system.