Smell plays a large role in flavor perception, both before, during and after ingestion of food. When coming across an odorant food item, registration of the smell in the brain will first go through the nose. Odorants then enter the nasal cavity and bind to the olfactory epithelium. This type of smell is called orthonasal smell. As you can see in the image below, the mouth and nasal cavity are connected, allowing odorants to travel to from the mouth to the olfactory epithelium in the nasal cavity as you chew. This type of smell is called retronasal smell. It travels from the mouth to the oral cavity through retronasal passage to the olfactory epithelium.

The sense of smell is achieved through the activities of the olfactory system. It starts when an airborne odorant enters the nasal cavity and reaches olfactory epithelium (OE). The OE is protected by a thin layer of mucus, which also serves the purpose of dissolving more complex compounds into simpler chemical odorants. The size of the OE and the density of sensory neurons varies among species; in humans, the OE is only about 9-10 cm2. The olfactory receptors are embedded in the cilia of the olfactory sensory neurons. Each neuron expresses only one type of olfactory receptor. However, each type of olfactory receptor is broadly tuned and can bind to multiple different odorants. For example, if receptor A binds to odorants 1 and 2, receptor B may bind to odorants 2 and 3, while receptor C binds to odorants 1 and 3. Thus, the detection and identification of an odor depend on the combination of olfactory receptors that recognize the odor; this is called combinatorial diversity.

When an odorant enters the nasal cavity and reaches the olfactory epithelium, the odorant binds to one of the olfactory receptors embedded in the cilia of the olfactory sensory neurons. Olfactory sensory neurons are bipolar cells with a single long axon that sends olfactory information up to the olfactory bulb. The olfactory bulb is a part of the brain that is only separated from the nasal cavity by the cribriform plate. Because of this small proximity between the nose and brain, nasal drugs are of great interest in cases where direct access to the central nervous system is preferred.

Within the olfactory bulb, axons from sensory neurons terminate in a specialized area called a  glomerulus. Sensory neurons with the same olfactory receptor type send their axons to the same one or two glomeruli. As a result, there can be thousands of axons from similar sensory neurons converging within a single glomerulus. Humans have at least 320 different odor receptor types. This interplay of different types of sensory neurons going to a glomerulus leads to olfactory diversity: smells can endlessly differ in odorant composition and intensity, yielding countless numbers of smells. This can be explained by the fact that odor is multidimensional: the quality and intensity of a compound interact. Whereas a color keeps its basic quality (green) with increasing or decreasing intensity (light green, dark green), odors often change their quality with higher or lower concentrations. With this, intensity effects can directly affect the quality of the sensation.

OLFACTORY CORTEX

From the olfactory bulb, the information is directly projected to the olfactory cortex. The olfactory cortex is a complex of several cortical areas that process olfactory information. One olfactory area, the cortical amygdala, influences emotional responses to smell. The second olfactory area is the orbitofrontal cortex is involved in the identification of odors and the reward value of odors and tastes. The entorhinal cortex, another olfactory cortical area, projects to the hippocampus, which is implicated in olfactory memory. The ability to detect and identify odors involves higher-order cortical areas. This direct link of the olfactory bulb to brain areas responsible for emotional response and memory illustrates the high emotional value of smell.