The chemical senses are the senses of smell (olfaction) and taste (gustation). Smell is a distant chemical sense, providing information about the chemical composition of substances before you come into the direct contact with them. Taste is an immediate chemical sense, providing information about potentially harmful substances before they enter your body.
How Chemical Senses Work
Molecules from food and other substances enter the nasal passages and mouth where are dissolved in watery mucus and fit into molecular slots, or pockets, in special receptor cells. The binding together of the molecule and receptor stimulates the cell to send electric signals along a pathway of nerve cells to the brain. Certain areas of the brain perceive odors and tastes and memorize people, places and events associated with them.
In human beings, the olfactory region occupies a small area of less than 1/3 square inch in each of the nasal passages. This area, however, contains approximately 50 million receptor cells, each with up to 20 minute, hairlike structures, called cilia. The cilia project downwards into a layer of mucus, into which odorant molecules dissolve. The human olfactory system can distinguish between thousands of smells, but odorant molecules must be at least partially soluble in water, and in fat, to be detected.
The taste receptor cells on the human tongue are arranged in taste buds — each of which containing between 50 and 150 individual receptor cells — on three projections, known as papillae. The circumvallate papilla is on the rear, or dorsal portion, of the tongue, the foliate papilla is on the sides and the fungiform papilla is on the top and sides. Collectively, the papillae can sense the characteristic tastes of saltiness, sourness, sweetness, bitterness and umami; umami is a meaty, or savory, taste sensation.
Similarities & Differences
The neural pathways for smell and taste are completely separate, but as complex substances stimulate different combinations of odor and taste receptors, the chemical senses often work together. The taste of food, for example, is partially due to food molecules stimulating olfactory receptors in the nose, rather than gustatory sensors in the mouth. Individually and collectively, the chemical senses can regulate eating and drinking, elicit emotional responses and form certain types of memories. Only five different types of taste receptors have been identified, whereas there are probably hundreds of different types of odor receptors.