Taste and smell are chemical senses. As light waves stimulate vision and sound waves stimulate sound, chemicals stimulate taste and smell.
Taste, or gustation, happens when chemicals stimulate receptors in the tongue and throat, on the inside of the cheeks, and on the roof of the mouth. These receptors are inside taste buds, which in turn are inside little bumps on the skin called papillae. Taste receptors have a short life span and are replaced about every ten days.
For a long time, researchers believed in the existence of four tastes: salty, sweet, sour, and bitter. Recently, researchers have suggested the presence of a fifth taste called umami. The spice monosodium glutamate (MSG) has an umami taste, as do many protein-rich foods. Taste is also strongly influenced by smell.
Smell, or olfaction, happens when chemicals in the air enter the nose during the breathing process. Smell receptors lie in the top of the nasal passage. They send impulses along the olfactory nerve to the olfactory bulb at the base of the brain. Researchers theorize that there are a great many types of olfactory receptors. People perceive particular smells when different combinations of receptors are stimulated.