Taste and Smell Together
It's hard to address either taste or smell individually, since they're so closely tied together. Both the gustatory nerves in the taste buds and the olfactory nerves in the nose react to chemicals in the environment and send messages about them to the brain, which interprets them as tastes and smells. Those taste buds can really only detect four different flavors: sour, bitter, salty, and sweet. All the other rich rewards of flavorful foods — as much as three-quarters of what we experience as “flavor” — are provided by the olfactory component.
It's not surprising that the sense of smell provides so much more input than the sense of taste, since the latter concentrates on just four variations and the former can detect some 400,000. As you've no doubt found if you've had a bad head cold, the loss of smell means the loss of most of what tastes good about food.
You may also have had the experience of smelling something so strongly you could taste it, too. The proximity of the nose and mouth and the fact that both taste and smell respond to chemicals in the environment cause a great deal of overlap.
Sense of “Cool” and “Sting”
Smell and taste aren't the only things that contribute to your experience of flavor. Another type of nerve that reacts to chemicals, called the common chemical sense, occupies the same territory as olfactory and gustatory nerve endings, and is also found in the membranes around the eyes.
These nerves record sensations that aren't quite smell, aren't quite taste, but add to your perception of both. Think of the coolness you feel when smelling a menthol chest rub, the sting of sour lemon that makes you squint your eyes when it's on your tongue, or the heat of a super-spicy salsa.
Together, the sense of smell and taste plus the common chemical sense are called the chemical sensing system, or chemosensation. These make up the only senses that operate through interaction with chemical substances, as opposed to things like light waves, sound waves, or physical contact.
Taste and Memory
Chemosensation gives you your physical perception of flavor, but there's one more element that enters into your sensory enjoyment of food and odors: memory. When the brain interprets the input from the gustatory and olfactory nerves, it interprets them with a strong emphasis on past experiences of those sensations. Apple pie doesn't just smell good, it makes you feel the warmth and comfort of your mother's kitchen. These memories may not work on a conscious level, but they bring depth and emotion to otherwise straightforward chemical reactions.
That's one reason why you have to be so careful when dealing with your child's sensory problems with taste and smell. Fighting with your child over a food or an odor creates a stressful, scary memory that will be replayed every time that sensory information gets processed, leading to more battles. Whatever you may feel you need to do to get your child enough nourishment or to function in places that may have disturbing smells, think about what kind of memory you want to create.