The Sense of Smell / All About the Sense of Smell

Smell is often considered to be the least important of all the senses, but it may be one of the oldest, and probably acts on the subconscious more than the other senses. There is little doubt that scents have important roles in human behavior. The body is provided with glands to produce specific odors, many of which appear to be associated with sexual attraction and excitement, and others that have considerable significance as well.

The nose contains millions of receptor cells, which mediate the olfactory system’s high sensitivity to an extraordinary range of odors. Olfactory receptors cells are nerve cells which communicate directly with the brain. Inside the nose, odorant molecules interact with receptor proteins located in cilia extending from exposed ends of the receptor cells. This activates the formation of so-called “second messengers,” that modulate passage of ions through channels in the cell membrane, in turn generating nerve impulses that are transmitted directly to the olfactory bulbs.

The bond between a baby and its mother is thought to be tightened by a form of "scent imprinting." In it, a baby which is suckling at the mother's breast pushes his or her face into a bank of scent organs that surround the nipple. A further sign of the importance of the sense of smell is the way it becomes a major source of information when other senses are not working, especially sight. Only a small part of the nose and nasal cavity is taken up by the organs of smell; the rest of it is mainly concerned with processing the airflow on its way through to the lungs. The walls of the nasal cavity, and particularly the flaplike middle and inferior conchae, are coated with respiratory mucous membranes which incorporate a vast number of tiny hairlike cells which act to move waves of mucus toward the throat.

Dust, bacteria, and chemical particles which are inhaled from the air are trapped by the mucus, carried back and swallowed; they are then taken care of by gastric juices to nullify any potential harm. The sense organs themselves are made up of two yellowish-gray patches of tissue, called the olfactory membranes, each about the size of a postage stamp. They are located in a pair of clefts just under the bridge of the nose and at the top of the nasal cavity. The reasons for the coloration are not completely clear, but it seems to be necessary for the membrane to work. During normal breathing, most of the air flows through the nose, with only a small part reaching the olfactory clefts, but this is enough to get a response to a new smell. When a person "sniffs the air" to detect smells, the air moves through the nose much faster, increasing the flow that makes its way to the olfactory clefts and so carrying more odor to those sensors. If you "follow your nose," you are taking a route that lies straight ahead and is obvious (as the nose on your face), or else you are going ahead without a plan, that is, following wherever instinct leads.

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Additional information regarding conditions of the special senses can be found here.