Olfaction is one of the two chemical senses: smell and taste. Both arise from interaction between chemical and receptor cells. In olfaction, the chemical is volatile, or airborne. Breathed in through the nostrils or taken in via the throat by chewing and swallowing, it passes through either the nose or an opening in the palate at the back of the mouth, and moves toward receptor cells located in the lining of the nasal passage.
As the chemical moves past the receptor cells, part of it is absorbed into the uppermost surface of the nasal passages called the olfactory epithelium, located at the top of the nasal cavity. There, two one-inch-square patches of tissue covered with mucus dissolve the chemical, stimulating the receptors, which lie under the mucus. The chemical molecules bind to the receptors, triggering impulses that travel to the brain.
There are thousands of different receptors in the cells of the nasal cavity that can detect as many as 10,000 different odors. Each receptor contains hair-like structures, or cilia, which are probably the initial point of contact with olfactory stimuli.
Research suggests that the sensitivity of the olfactory system is related to the number of both receptors and cilia. For example, a dog has 20 times as many receptor cells as a human and over 10 times as many cilia per receptor.
The cribriform plate forms the roof of the nasal cavity. The olfactory nerve passes through openings in this bone and ends in the olfactory bulb, a neural structure at the base of the brain. From there, olfactory signals are diffused throughout the brain to areas including the amygdala, hippocampus, pyriform cortex (located at the base of the temporal lobe), and the hypothalamus.
Olfaction is the only sense that does not involve the thalamus. Olfaction messages are especially intensive in the amygdala, a part of the brain responsible for emotions, which may help the unusual power of certain smells to trigger emotions and recollections based on memories from the past.
Further, a person’s reaction to smell is mediated by context. For example, the same smell present in body odor is responsible for the flavor of cheese. In the first case, the smell is perceived as negative, in the second, it is positive.
In humans, olfaction intensifies the taste of food, warns of potentially dangerous food, as well as other dangers (such as fire), and triggers associations involving memory and emotion. Olfaction is an especially important sense in many animals. A predator may use it to detect prey, while prey may use it to avoid predators.
It also has a role in the mating process through chemicals called pheromones, which can cause ovulation in females or signal a male that a female is in a sexually receptive state. Although the existence of human pheromones has not been verified, olfaction still plays a role in human sexual attraction, as well as in parenting.
Mothers can usually identify their newborn infants by smell, and breast-feeding babies can distinguish between the smell of their mothers and that of other breast-feeding women. Researchers have also found that children are able to recognize their siblings by smell and parents can use smell to distinguish among their own children.
However, as people age the sense of smell diminishes, especially for men. By age 80, many men have almost no ability to detect odors. The intensity of a particular odor is strongly affected by adaptation. Odors may become undetectable after only a brief period of exposure.
The sense of smell also plays an important role in the discrimination of flavors, a fact demonstrated by the reduced sense of taste in people with colds. The enjoyment of food actually comes more from odors detected by the olfactory system than from the functioning of the taste system.
The olfactory and gustatory (taste) pathways are known to converge in parts of the brain, although it is not known exactly how the two systems work together. While an aversion to certain flavors (such as bitter flavors) is innate, associations with odors are learned.