The sense of smell uses the nose to identify unique chemical signatures of odors, with olfactory receptor neurons located in the olfactory epithelium. Smell is the oldest sense, mainly related to food and sexual bonding, and is associated with memory formation. The olfactory nerve projects axons into the olfactory bulb, which ultimately converges on only about 100 neurons, allowing humans to distinguish between several thousand odors.
The sense of smell, known as olfaction, uses a sensory organ called the nose to relay information about smell to the olfactory cortex in the brain. Diffuse suspensions of relevant molecules, called odors, are analyzed by the nose using a lock-and-key molecular scheme whereby odors are identified by their unique chemical signatures.
Sense evolved as a means of detecting survival-relevant information about the outside world, especially evaluating food. Smell is the oldest of the senses, with analogues dating back to early animals 600 million years ago. One of the five primary senses, smell is most intimately associated with the formation of memories.
Olfactory receptor neurons, the cells responsible for smell, are found on a 1-by-2-inch strip of tissue called the olfactory epithelium, located about 3 inches above and behind the nares. The human olfactory epithelium is approximately 16 cm², in contrast to some dogs which have 150 cm².
With advanced color vision, vision can be considered the primary human sense: smell plays a more limited role, mainly related to food and sexual bonding. Olfactory receptor neurons are surrounded by support cells that expel mucus, making it easier for odor molecules to pick up. The nostrils are covered with hairs to prevent the intrusion of unwanted organisms and inanimate material from the environments.
Olfactory receptor neurons, each of which can detect different molecules, project axons into the brain via the olfactory nerve. These projections converge on a small (~50 micron) structure called the olfactory bulb, which ultimately converges on only about 100 neurons. Considering the degree of convergence found in the olfactory portion of the nervous system, it may seem a wonder that so many odors can be distinguished, but seemingly arbitrary firing patterns among the 100 neurons are enough to get the job done. Most humans are able to distinguish between several thousand odors and varying degrees of intensity and combinations of odors within them.
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