Olfaction is the sense that gives people and animals an ability to smell. Human olfactory receptors, i.e. the sensory cells that register smell substances, are gathered in a small organ at the top of the nose, the so-called area olfactoria. This epithelium area consists of the olfactory nerve cells, supporting and basal cells.
Olfactory neurons have cilia which receptors are located on, in order to increase the surface of the epithelium. Each olfactory nerve cell is linked to only one kind of receptor. The tissue also contains glands that produce mucus to protect the cilia and also help to capture the fragrances. Insect olfactory organs tend to be placed on sensory whip. Olfaction area is located on top of the nasal cavity ceiling, at nervus olfactorius. It is an outgrowth of the brain that receive olfactory neuronal axion and connects them to the brain’s nerve cells.
The olfactory organ in humans and other higher organisms can be thought of as a complex molecule detector and as well as a form of chemoreceptor. With the sense of smell, we can discern about 10 000 350 proteins and combinations of these, of which 80% are unpleasant smells.
Compared to the human sense of smell, the smell of other animals is both more complex and more specialized in other scents. Smell center of the dog occupies about a third of dog’s brain with only one twentieth in humans.
Lucretius (1st century BC) postulated that the olfactory system worked by recognizing various shapes and sizes of odor particles. The modern theory was verified by Richard Axel and Linda B. Buck, who in 2004 won the Nobel prize in Physiology or medicine for demonstrating how smell works by cloning of olfactory receptor protein and then pairing them with various odoriferous substances.
Humans have about 350 different types of odor receptors, but we can recognize (distinguish) approximately 10,000 different smells.
The G protein-coupled receptors that are located on the olfactory receptor cell surface represent the largest class of protein and about three percent of our genes are used to produce the odor receptors. A fragrance substance binds to the G-receptors, which in turn activates cAMP that binds directly to the Ca+ channels, which open and depolarize the cell membrane by chloride ions. Depolarization starts a potential action if it is strong enough to reach over a threshold. Olfactory receptor cells’ axons leads to different glomerulus where other neurons signal to the brain through the olfactory nerve (cranial nerve I, n. olfactorius).
Signals from odorant receptors go to the brain’s different olfactory centers. In addition, there are nerves leading directly from glomerulus to the limbic system.
They are responsible for developing emotions. The sense of smell is the only sense whose nerves are not forwarded in talamus on its way to the cerebrum.
That we perceive as taste is largely due to our olfactory impression. While taste receptors can only distinguish between six different basic tastes (sweet, salty, sour, bitter, umami and fatty), the sense of smell is able to distinguish hundreds of different scents in extremely small quantities.
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