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The olfactory epithelium is a layer of odor-sensitive cells located inside the depths of the nose. These cells react to odors as they enter the nose, sending signals to the olfactory bulb. Level of sensitivity to smell is determined in part by the size of the olfactory epithelium and the number of cells present; dogs, for example, have far more cells than humans, which allows them to have a much better sense of smell. Nasal structure can also play a role.
Several cell types can be found in the olfactory epithelium. One is a specialized neuron known as a ciliated receptor or brush cell that responds to odors with an electrical impulse directed to the olfactory bulb. The exact mechanisms of how these cells respond to smells are not totally understood, but they clearly react to with varying degrees of sensitivity and send signals to help the body interpret olfactory sensations.
Supporting cells provide a variety of functions to keep the olfactory epithelium in working order, including secretions and a tissue matrix. In addition, basal cells are stem cells with the ability to divide either into supporting or ciliated receptor cells. This makes the olfactory epithelium somewhat unique in the body, as it is rare for mature neurons to be replaced by stem cells once people are fully grown. In the olfactory epithelium, the upper layers are continuously replaced over time to maintain healthy, functioning cells.
Studies on the olfactory epithelium in various animals provide a wealth of information about the cells involved and how they function. Researchers are also interested in size differences to learn more about which animals have an acute sense of smell and why it may have developed. Predators, for example, can have an advantage when their sense of smell is very powerful, but so can prey who might want to avoid becoming lunch.
Damage to the olfactory epithelium and processing pathways can cause disorders. Some patients have a condition called anosmia, where they have no sense of smell at all. Others may misinterpret odors or have trouble detecting specific smells because their noses lack the necessary sensitivity. For example, many people react strongly to the smell of skunk because it can be a powerful stench, but some people cannot smell it at all, although they can perceive other odors.
Changes to the sense of smell, or recurrent olfactory hallucinations, can be indicators of brain injuries. People with tumors, venous malformations, and other disorders in the brain may experience disruptions to their sense of smell which can grow worse over time. This can also be a potential complication of brain or sinus surgery, where the patient’s sense of smell may be permanently altered by accident.