ear, nose, and throat disorders
Overview of Smell and Taste Disorders
Introduction
Because disorders of smell and taste are rarely life threatening, they may not receive close medical attention. Yet, these disorders can be frustrating because they can affect the ability to enjoy food and drink and to appreciate pleasant aromas. They can also interfere with the ability to notice potentially harmful chemicals and gases and thus may have serious consequences. Occasionally, impairment of smell and taste is due to a serious disorder, such as a tumor. Smell and taste are closely linked. The taste buds of the tongue identify taste, and the nerves in the nose identify smell. Both sensations are communicated to the brain, which integrates the information so that flavors can be recognized and appreciated. Some tastes—such as salty, bitter, sweet, and sour—can be recognized without the sense of smell. However, more complex flavors (such as raspberry) require both taste and smell sensations to be recognized. A partial loss of smell (hyposmia) and complete loss of smell (anosmia) are the most common disorders of smell and taste. Because distinguishing one flavor from another is based largely on smell, people often first notice that their ability to smell is reduced when their food seems tasteless. To distinguish most flavors, the brain needs information about both smell and taste. These sensations are communicated to the brain from the nose and mouth. Several areas of the brain integrate the information, enabling people to recognize and appreciate flavors. A small area on the mucous membrane that lines the nose (the olfactory epithelium) contains specialized nerve cells called smell receptors. These receptors have hairlike projections (cilia) that detect odors. Airborne molecules entering the nasal passage stimulate the cilia, triggering a nerve impulse in nearby nerve fibers. The fibers extend upward through the bone that forms the roof of the nasal cavity (cribriform plate) and connect to enlargements of nerve cells (olfactory bulbs). These bulbs form the cranial nerves of smell (olfactory nerves). The impulse travels through the olfactory bulbs, along the olfactory nerves, to the brain. The brain interprets the impulse as a distinct odor. Also, the area of the brain where memories of odors are stored—the smell and taste center in the middle part of the temporal lobe—is stimulated. The memories enable a person to distinguish and identify many different odors experienced over a lifetime. Thousands of tiny taste buds cover most of the tongue’s surface. A taste bud contains several types of taste receptors with cilia. Each type detects one of the five basic tastes: sweet, salty, sour, bitter, or savory (also called umami, the taste of monosodium glutamate). These tastes can be detected all over the tongue, but certain areas may be slightly more sensitive for each taste: sweetness at the tip of the tongue, saltiness at the front sides of the tongue, sourness along the sides of the tongue, and bitter sensations in the back one third of the tongue. Food placed in the mouth stimulates the cilia, triggering a nerve impulse in nearby nerve fibers, which are connected to the cranial nerves of taste (the facial and glossopharyngeal nerves). The impulse travels along these cranial nerves to the brain, which interprets the combination of impulses from the different types of taste receptors as a distinct taste. Sensory information about the food’s smell, taste, texture, and temperature from the smell and taste receptors is processed by the brain to produce a distinct flavor when food enters the mouth and is chewed. 