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.
How People Sense Flavors
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. |
Smell
The ability to smell can be affected by changes in the nose, in the nerves leading from the nose to the brain, or in the brain. For example, if nasal passages are stuffed up from a common cold, the ability to smell may be reduced because odors are prevented from reaching the smell receptors (specialized nerve cells in the mucous membrane lining the nose). Because the ability to smell affects taste, food often does not taste right to people with a cold. Smell receptors can be temporarily damaged by the influenza (flu) virus. Some people cannot smell or taste for several days or even weeks after a bout of the flu, and rarely, loss of smell or taste becomes permanent. Sudden loss of smell may also be an early symptom of COVID-19, an acute respiratory illness that can be severe. COVID-19 is caused by a coronavirus called SARS-CoV-2. (See Loss of Smell.)
Did You Know?
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Oversensitivity to smell (hyperosmia) is much less common than loss of smell. Pregnant women commonly become oversensitive to smell. Hyperosmia can also be psychosomatic. People with psychosomatic hyperosmia have no apparent physical disorder. Psychosomatic hyperosmia is more likely to develop in people who have a histrionic personality (characterized by conspicuous seeking of attention with dramatic behavior).
Some disorders can distort the sense of smell, making innocuous odors smell disagreeable (a condition called dysosmia). These disorders include the following:
Infections in the sinuses
Partial damage to the olfactory nerves
Poor dental hygiene
Mouth infections
Depression
Viral hepatitis, which may cause dysosmia that results in nausea triggered by otherwise inoffensive odors
Nutritional deficiencies
Seizures originating in the part of the brain where memories of smell are stored—the middle part of the temporal lobe—may produce a brief, false sensation of vivid, unpleasant odors (olfactory hallucinations). These odors are part of the intense feeling that a seizure is about to begin (called an aura) and do not indicate a smell disorder. Brain infections due to herpesviruses (herpes encephalitis) may also cause olfactory hallucinations.
Taste
A reduction in the ability to taste (hypogeusia) or loss of taste (ageusia) usually results from conditions that affect the tongue, usually by causing a very dry mouth. Such conditions include Sjögren’s syndrome
In Bell palsy (a disorder in which half of the face is paralyzed), the sense of taste is often impaired on the front two thirds of one side of the tongue (the side affected by the palsy). But this loss may not be noticed because taste is normal or increased in the rest of the tongue.
Burns to the tongue may temporarily destroy taste buds. Neurologic disorders, including depression and seizures, may impair taste.
A distortion of taste (dysgeusia) may be caused by inflammation of the gums (gingivitis) or by many of the same conditions that result in loss of taste or smell, including depression and seizures. Taste may be distorted by some medications, such as the following:
Antibiotics
Antiseizure medications
Antidepressants
Certain chemotherapy medications
Diuretics
Medications used to treat arthritis
Thyroid medications
