Because distinct flavors depend on aromas to stimulate the olfactory chemoreceptors, smell and taste are physiologically interdependent. Dysfunction of one often disturbs the other. Disorders of smell and taste are rarely incapacitating or life threatening, so they often do not receive close medical attention, although their effect on quality of life can be severe.
Taste
Although abnormal taste sensations may be due to mental disorders, local causes should always be sought. Glossopharyngeal and facial nerve integrity can be determined by testing taste on both sides of the dorsum of the tongue with sugar, salt, vinegar (acid), and quinine (bitter).
Drying of the oral mucosa caused by heavy smoking, Sjögren syndrome, radiation therapy of the head and neck, or desquamation of the tongue can impair taste, and various drugs (eg, those with anticholinergic properties and vincristine) alter taste. In all instances, the gustatory receptors are diffusely involved. When limited to one side of the tongue (eg, in Bell palsy Facial Nerve Palsy Facial nerve (7th cranial nerve) palsy is often idiopathic (formerly called Bell palsy). Idiopathic facial nerve palsy is sudden, unilateral peripheral facial nerve palsy. Symptoms of facial... read more 
), ageusia (loss of the sense of taste) is rarely noticed. Sudden loss of taste may be an early symptom of coronavirus disease 2019 (COVID-19 COVID-19 COVID-19 is an acute, sometimes severe, respiratory illness caused by the novel coronavirus SARS-CoV-2. Prevention is by vaccination and infection control precautions (eg, face masks, handwashing... read more ), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Smell
The inability to detect certain odors, such as gas or smoke, may be dangerous, and several systemic and intracranial disorders should be excluded before dismissing symptoms as harmless. Whether brain stem disease (involvement of the nucleus solitarius) can cause disorders of smell and taste is uncertain, because other neurologic manifestations usually take precedence.
Anosmia Anosmia Anosmia is complete loss of smell. Hyposmia is partial loss of smell. If unilateral, anosmia is often unrecognized. Most patients with anosmia have normal perception of salty, sweet, sour, and... read more (complete loss of the sense of smell) is probably the most common abnormality. Hyperosmia (increased sensitivity to odors) usually reflects a neurotic or histrionic personality but can occur intermittently with seizure disorders. Dysosmia (disagreeable or distorted sense of smell) may occur with infection of the nasal sinuses, partial damage to the olfactory bulbs, or mental depression. Some cases, accompanied by a disagreeable taste, result from poor dental hygiene. Uncinate epilepsy can produce brief, vivid, unpleasant olfactory hallucinations. Hyposmia (partial loss of smell) and hypogeusia (diminished sense of taste) can follow acute influenza, usually temporarily. Sudden loss of smell also may be an early symptom of COVID-19 COVID-19 COVID-19 is an acute, sometimes severe, respiratory illness caused by the novel coronavirus SARS-CoV-2. Prevention is by vaccination and infection control precautions (eg, face masks, handwashing... read more .
How people sense flavors
To distinguish most flavors, the brain needs information about both smell and taste. These sensations are communicated to various areas of the brain from receptors in the nose and mouth. The olfactory epithelium is an area of the nasal mucosa in the upper part of the nasal cavity. The smell receptors in this epithelium are specialized nerve cells with cilia that detect odors. Airborne molecules entering the nasal passage stimulate the cilia, triggering a nerve impulse that is transmitted upward through the cribriform plate and across a synapse within the olfactory bulbs (the distal ends of the 1st cranial nerves—olfactory nerves). The olfactory nerves transmit the impulse to the brain, which interprets the impulse as a distinct odor. Information is also sent to the middle part of the temporal lobe—the smell and taste center, in which memories of odors are stored. Thousands of tiny taste buds cover most of the tongue’s surface. A taste bud contains several types of ciliated taste receptors. 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 are more sensitive for each taste. Sweetness is most easily identified by the tip of the tongue, whereas saltiness is best appreciated at the front sides of the tongue. Sourness is best perceived along the sides of the tongue, and bitter sensations are readily detected in the back one third of the tongue. Nerve impulses from the taste buds are transmitted to the brain through the facial and glossopharyngeal nerves (cranial nerves VII and IX). The brain interprets the combination of impulses from the olfactory and taste receptors along with other sensory information (eg, the food’s texture and temperature) to produce a distinct flavor when food enters the mouth and is chewed. |
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