Taste Nerve Pathways

Sensory impulses from taste receptors located in the anterior two-thirds of the tongue travel on fibers of the facial nerve (VII); impulses from receptors in the posterior one-third of the tongue and the back of the mouth pass along the glossopharyngeal nerve (IX); and impulses from receptors at the base of the tongue and the pharynx travel on the vagus nerve (X) (see chapter 11, pp. 430-431). These cranial nerves conduct the impulses into the medulla oblongata. From there, the impulses ascend to the thalamus and are directed to the gustatory cortex of

A fifth taste, called "umami" (oo-MOM'-ee), has long been recognized in Japan, but has only recently come to the attention of western taste researchers. Apparently difficult to describe, umami has been defined as "savory," "pungent," "meaty," or simply "delicious" or "perfect," and seems to involve all of the other taste sensations. Umami is thought to arise from taste receptors that are specific for monosodium glutamate (MSG), which is present naturally in many foods, and added to others to enhance flavor. MSG is a salt of the amino acid glutamate.

Smell and Taste Disorders

Imagine a spicy slice of pizza, or freshly brewed coffee, and your mouth waters in anticipation. But for millions of people, the senses of smell and taste are dulled, distorted, or gone altogether. Many more of us get some idea of their plight when a cold temporarily stifles these senses.

Compared to the loss of hearing or sight, being unable to taste or smell normally may seem more an oddity than an illness. But those with such ailments would probably disagree. In some situations, a poor or lacking sense of smell can even be dangerous. One patient died in a house fire because he did not smell the smoke in time to escape.

The direct connection between the outside environment and the brain makes the sense of smell very vulnerable to damage. Smell and taste disorders can be triggered by colds and flu, allergies, nasal polyps (swollen mucous membranes inside the nose), a head injury, chemical ex posure, a nutritional or metabolic problem, or a disease. In many cases, a cause cannot be identified.

Drugs can alter taste and smell in many ways, affecting cell turnover, the neural conduction system, the status of receptors, and changes in nutritional status. Consider what happened to twelve hikers touring Peru and Bolivia. A day before a long hike, three of them had begun taking acetazolamide (Di-amox), a drug that prevents acute mountain sickness. The night after the climb, the group went out for beer. To three of the people, the brew tasted unbearably bitter, and a drink of cola to wash away the taste was equally offensive. At fault: acetazolamide.

Drugs containing sulfur atoms squelch taste. They include the anti-inflammatory drug penicillamine, the antihypertensive drug captopril (Capoten), and transdermal (patch) nitroglycerin to treat chest pain. The antibiotic tetracycline and the an-tiprotozoan metronidazole (Flagyl) cause a metallic taste. Cancer chemotherapy and radiation treatment often alter taste and smell.

Exposure to toxic chemicals can affect taste and smell, too. A forty-five year-old woman from Al-toona, Pennsylvania, suddenly found that once-pleasant smells had become offensive. Her doctor traced her problem to inhaling a paint stripper. Hydrocarbon solvents in the product — toluene, methanol, and methylene chloride — were responsible for her cacosmia, the association of an odor of decay with normally inoffensive stimuli. ■

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