Smell and Taste Disorders

Physiology of Smell

Smell and taste are closely linked. The taste buds of the tongue identify taste; 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 (raspberry, for example) require both taste and smell sensations to be recognized.

The human sense of smell depends on the functioning of not only cranial nerve I (olfactory nerve) but also portions of cranial nerve V (trigeminal nerve). Qualitative odor sensations (e.g., the smell of a rose, lemon or grass) are mediated by cranial nerve I , whereas somatosensory overtones of odorants (e.g., warmth, coolness, sharpness and irritation) are mediated by the ophthalmic and maxillary divisions of cranial nerve V.

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) that 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 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. Food placed in the mouth stimulates taste receptors in the taste buds. Taste receptors have cilia that detect tastes. Food molecules stimulate 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 impulse as a distinct taste. Taste buds can detect sweetness, saltiness, sourness, and bitterness. Combinations of the four basic tastes produce a wide spectrum of tastes.

Smell receptors are located within the olfactory neuroepithelium, a region of tissue found over the cribiform plate, the superior septum and a segment of the superior turbinate. The free nerve endings of cranial nerve V are located diffusely throughout the nasal respiratory epithelium, including regions of the olfactory neuroepithelium. It is important to remember the distinctive nature of these two neural systems, because some odorants (e.g., ammonia) are sensed largely by the trigeminal nerve.

Once odorants enter the nose, they must move to the nasal vault and dissolve within the covering mucous layer in order to stimulate the olfactory receptors.  Mucous has an important role in dispersing scents to the underlying receptors. The nasal turbinates are also important because they provide moderate resistance and a moist environment, thereby allowing optimal stimulation of olfactory neurons by airborne compounds.

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