by Dr. Gary Farr on 17 June 2002
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Taste or gustation is the ability to respond to dissolved molecules and ions (as contrasted with the sense of smell which detects airborne molecules). Humans detect taste with taste receptor cells. These are clustered in taste buds. Each taste bud has a pore that opens out to the surface of the tongue enabling molecules and ions taken into the mouth to reach the receptor cells inside.
The tongue, a muscular organ located on the floor of the mouth, is an extremely mobile structure in humans and an important accessory organ in such motor functions as speech, chewing, and swallowing. In conjunction with the cheeks, it is able to guide and maintain food between the upper and lower teeth until mastication is completed. The tongue's motility aids in creating a negative pressure within the oral cavity, thus enabling mammals to suckle.
The mucous membrane that covers the tongue varies greatly. Especially important as a peripheral sense organ, it contains groups of specialized epithelial cells, known as taste buds, that carry stimuli from the oral cavity to the central nervous system. Furthermore, the tongue's glands produce some of the saliva necessary for swallowing.
The mammalian tongue consists of a mass of interwoven, striated (striped) muscles covered with mucous membrane and interspersed with glands and a variable amount of fat. By its extrinsic muscles, the tongue is attached to the lower jaw, the hyoid bone (a U-shaped bone between the lower jaw and the larynx), the skull, the soft palate, and the pharynx. It is bound to the floor of the mouth and to the epiglottis (a plate of cartilage that serves as a lid for the larynx) by folds of its mucous membrane.
Although best recognized when tasting salty, sweet, bitter, or sour substances, this sense serves various purposes. The gustatory system responds to chemical substances in the oral cavity and, in turn, regulates the interaction between ingestive behavior and internal milieu. The sense of taste also serves as a gateway for monitoring and controlling the ingestion of food (Smith, 1997).
How Do We Taste?
The sense of taste is often associated solely with the tongue, but the sensory organs of taste also include the palate, pharynx, and epiglottis (Anonymous, 1997c). The most widely known roll-player in the gustatory system is the taste bud. Taste buds are specially modified epithelial cells and are distributed throughout the oral, pharyngeal, and laryngeal mucosa in a topographic, overlapping manner. Within the taste buds are gustatory receptor cells (Smith, 1997) which are stimulated by chemicals dissolved in saliva (Matsuo et al, 1994). The receptor cells depolarize and make synaptic contact with first order fibers of either {nervous_system_cranial_d} cranial nerve VII (serving anterior 2/3 of tongue), {nervous_system_cranial_e} IX (serving posterior 1/3), or {nervous_system_cranial_e} X (serving the throat, glottis, epiglottis, and pharynx.) These fibers are then met by the second order projections and connect to the thalamus. From the thalamus, sensory information is sent to the primary gustatory cortex in the ventral parietal lobe of the brain.
Taste is categorized into four properties: salty, bitter, sour, and sweet (Anonymous, 1997c). Sour taste is produced by acids; sweet by glucose, sucrose, and other carbohydrates; bitter by Ca2+ and IP3; and salty taste is produced by Na+ (Jacobs, 1997). These sensations arise from neural activity within the central nervous system. The neurons are not specific to any one of the four stimuli, but respond best to one of the four (Smith, 1997). The taste buds on the tip of the tongue respond best to sweetness, on the sides to sourness and saltiness, and on the back of the tongue, taste buds are most sensitive to bitterness (Anonymous, 1997c).
Taste Disorders
Of the five senses, gustation is one of only two that possesses the ability to regularly replace cells throughout the life span (Beidler, 1970). However, an estimated two million Americans still suffer from disorders of taste and smell. Taste disorders include: Ageusia, complete loss of taste; Hypogeusia, diminished taste sensitivity; Hypergeusia, enhanced taste sensitivity; and Dysgeusia, distortion in taste perception (Seiden, 1997).
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