Anatomy & Physiology

Anatomy & Physiology Study Guide

16.6 Gustation Gustation, or taste, provides information about the foods and liquids we consume. Taste receptors, or gustatory receptors , are distributed over the superior surface of the tongue and adjacent portions of the pharynx and larynx. Taste receptors and specialized epithelial cells create sensory structures called taste buds . An adult has about 3000 taste buds. The superior top of the tongue bears epithelial projections called lingual papillae. The human tongue has three types of lingual papillae: filiform papillae, fungiform papillae, and circumvallate papillae. Filiform papillae provide friction that helps the tongue move objects around in the mouth but do not contain taste buds. Each small fungiform papilla contains about five taste buds; each large circumvallate papilla contains as many as 100 taste buds. Taste buds are recessed into the surrounding epithelium. Each taste bud contains about 40 slender, spindle-shaped cells of at least four different types. A typical gustatory cell survives for only about ten days before it is replaced. Gustatory Discrimination There are four primary taste sensations: sweet, salty, sour, and bitter. There is some evidence for differences in sensitivity to tastes along the axis of the tongue, with the greatest sensitivity to salty- sweet anteriorly, and sour–bitter posteriorly. However, no structural differences exist amongst the taste buds, and all the taste buds of the tongue provide all four primary taste sensations. Dissolved chemicals contacting the taste hairs bind to receptor proteins of the gustatory cell. The result of taste receptor stimulation is the release of neurotransmitters by the receptor cell. Taste receptors adapt slowly, but the sensitivity to a new taste is reduced by central adaptation. There is variation in the threshold for the primary taste receptors, and they respond more readily to unpleasant than to pleasant stimuli. For example, the tongue is almost a thousand times more sensitive to acids, which taste sour, than to either salty or sweet chemicals, and it is a hundred times more sensitive to bitter compounds than to acids. More than 10,000 taste buds are present at birth, but the number declines dramatically starting around age 50. The sensory loss becomes especially significant because aging individuals also experience a decline in the number of olfactory receptors. As a result, many geriatric individuals find that their food tastes bland and unappetizing, whereas children tend to find the same foods too spicy. 16.7 Sight Vision is the most relied upon the sense of all the special senses. The visual receptors are contained within the eyes. The eyes are elaborate structures that enable the brain to detect light and create detailed visual images. Internal eye structures contribute to vision, while accessory eye structures provide protection.

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