Taste and Smell
Aging And Taste
Older people lose the ability to detect very low concentrations of bitter and salty substances. In contrast, the perception of sweet and sour is robust even in extreme old age. Not all bitter compounds are affected equally, however.
Age-related taste deficits are most pronounced when testing is localized to specific areas on the tongue. Instead of whole mouth tasting, the stimulus is applied locally to spots on the tongue by using a special apparatus or by applying the tastant with a cotton swab. Scientists believe that whole mouth perception may compensate for some of the regional losses of taste function with age.
Regional losses in taste might be expected, given the anatomy of the taste system. Tastes are detected in the mouth by specialized receptor cells located in the upper part of taste buds and near the taste pore. Taste buds are located not only on the tongue but also in the throat and on the roof of the mouth (soft palate). Taste buds are made up of thirty to fifty individual cells, which are organized into an oblong sphere, much like the segments of an orange. The entire taste bud is regenerated every two weeks. Individual taste cells live for only ten to twelve days, and new cells below them evolve to replace them as they die.
Taste buds in the front of the tongue are located in tiny, mushroom-shaped pieces of tissue known as fungiform papillae. These appear as small, round, pink bumps on the tongue surface. In humans each fungiform papilla contains a number of taste buds, each one opening to the outside through a tiny taste pore. Some people may have twenty-five or more taste buds per papilla. There are also regional differences in taste bud density. Fungiform papillae on the tip of the tongue have more taste buds than do those in the middle region of the tongue. A branch of the facial nerve innervates the fungiform papillae, and carries information about taste on the front of the tongue to the brain.
Papillae in the rear of the tongue, known as circumvallate papillae, are arranged in an inverted V shape and are level with the surface of the tongue. Information about taste in the rear of the tongue is carried to the brain by the glossopharyngeal nerve.
More taste buds are located in tissue folds on the sides of the tongue, just in front of the circumvallate papillae. These foliate papillae are mostly innervated by the glossopharyngeal nerve, but some appear to be innervated by the same nerve as papillae at the front of the tongue. Taste buds in the soft palate are innervated by a branch of the facial nerve, and taste buds in the throat are innervated by the glossopharyngeal nerve and the vagus nerve.
Because taste buds are found all over the oral cavity, the perceived taste of food appears to come from the entire mouth and not from isolated patches on the tongue, throat, and roof of the mouth. It is the sense of touch that serves to localize taste perception, such that the taste is perceived to be coming from the area that is stimulated by touch. Smelling chocolate odor while chewing on tasteless gum is interpreted by the brain as eating chocolate.