The ChelUical SensesTaste and SlUell
'IDe senses of taste and sUlell allow us to separate undesirable or even lethal foods frOUl those that are pleasant to eat and nutritious. 'IDe sense of slIlell also allows aniUlalS to recognize the proxiUlity of other aniITIals or even individuals alIlong anilIlals. Finally, both senses are strongly tied topriITIitive eUlotional and behavioral functions of our nervous systelIls.
Sense of Taste
Taste is Ulainly a function of the taste buds in the IIlOUth, but it is COUlUlon experience that one's sense of sUlell also contributes strongly to taste perception. In addition, the texture of food, as detected by tactual senses of the Ulouth, and the presence of substances in the food that stiITIulate painendings, such as pepper, greatly alter the taste experience. 'IDe iUlportance of taste lies in the fact that it allows a person to select food in accord with desires and often in accord with the body tissues' Uletabolic need for specific substances.
Primary Sensations of Taste
'IDe identities of the specific cheUlicals that excite different taste receptors are not all known. Even so,psychophysiologic and neurophysiologic studies have identified at least 13 possible or probable cheUlical receptors in the taste cells, as follows: 2 sodiuUl receptors, 2 potassiUUl receptors, 1 chloride receptor, 1 adenosine receptor, 1 inosine receptor, 2 sweet receptors, 2 bitter receptors, 1 glutaITIate receptor, and 1 hydrogen ion receptor. For practical analysis of taste, the aforeUlentioned receptorcapabilities have also been grouped into five general categories called the pritnary sensations of taste. 'Ib.ey are sour, salty, sweet, bitter, and "unzarni. n A person can perceive hundreds of different tastes. 'IDey are all supposed to be cOUlbinations of the eleUlentary taste sensations,just as all the colors we can see are cOUlbinations of the three priITIary colors, as described in Chapter 50.Sour Taste. 'IDe sour taste is caused by acids, that is, by the hydrogen ion concentration, and the intensity of this taste sensation is approxiITIately proportional to the logarithtn ofthe hydrogen ion concentration. 'IDat is, the Ulore acidic the food, the stronger the sour sensation becoUles.
Salty Taste. 'IDe salty taste is elicited by ionized salts, Ulainly by the sodiUUl ionconcentration. 'IDe quality of the taste varies sOUlewhat frOUl one salt to another, because SOUle salts elicit other taste sensations in addition to saltiness. 'IDe cations of the salts, especially sodiuUl cations, are Ulainly responsible for the salty taste, but the anions also contribute to a lesser extent.
Sweet Taste. The sweet taste is not caused by any single class of cheITIicals. SOUle
of thetypes of cheITIicals that cause this taste include sugars, glycols, alcohols, aldehydes, ketones, aUlides, esters, SOUle anrino acids, sOlIle sUlall proteins,
The Nervous System: B. The Special Senses
Relative Talle Indices of Dlllereni Substances
Hydrochloric acid Formic acid Cbloracetic acid
: .. , .•.-•...•.. -.•- . •
FI. . .
.. •..i"-"::::='_~;;_::f.-_T sla ceUs i a
--Subepithelial oonnectiv9 tissue
is about 10 days in lower mammals but is unknown for humans. The outer tips of the taste cells are arranged around a minute taste pore, shown in Figure 53-1. From thetip of each taste cell, several microvilli, or taste hairs, protrude outward into the taste pore to approach the cavity of the mouth. These microvilli provide the receptor surface for taste. Interwoven around the bodies of the taste cells is a branching terminal network of taste nerve fibers that are stimulated by the taste receptor cells. Some of these fibers invaginate into folds of the taste...