flowchart of food on the tongue to recognition of its taste
Answers
t is appropriate that we consider taste and smell together because they are so intertwined in our experience that most people are unaware that most of what they call taste is really an olfactory experience. The sensations evoked by a substance put into the mouth are complex and involve much more than taste. For example, a mouthful of orange soda pop gives one a taste that is a combination of sour and sweet; it is cold; it may sting or tingle a bit if it is carbonated; it evokes a complex touch sensation in the mouth, and it smells fruity or fragrant. The only part of this complex of sensations that is taste in origin, i.e., resulting from receptors on the tongue, palate or pharynx is the sensation of sweetness or sourness. Anyone who has ever had a head cold can attest to the flatness or blandness of his diet during that time. This is due to the fact that access of odorants to olfactory receptors is blocked by the copious secretions of mucus.
Fig. 10-1. The distribution of gustatory papillae, their innervation, and the regions of maximum sensitivity to different submodalities of taste on the human tongue. (Altner H: Physiology of taste. In Schmidt RF [ed]: Fundamentals of Sensory Physiology. New York, Springer-Verlag, 1978)The gustatory system is much simpler than the olfactory system. Four primary taste submodalities are generally recognized: sweet, sour, salty, and bitter. Different regions on the tongue exhibit different maximal sensitivities to the four taste submodalities (Figure 10-1 which also shows the pattern of innervation of the tongue). The tip of the tongue is the most sensitive to sweetness and saltiness. The sensation of sourness is experienced best on the lateral aspects of the tongue, and bitterness is experienced best and perhaps only on the back of the tongue. Next time you put some bitter substance such as tonic water (quinine) into your mouth, you can verify this for yourself.
The chemical senses are difficult to study experimentally because the stimuli that lead to gustatory sensations are not well understood. Sour tastes are evoked by all acids in dilute solutions. Apparently, it is the hydrogen ion that activates taste receptors and leads to a sensation of sourness. Accounting for salty tastes is more difficult. Sodium chloride is the only substance known to evoke a purely salty taste in any concentration that is suprathreshold. With other compounds, having one of the cations, Na+, K+, Li+, Ca2+, and one of the anions, Cl-, Br-, I-, SO42-, NO32-, CO22-, the saltiness varies with concentration. For example, a very dilute solution of KCl tastes sweet and, as the concentration increases, it becomes first bitter, then both bitter and salty, and finally purely salty. Both the anion and the cation are apparently involved in evoking the salty sensation.
The basis for sweet and bitter tastes is not known. Sucrose is a carbohydrate, a disaccharide formed from one molecule each of fructose (a fruit sugar) and glucose. Glucose, fructose, and starch are also carbohydrates. Fructose (grape sugar) is the sweetest; glucose is less sweet than sucrose or fructose; and starch is not sweet at all. Some alcohols are very sweet. For example, xylitol is used as a sweetener in Europe and in some chewing gum in the United States. Saccharin is also sweet (and bitter), but chemically it bears little similarity to either the sugars or the alcohols. The only known commonality for all these substances is that they excite gustatory receptors leading to sensations of sweetness.