Hypotheses about the peripheral basis for the sense of taste in mammals have been based to a considerable degree on the determined sensibilities of the nerve fibers in the chorda tympani of the rat to chemical stimulation of the anterior tongue. Yet, whether neuron types exist in this nerve, the nature of the basic mechanisms of taste reception that are tapped by this nerve and the form in which information about stimulus quality and intensity is transmitted to the central nervous system by this nerve are, at present, unresolved issues. These issues are addressed in the present study, which is a detailed analysis of the responses of rat chorda tympani nerve fibers that are sensitive to ionic stimuli; solutions applied to the anterior tongue included a range of concentrations of four chemical compounds (sucrose, sodium chloride, hydrochloric acid, and quinine hydrochloride) that represent widely different taste qualities to man or rat. Of the 44 nerve fibers sampled, 40 were stimulated most by one of the two ionic stimuli at test concentrations reported to be equally effective: 21 fibers were most responsive to 0.1 M NaCl and named N units; 19 fibers were most responsive to 0.01 M HCl and named H units. Although many N and H units responded to both HCl and NaCl, the distribution of NaCl-HCl response ratios was bimodal, indicating there are two varieties of units sensitive to ionic taste stimuli in the rat chorda tympani. Sucrose (0.5 M) affected 4 of the nerve fibers and was the most effective stimulus for 3 of them; 0.02 M quinine affected 13 of the fibers but 10 of these were H units. H units were less "specifically tuned" than N units; they were more likely to respond to several of the chemicals. Although the absolute sensitivity to NaCl in N units or to HCl in H units varied more than 10-fold, the relative effects of the four stimuli (response profiles) were generally similar for units of a given type. Exceptions occurred when H unit responses to NaCl or quinine were suppressed by prolonged effects of preceding HCl stimulation. The similarity in response profiles is reflected in high and significant correlations between responses to each pair of effective stimuli across either H or N units.(ABSTRACT TRUNCATED AT 400 WORDS)
Humans cannot reliably identify the distinctive characteristic odors of components in mixtures containing more than three compounds. In the present study we demonstrate that selective adaptation can improve component identification. Characteristic component odors, lost in mixtures, were identifiable after presenting other mixture constituents for a few seconds. In mixtures of vanillin, isopropyl alcohol, l-menthol and phenethyl alcohol, this rapid selective adaptation unmasked each component. We suggest that these findings relate directly to how olfactory qualities are coded: olfactory receptors do not act as detectors of isolated molecular features, but likely recognize entire molecules closely associated with perceived olfactory qualities or "notes." Rapid and focused activation of a few distinct receptor types may dominate most odor percepts, emphasizing the importance of many dynamic and specific neural signals. An interaction between two fundamental coding strategies, mixture suppression and selective adaptation, with hundreds of potential olfactory notes, explains humans experiencing the appearance and disappearance of identifiable odors against ambient mixture backgrounds.
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