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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOCi' I 



which is sour to ail persons. Many of the substances 

 with whicli taste blindness may be demonstrated are 

 antithyroid compoimds (99). Taste blindness is not 

 correlated with sensitivity for other bitter stimuli or 

 the other taste qualities. This suggests a high degree 

 of specificity for the particular chemical linkage to- 

 gether with some feature of the receptor mechanism. 

 An attempt has been made to relate taste blindness 

 to solubility of PTC in saliva (47). In view of the 

 specificity of the linkage this does not seem to be a 

 likely explanation. 



The fact that sweet and bitter sensitivity are often 

 associated in the case of certain stimuli and that 

 both tend to be inactivated by the action of drugs or 

 narcotic agents led one in\estigator to propose that 

 both depend upon the action of a single receptor 

 mechanism (194). Of all the taste mechanisms, that 

 underlying bitter sensitivity is least well understood. 



ELECTRIC TASTE. That electric currents can stimulate 

 the sense of taste has been known almost since the 

 discovery of electricity. \'olta noted, for example, a 

 sour taste when a circuit with two dissimilar metals 

 made contact with the tongue. Such taste is elicited 

 not only at the make and ijreak of current but by 

 the steady flow as contrasted with the more familiar 

 stimulation of nervous tissue by short duration pulses. 

 Anodal unipolar stimulation of the tongue with an 

 indifTerent electrode elsewhere on the body elicits a 

 sour taste; cathodal stimulation yields a complex 

 alkaline quality but, at cathodal break, sour is re- 

 ported (198). 



Early investigators often employed metallic polariz- 

 able electrodes. With an inert electrode like platinum, 

 the following electrolytic change occurs when current 

 flows through a weak salt solution. Electrons are 

 introduced at the cathode toward which the positive 

 hydrogen and sodium ions are attracted. A higher 

 voltage is usually required to discharge the Na ion 

 so that H ions are discharged and H2 is liberated, 



ae + 2H2O -^ 2OH- + H.. 



leaving Na+ and OH" ions. At the positive pole 

 electrons come primarily from the OH~ ions which 

 by their discharge leave an excess of H+ ions. 



4OH- 



2H,0 + O2 + 2e 



These together with the remaining CI" ions form a 

 dilute solution of hydrochloric acid. These chemical 

 effects would appear to account for the sour taste at 



the anode and alkaline taste at the cathode. In addi- 

 tion there is movement of cations toward the cathode 

 and anions toward the anode with a resulting change 

 in concentration in the vicinity of the electrodes. 



With a nonpolarizable reversible electrode, electron 

 transfer at the electrode is derived from the reaction 

 Ag ^ Ag+ -f e. No discharge of OH" or H+ ions 

 occurs. There is no electrolysis, l)ut the subject still 

 reports sour at the anode (40). Further work is de- 

 sirable in view of one preliminar)- report that the 

 anode produces a salty taste when a carefully con- 

 structed reversible electrode is utilized C71). 



In general two hypotheses have been proposed to 

 account for the electric taste. The first is the chemical 

 theory in which it is belie\ed that taste buds are 

 stimulated by the concentration of ions resulting from 

 electrolysis. Thus the sour of the platinum electrode 

 is said to be due to the excess of hydrogen ions. The 

 appearance of the same taste with a reversible elec- 

 trode suggests the second \iew, namely that direct 

 depolarization of the taste membrane occurs by 

 virtue of the ionic transfer in the cell and across the 

 cell membrane. In both cases, of course, the pa.ssage 

 of current is an electrochemical reaction. 



Direct stimulation of the nerve fibers so that the 

 receptor organ itself is 'by-passed' can be ruled out, 

 at least for direct current anodal currents. The taste 

 threshold current is lower for the anode than the 

 cathode, which is the reverse of the relation found 

 for direct nerve stimulation. Furthermore the eleva- 

 tion of threshold after the topical applications of 

 tetracaine was much greater for the anode than the 

 cathode, suggesting that the anode stimulated the 

 more superficial receptor but that the cathode stimu 

 la ted the deeper nerve fibers. The strength duration 

 curses of taste indicate a longer time constant for 

 the anode than the cathode (38, 40). 



Electrophysiological recordings of the single taste 

 fibers show that anodal polarization of the tongue 

 surface causes a discharge like that to chemical stimu- 

 lation, except that the latency to the electrical stimulus 

 is approximately 5 to 7 msec, whereas that to chemi- 

 cal stimulation is approximately 35 to 50 msec. (160). 

 Thus the anodal electrical stimulus appears to act 

 via the receptor cell but with a much shorter latency 

 as though some initial step were by-passed. 



The discharge to a steady anodal current continues, 

 after an initial decrement, as long as the current flows. 

 The same magnitude of cathodal current, however, 

 causes an immediate inhibition of activity which lasts 

 as long as the current flows. Upon break of the 

 cathodal current, there follows a transient burst of 



