512 HANDBOOK OF PHYSIOLOGY ^^ NEUROPHVSIOLOGV 



FREQ 

 40- 



RAT 



SINGLE ELEMENTS 



(01) 



.03N 

 HCI 



IM 

 KCI 



.1 M 

 NoCI 



OIM 

 Q u. 



1.0 M 

 Sue. 



03N 

 H CI 



IM 

 KCI 



.1 M 

 NaCI 



OIM lOM 

 Qu. Sue. 



FIG. 6. Bar graphs summarizing frequency of response during the first second to fi\'e standard 

 taste solutions in nine different single fiber preparations in the rat. Sucrose of 0.3 m was used as 

 test solution in eleinents D and /, 0.01 M HCI in element /. In all other cases concentrations are 

 as shown on abscissa. Cross-hatehrd bar graph superimposed on figure for element E shows relative 

 magnitude of integrator response for test solutions. Figures in parentheses give magnitudes in arbi- 

 trary units. Note that only elements D and G resemble the response of the total nerve. [From Pfaff- 

 mann (164).] 



receptor cell. The proijlcm of chemical specificity is 

 one of specificity within the individual cells, i.e. to 

 different sites or loci on the cell membrane. 



The specificity of the receptor unit cannot be ade- 

 quately described by the response to only one con- 

 centration of a test stimulus. Figure 7 shows that 

 fiber B (the same as fiber B in fig. 6) can be stimu- 

 lated by sodium chloride at concentrations of o. i m 

 and higher. This might be labelled as a salt-sugar 

 unit. Since gymnemic acid applied to the tongue 



leads to a clear-cut decrement in the response to sugar 

 with no change for sodium chloride, it appears that 

 only the sucrose sites on the cell is blocked, not the 

 'salt-sucrose' cell itself. The differential suppression of 

 sugar sensitivity, often cited as evidence for separate 

 modalities of taste, can be equally well encompassed 

 by a theory of specific sites on the cell membrane. 



The two fibers, A and B of figure 7, respond to 

 ijoth sodium chloride and sucrose, but A is more 

 reactive to sodium chloride and B is more reactive to 



