528 



HANDBOOK OF PHYSIOLOGY' 



NEUROPHYSIOLOGY I 



with 5 per cent extra salt in diet and lo per cent 

 extra salt. The basic preference-aversion response 

 (TV) is shifted systematically by these changes in 

 salt need. In the normal animal, the preference 

 threshold lies above the concentration at which a 

 discharge of nerve impulses can be detected. In the 

 adrenalectomized rat the preference and electro- 

 physiological thresholds are more nearly equal. The 

 normal animal thus appears to taste the salt but does 

 not ingest it. The adrenalectomized rat takes the 

 salt solution when he can taste it (165). 



The neural response curve of the receptor appears 

 to be a stable properly of the taste bud. The threshold, 

 i.e. a minimum concentration necessary to elicit a 

 discharge of the taste receptors, is essentially the same 

 in normal and adrenalectomized rats (166). This has 

 been confirmed in studies using the conditioned reflex 

 method (.15, 98). Thus the change in self-selection 

 behavior cannot be explained by a peripheral change 

 in sensitivity of the taste receptors. Of similar import 

 is the finding of the constancy of the chorda tympani 



response in insulin hypoglycemia (167). Insulin in- 

 jection typically leads to a striking increase in the 

 preference for sugar solutions in a free choice situa- 

 tion. 



Richter has presented evidence that the compensa- 

 tory increase in intake fails when the sensory nerves 

 to the tongue are surgically removed (175). The 

 attenuation in preference behavior in the normal 

 animal after combined chorda tympani-ninth nerve 

 deafferentation further supports the view that taste 

 stimulation triggers the response to taste solutions 

 (162). A rat with an esophageal fistula will show the 

 salt preference even when the .solution does not enter 

 the stomach and cannot ha\e a metabolic effect (192). 

 At the same time the ingestion of water or salt solu- 

 tions can be modified by stomach loading by intuba- 

 tion with sodium chloride solutions which 'by-passes' 

 taste, but the effect is less than when the same amount 

 of .solution is taken by mouth so that the taste recep- 

 tors are stimulated (132). Thus both taste and intra- 

 gastric factors may influence drinking C204). 



% 



100. 



80. 



60. 



40- 



20- 



r 

 -3 



o es 



-8.0 



-1.5 



-1.0 



- .5 



1 



-2 



NoCI LOG M Cone. 



"T" 

 



FIG. 17. A composite graph of the neural response (broken line) and the preference curves (solid 

 lines') for different concentrations of NaCI. The neural response curve (ordinate to the right) shows 

 the magnitude of the electrical activity in the chorda tympani nerve of a normal rat. Each open 

 circle is a measure of the integrated electrical activity (in milliamperes, ma.) of the discharge at 

 each concentration. The preference curves were obtained from four different groups of animals: 

 adrenalectomized (adren.), normal (.V), normal with 5 per cent additional salt in the diet (j'/c) 

 and normal with 10 per cent additional salt in the diet (/o%), respectively. Each point is the av- 

 erage preference (or aversion) indicated as a percentage (cc intake salt) -f- (cc salt -|- cc H-.O) at 

 each concentration. Each point is the average consumption for a 48-hr. period when both water 

 and salt solution were continuously available. Salt solutions were presented in ascending order of 

 concentration. [From Pfaflfmann (165).] 



