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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



associated with severe degeneration throug;hout the 

 arcuate nucleus except for the dorsomedial tip (155). 



The close association of taste pathways with the 

 somatosensory and also motor mechanisms appears 

 to hold true at the cortical level. Bremer (31) showed 

 that ablation of the masticatory cortex in rabbits is 

 associated with a taste deficit. Changes in the elec- 

 trocorticogram from this same area were observed in 

 unanesthetized rabbits when quinine solutions were 

 placed on the tongue (72). A corresponding area on 

 the orbital surfaces of the rat ijrain (25) was identi- 

 fied by the e\oked potential method following elec- 

 trical stimulation of the chorda tympani and glosso- 

 pharyngeal nerves. This corresponds to the region 

 from which masticatory movements could be elicited 

 by electrical stimulation (130) and thus is a sensory 

 motor area. Ablation of this area in the rat led to an 

 elevation of the two-bottle preference thresholds for 

 quinine .solutions. No taste deficits were noted in two 

 animals in which most of the neocortex except for the 

 combined chorda tympani and ninth nerve receiving 

 areas was removed. Further studies of the deficits 

 produced by cortical aijlation indicate that under 

 certain high drive states no apparent taste deficit 

 can be demonstrated. A thirsty normal animal and 

 a thirsty animal with a cortical lesion will show the 

 same aversion to quinine. Both have higher thresholds 

 for quinine than the normal animal with water pres- 

 ent ad libitum. Thus, the removal of the cortical 

 taste area does not make the animal ageusic but in- 

 stead renders the animal less discriminating in an ad 

 liijitum situation (23, 24). 



The chorda tympani nerve area of the rat and cat 

 lies in the face somatic area (156, 201). Much of the 

 surface-positive cortical response to electrical stimu- 

 lation of the chorda tympani is due to the tactile af- 

 ferent fibers in that nerve. It has not been possible to 

 record an evoked cortical potential with gross sur- 

 face electrodes with adequate taste stimulation. How- 

 ever, ultramicroelectrodc probings in the tactile 

 tongue area of the cat did yield single units that re- 

 sponded to taste but not to touch or temperature. 

 Other units in this area showed convergence of tactile, 

 thermal and gustatory impulses (128). The taste units 

 appeared to be less chemically specific than the single 

 afferent fibers for they responded to almost all types 

 of gustatory stimulation Cso)- 



In monkeys and chimpanzees, lesions of the face 

 motor and sensory areas along the free surface of the 

 lower Rolandic cortex did not produce taste deficits 

 in preference tests (155). Taste deficits occurred only 

 when the lesions involved the buried opercular and 



parainsular cortex. Bagshaw & Pribram (13) have 

 shown that the insular and anterior supratemporal 

 as well as parainsular cortex all must lie included to 

 lead to an elevation of threshold. Some elevation of 

 threshold followed ablation of the operculum plus 

 insula, but not with restricted ablation of the insula 

 or insula and anterior supratemporal plane. 



In man, a series of patients with bullet wounds of 

 the inferior postcentral region showed reduced gusta- 

 tory and tactual sensibility of the tongue (30). Pen- 

 field & Boldrey (159) elicited gustatory sensations in 

 conscious human patients by electrical stimulation 

 of the lower end of the postcentral gyrus. Thus, the 

 evidence implicates the region of the cortex of the 

 operculum, insula and supratemporal plane of the 

 temporal lobe. 



Patton (155) notes that not only is there the close 

 approximation of the taste to the somatosensory 

 system but that taste localization fits into its orderly 

 topographical arrangement. Taste does not have a 

 special primary cortical receiving zone with exclusive 

 gustatory functions. 



REGEPTOR MECHANISMS 



Functional Characteristics 



No simple relation can be established between 

 chemical stimuli and taste quality except perhaps in 

 the case of acid. Equally sour concentrations of 

 hydrochloric, sulfuric, nitric, phosphoric and acetic 

 acids are said to be indistinguishable from each other 

 when odor is excluded (56); but sucrose, dextrose 

 and lactose do not ha\e exactly the same taste (44); 

 and stimuli that elicit the bitter taste can be dis- 

 criminated from each other. The taste qualities of in- 

 organic salts are complex and only sodium chloride 

 has a pure saline taste, yet in threshold solutions this 

 is variously reported as .sweet or bitter (173, 181). 



The tongue surface is not uniformly sensitive to 

 punctate stimulation. The middorsum is insensitive 

 to all tastes. Sweet sen.sitivity is greatest at the tip, 

 sour at the sides, bitter at the jjack, while salt sen.si- 

 tivity is relatively homogeneous but greatest at the 

 tip (96). Indi\idual papillae have been found to 

 react exclusively to salt, to sweet or to sour, or to 

 come combination of two, three or four of the basic 

 taste stimuli (120). Certain drugs have a differential 

 effect on sensitivity. Gymnemic acid, an extract of 

 the leaves of an Indian plant Gynmema sylvestre, reduces 

 sensitivitv for sweet and bitter but leaves salt and sour 



