DYNAMICS OF TASTE CELLS 137 



the surface of the tongue enter between the cells of the taste bud. This is in 

 agreement with many physiological experiments that indicate difficulty in 

 penetration of substances through the tongue surface. For example, 

 mitotic division is blocked within 15 min after colchicine is injected intra- 

 peritoneally into the rat although no blockage is seen 24 hr after much 

 higher concentrations of colchicine remain on the tongue surface. The 

 microvilli of the taste cells, on the other hand, extend into the taste pore 

 and are in direct contact with the saliva. For the above reasons it is 

 assumed that the microvilli are the structures that are stimulated by taste 

 solutions. 



A single thickness membrane of about 80A encloses the contents of each 

 microvillus as far down as the crypts where the membrane thickness doubles 

 (de Lorenzo, 1962). The gross chemical constituency of this membrane is 

 not known, but it is probably similar to that of other cell membranes ; 

 namely oriented protein and phospholipid. Direct chemical or physical 

 analysis of the membrane is not presently possible but the chemical stimulus 

 can be used as an ionic or molecular probe of the surface of the microvilli. 

 This was attempted in 1951 by setting up a theoretical approach to the 

 binding of the stimulus to the receptor, showing its agreement with experi- 

 mental data, and then calculating the binding forces involved (Beidler, 

 1954). 



The ionic or molecular stimulus. A, can be assumed to be adsorbed to 

 sites on the microvillus surface, B, to form AB. The equilibrium constant, 

 K, for such a reaction is 



[AB] 



K 



[A] [B] 



It is assumed that the magnitude of taste receptor response, R, is pro- 

 portional to the number of sites filled, AB, and that the maximum possible 

 response, R^, occurs when all the available sites are filled. From such 

 reasoning, the equation 



C _ C 1 



R ~ \ '^ KR, 



is found where C is the concentration of the stimulus. This taste equation 

 was found to satisfy much of the experimental data and a distinct value for 

 K, which determines the affinity of the taste receptor site for the taste sub- 

 stance, was obtained for each type of stimulus (Beidler, 1954). 



The magnitude of maximum response, R^, can also be determined and 

 is a measure of both the number of total receptor sites available for the 

 particular stimulus and the intrinsic activity of each occupied site. The 

 intrinsic activity is a function of the ability of the stimulus-receptor 

 combination to initiate a response, presumably by producing a small local 



