THE GUSTATORY RELAY IN THE MEDULLA 389 



tap water (9°-37°C) were routinely employed after a unit responding to 

 chemical stimulation had been isolated. A Grass kymograph camera 

 photographed the action potential traces. 



Figure 6 shows functional relations between the average frequency of 

 firing and the concentration of the chemical stimulus for one unit. The 

 response to sodium chloride is a logarithmic function of concentration, 

 until a decline occurs with the strongest concentrations. A decline in the 

 response to strong NaCl, similar to that found in this single unit, has 

 also been observed in some single chorda tympani fibers (Pfaffmann, 

 1955). Potassium chloride and sucrose were also effective stimuli. The 

 responses to HCl, Na saccharin, and quinine hydrochloride (not shown 

 in this figure) were about equal in magnitude to those of the KCl 

 function. The activity to water of diff'erent temperatures indicates that 

 there was a slight depression to warm tap water and a significant response 

 to cool tap water at lOX. Mechanical stimulation of the tongue resulted 

 in a phasic discharge. 



This neuron is similar to many of the rat's second order taste units in 

 that it responded to a number of different chemicals as well as to cooling 

 and mechanical stimulation of the tongue. How does the nervous system 

 detect which of these stimuli activated the single unit ? Using an average 

 frequency measure and considering only this fiber, any one of the six 

 chemicals applied could have produced a frequency of 20 impulses per 

 second. If the cells are not highly specific to certain chemicals, perhaps 

 there are clusters of cells with similar sensitivity profiles. Data bearing on 

 this possibility are shown in Fig. 7. This is a three-dimensional array of 

 26 cutouts, each representing the response of a single taste unit, tested 

 with each of the six stimuli shown on the front of the model. The vertical 

 height of the black and white bars indicates the response magnitude to 

 the particular stimulus. (No special significance should be attached to 

 the order of presentation of these histograms in the model.) It is apparent 

 that there are no particular clusters or groups of histograms that might 

 correspond to " fiber or cell types ". Another analysis was attempted to 

 determine whether any significant specificity of single taste units to a 

 particular chemical could be demonstrated. The tongue was treated with 

 a water extract of Gymnema sylvestre, which in human psychophysical 

 studies produces a selective depression of the sensitivity to bitter and 

 sweet chemicals. Yet, no selective depression of firing was found here, 

 only a generahzed depression in the response of single units to all chemicals. 



The most impressive diff^erence among the 26 units is their overall level 

 of reactivity. If a unit was relatively insensitive to one of the taste stimuh, 

 it was similarly unresponsive to other chemicals, and conversely, if it was 

 highly sensitive to one chemical, it was highly sensitive to most others. 

 At the present time, it is not clear why there should be such a wide range of 



