
to NaCl recorded from the rat chorda tympani nerve typically show an initial phasic portion followed by a slowly declining tonic component (14) that is proportional to stimulus concentration. This phasictonic response is also characteristic of single chorda tympani neurons -( 1, 1 l)+ There has been some suggestion that the initial phasic burst of activity is particularly important in the representation of gustatory information (5, 7, 8, 20) . For example, rats can make taste quality discriminations within the duration of the transient portion of the chorda tympani discharge, and cross-adaptation procedures, which result in a loss of taste sensation in human psychophysical studies (Zl), have their effect on the phasic portion of the rat chorda tympani response (20) . Although the transient portion of the neural discharge appears to be implicated in the representation of sensory information, the mechanisms underlying the production of this phasic burst of activity and the subsequent decline termed adaptation are not well understood.

The time course of the chorda tympani response has received recent attention with regard to its implications for theories of gustatory stimulation (4a, 5, 6, 8 The anterior portion of the rat's tongue was enclosed in a flow chamber fitted with a rubber dam so that saliva was excluded and solutions were delivered from a system of overhead funnels at a rate of 3 ml/s. Even though the funnel containing the stimulus solution almost emptied during the 120-s stimulation period, the flow rate varied by less than 4 % throughout this period. The stimulus solutions, made from reagent grade NaCl and distilled water, were the following:

1.0, 0.6, 0.3, 0.1, 0.06, 0.03, and 0.01 M NaCl. The stimuli were maintained and delivered at room temperature (2 1 "C). Experimental procedure. Each NaCl concentration was presented to the tongue of each preparation for a period of 2 min. During data collection, each stimulus was preceded and followed by at least a Z-min distilled water rinse. Since rather drastic changes can sometimes occur in the responsiveness of a chorda tympani preparation, even though the base-line response appears to be stable, a control stimulus of 0.1 M NaCl was presented for 15 s between stimulus solutions to monitor the stability of the preparation. The four preparations included in this study showed a consistent response to the control stimulus throughout the period of data collection, i.e., the amplitude of the transient response to 0.1 M NaCl did not vary more than 10 %.

Data analysis. The stored neural responses were integrated using a resistor-capacitor circuit similar to that described by Beidler (1). These integrated responses were displayed on a strip-chart recorder (Sanborn 320) with a rapid (20 mm/s) paper speed. Both the integrating circuit and the strip-chart recorder showed a faithful response to a series of square-wave inputs of varying amplitudes, i.e., their response characteristics were highly linear. The rise time of the strip-chart recorder was 20 ms, which did not contribute significantly to the response of the entire system. A very short integrator time constant (47 ms rise and fall) was used in this study, since the dynamics of the neural discharge as seen in the integrated response are very greatly affected by the time constant of the integrating circuit.

Short time constants reveal rapid fluctuations in the neural response that go unnoticed using longer time constants (5). From the strip-chart records, the amplitudes of the integrated responses at various times beginning with the peak of the transient response were calculated in millimeters. Data were obtained at lOO-ms intervals up to 2.5 s after the peak of the transient response, then at 500-ms intervals up to 5 s, at l-s intervals up to 10 s, at 5-s intervals up to 30 s, and at 10-s intervals up to 120 s. The value at 120 s was taken as the steady-state response value for purposes of the computer analysis, even though the response was still declining slowly at this point. These data were normalized and averaged across preparations for each stimulus concentration.

The mean responses at each concentration were then computer analyzed to obtain a mathematical description of their time course.
RESULTS
Stimulation of the tongue of the rat with NaCl at concentrations ranging from 0.01 to 1 .O M results in a chorda tympani discharge characterized by an initial rapidly declining transient phase followed by a slowly declining steady-state component that is proportional to NaCl concentration. Figure  1 shows the mean integrated response to these NaCl concentrations initially (transient) and at various times following the peak of the transient response. It is evident from Fig. 1 Fig. 2 , where the response to each succeeding concentration of NaCl is displaced to the right for greater clarity. Here it can be seen that the initial decline from the peak of the phasic response is 72) and th e manner in which they combine to influence the total response as a function of time are graphically illustrated in Fig. 3 . The time course of the neural response after the peak of the transient burst of activity, according to the model shown in Fig. 3 Table 1 . Thus, the stronger the NaCl concentration the greater the amplitude of both the gradual second decline of the adaptation process and the steady-state value of the response. However, the amplitude of the initial rapidly declining exponential is independent of stimulus intensity.
DISCUSSION
The results of these analyses suggest that adaptation of the chorda tympani response reflects at least two underlying processes. While it is perhaps inappropriate to make inferences about specific receptor mechanisms from integrated whole-nerve responses, correlations between these data and other relevant literature can provide support for certain hypotheses regarding the nature of the transduction process in taste. For example, the rate of solution flow has been shown to have an effect on both the amplitude of the transient portion of the chorda tympani response (19, %a) and human estimates of taste intensity (9). These findings support the idea that the initial phasic burst of chorda tympani activity reflects the rate of stimulus adsorption (8), which should vary directly with the rate of solution flow. These considerations from the literature led to the consideration of the following physical model of the taste receptor system. First, it is assumed that there is a diffusion process that must take place in order for the chemical to reach the receptor, and second, that the change in chemical concentration at the surface of the tongue may be described as shown in Fig. 4 . This model allows one to include the effect of flow rate on the response since the time, T1, re- quired to reach the final concentration, K, will be smaller for higher flow rates. For notational simplicity in the following mathematical development, the slope of the rise in concentration is defined as m = K/T1 , which will be proportional to flow rate. Finally, it is assumed that the response is proportional to the rate of complex formation, as in the model proposed by Heck, and Erickson (8) . All of this development is similar to that discussed by Beidler (3) and by Faull (4a). While Beidler assumed, as in the present discussion, that the diffusing material cannot go further than the thickness of the "slab," Faull made the assumption that the material could continue to diffuse further. The present results do not distinguish between these two assumptions, but do support the validity of this physical model. Neither Beidler's nor Faull's theoretical development includes the rate of change in concentration of the solution flowed over the tongue as is done here (see Fig. 4 ). Since the present development includes flow rate, it allows prediction of the relationship of the coefficients to flow rate as well as conten tration.
DISCUSSION
The described one-dimensional diffusion problem may be concisely stated mathematically as :
DISCUSSION
d2C ( 
DISCUSSION
In this study, we are interested in the concentration at the receptor, C(a, t). Then Co(a, t) simplifies to:
DISCUSSION
Co(a, t) = mt -F (aa)
DISCUSSION
Thus, the physical model leads to the following mathematical model for the concentration of solute at the receptor:
DISCUSSION
The following observations may be made about the solution described by equations 11 and 12: I) for t > Tl , equation 12 -is a constant depending on the concentration, K = rnT1 plus a sum of exponentials.
DISCUSSION
2) The coefficients of the exponential terms are inversely proportional to n3, making the first term far larger than any other. 3) The time constants (1 /X,z> are inversely proportional to 9, making the first term last far longer than any others. If equation 12 is used in conjunction with the Heck and Erickson model, the predicted response is given by: aW) = A0 + AJ"' + 2 B, Pm
DISCUSSION
n=l 1) AO should be proportional to the concentration of the solution used to bathe the tongue (m Tl = K). 2) A1 should be a function of both concentration and rate of stimulus onset (m), i.e., the flow rate of the solution.
DISCUSSION
Thus, it will not be simply proportional to either parameter.
DISCUSSION
3) I31 should be proportional to the rate of onset, m. 4) B2 and higher These include the slope, m, the diffusion constant, a2, and the diffusion distance, x.
DISCUSSION
Initial analysis of the data collected in this study suggested that two exponential terms were present in addition to the constant term. This is easily confirmed by a semilogarithmic plot of the data. Mathematical models using a constant plus two exponentials have been used to describe the response of other sensory systems (15). If some sort of neural adaptation were proposed as the mechanism underlying the second exponential decay, two properties could reasonably be hypothesized for this function. First, the amount (amplitude) of adaptation could be expected to be positively correlated with the concentration of the stimulus, and second, the time constant would be expected to be much larger than that associated with the initial response. Thus, the preceding development leads to the proposed mathematical model consisting of three terms, A* and Ale-t'7L, reflecting the chemical dynamics of the system and A&172, reflecting neural adaptation. In the mathematical model proposed, the amplitude of the initial transient response is equal to the sum of the values of A0 , Al , and A2 . The values of A0 and A2 are correlated with NaCl concentration, as predicted by the theoretical considerations.
DISCUSSION
Thus, the maximum transient response magnitude (A0 + Al + AZ) is correlated with stimulus concentration in agreement with the results reported by Halpern and Marowitz (6). Further, A1 is not correlated with NaCl concentration, as predicted by the mathematical model. The value of Al is equal to the initial response value minus the sum of A0 and AZ, both of which increase with increasing NaCl concentration. If the initial transient response were a function of the rate of stimulus adsorption (8), then th e amplitude (Al) of the initial exponential might be expected to show a decrease with increasing NaCl concentration.
DISCUSSION
For example, if the initial rate of binding were to approach its maximum more rapidly than did the equilibrium rate, then the phasic portion of the response would show a decrease relative to the tonic component at higher stimulus concentrations.
DISCUSSION
In fact, Fig. 1 shows that the

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