Neural responses to bitter compounds in rats

To determine whether the idiosyncratic distribution of transduction mechanisms for bitter tastants in rat taste receptor cells (TRCs) could be inferred from the neural activity they evoke, single neuron responses to ten bitter-tasting compounds were recorded from rat glossopharyngeal (n=30) and chorda tympani (n=22) neurons. Responses to several ‘bitter' alkaloids were obtained: 10 mM quinine-HCl, 50 mM caffeine, and 1 mM each nicotine, yohimbine, and strychnine, plus a number of non-alkaloid bitter-tasting compounds: 0.1 M KCl, 0.01 M MgCl₂, and 1 mM each phenylthiocarbamide (PTC),

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-tyrosine, and denatonium benzoate. To obtain some distinctions with other stimuli NaCl (0.1 M), HCl (pH 2.0), and capsaicin (10 μM) were also tested. It was found that individual neurons in both glossopharyngeal and chorda tympani nerves differed in their relative sensitivities to the various bitter stimuli. To determine relationships among these stimuli, the differences in the evoked responses between each stimulus pair were summarized in a multi-dimensional scaling space. In these analyses neither nerve showed any obvious similarity between the placements of quinine and the other bitter stimuli. Such data suggest that first-order gustatory neurons can discriminate among the above bitter stimuli. For glossopharyngeal neurons, some similarity to quinine was found only for nicotine and denatonium, and for chorda tympani neurons, some similarity to quinine was found only for KCl and MgCl₂. Of the bitter compounds tested, quinine evoked the greatest response from glossopharyngeal neurons. We propose this arises because quinine can activate TRCs by more transduction mechanisms than other bitter stimuli. The results from these studies were summarized in a qualitative model for the coding of bitter tastants where the variety of transduction mechanisms for bitters are distributed among various TRCs to account for the heterogeneous responses among the neurons. © 1997 Elsevier Science B.V. All rights reserved.     

Peripheral gustatory processing of sweet stimuli by golden hamsters

Behaviors and taste-nerve responses to bitter stimuli are linked to compounds that bind T2 receptors expressed in one subset of taste-bud receptor cells (TRCs); and behavioral and neural responses to sweet stimuli are linked to chemical compounds that bind a T1 receptor expressed in a different TRC subset. Neural and behavioral responses to bitter-sweet mixtures, however, complicate the ostensible bitter and sweet labeled lines. In the golden hamster, Mesocricetus auratus, quinine hydrochloride, the bitter prototype, suppresses chorda tympani (CT) nerve responses to the sweet prototype: sucrose. This bitter-sweet inhibition was tested with concentration series of sucrose and dulcin, a hydrophobic synthetic sweetener that hamsters behaviorally cross-generalize with sucrose. Dulcin, sucrose and other sweeteners activate one subset of CT fibers: S neurons; whereas, quinine activates a separate subset of CT fibers: E neurons. Whole-nerve and S-neuron CT responses to a sweetener concentration series, mixed with 0, 1, 3 and 10 mM quinine, were measured for 0-2.5 s transient and/or 2.6-10 s steady-state response periods. Ten-sec total single-fiber records, aligned at response onset, were averaged for 100 ms bins to identify response oscillations. Quinine inhibition of dulcin and sucrose responses was identical. Each log molar increment in quinine resulted in equivalent declines in response to either sweetener. Furthermore, sucrose response decrements paralleled response increments in quinine-sensitive CT neurons to the same quinine increases. A 1.43 Hz bursting rhythm to the sweeteners was unchanged by quinine inhibition or decreases in sweetener concentration. Taste-bud processing, possibly between-cell inhibition and within-cell negative feedback, must modify signals initiated by T1 receptors before they are transmitted to the brain.     

Responses of the hamster chorda tympani nerve to binary component taste stimuli: evidence for peripheral gustatory mixture interactions

Studies of taste mixtures suggest that stimuli which elicit different perceptual taste qualities physiologically interact in the gustatory system and thus. are not independently processed. The present study addressed the role of the peripheral gustatory system in these physiological interactions by measuring the effects of three heterogeneous taste mixtures on responses of the chorda tympani (CT) nerve in the hamster ( Mesocricetus auratus). Binary taste stimuli were presented to the anterior tongue and multi-fiber neural responses were recorded from the whole CT. Stimuli consisted of a concentration series of quinine. HCI (QHCI: 1–30 mM), sodium chloride (NaCl: 10–250 mM). sucrose (50–500 mM) and binary combinations of the three different chemicals. Each mixture produced a unique pattern of results on CT response magnitudes measured 10 s into the response. Sucrose responses were inhibited by quinine in QHCI-sucrose mixtures. Neural activity did not increase when quinine was added to 50–250 mM NaCl in QHCINaCl mixtures. However, the neural activity elicited by sucrose-NaCl mixtures was greater than the activity elicited by either component stimulus presented alone. The results demonstrate that gustatory mixture interactions are initiated at the level of the taste bud or peripheral nerve. Mechanisms for these interactions are unknown. The results are consistent with one component stimulus modifying the interaction of the other component stimulus with its respective transduction mechanism. Alternatively, peripheral inhibitory mechanisms may come into play when appetitive and aversive stimuli are simultaneously presented to the taste receptors.     

Gustatory-salivary reflex: neural activity of sympathetic and parasympathetic fibers innervating the submandibular gland of the hamster

Electrophysiological experiments were performed to clarify the neural control mechanisms subserving gustatory-salivary reflex in anesthetized and decerebrate hamsters. Efferent neural activities of postganglionic sympathetic and preganglionic parasympathetic fibers, innervating the submandibular gland, were recorded when taste stimuli were infused into the oral cavity. Neural activities of primary gustatory afferents were also recorded from the chorda tympani (innervating the anterior part of the tongue) and the glossopharyngeal nerve (innervating the posterior part of the tongue). The parasympathetic fibers showed a low rate of spontaneous discharges (about 0.3 Hz), and responded tonically in an excitatory manner to taste stimulation. The magnitude of parasympathetic activity was highly correlated with the magnitude of gustatory afferent responses of the chorda tympani rather than that of the glossopharyngeal nerve. On the other hand, the sympathetic fibers showed irregular burst discharges (1.5 burst/s), and the rate of burst discharges was increased in response to high concentrations of HCl (0.03 M) or NaCl (1 M) solutions. Deafferentation experiments suggest that the parasympathetic activity is mainly influenced by gustatory information via the chorda tympani, while the sympathetic activity can be evoked by both the chorda tympani and glossopharyngeal nerve.     

Effect of salivation on neural taste responses in freely moving rats: analyses of salivary secretion and taste responses of the chorda tympani nerve

The outer surface of the mammalian taste receptor cell is usually covered with saliva, which may affect the initial process of gustation. To ascertain the interaction between salivation and gustation, salivary secretion from the submandibular and parotid glands and taste responses of the chorda tympani nerve were analyzed in the rat, during grooming, eating, and licking of the four standard taste stimuli (sucrose, NaCl, HCl, and quinine hydrochloride). Regions of the tongue surface bathed by saliva secreted from the each gland were examined, and it was found that: (1) Rats frequently groomed, and the anterior part of the tongue, innervated by the chorda tympani nerve, was usually covered with a mixture of submandibular saliva and substances on the body surface. (2) Licking of acceptable sucrose and NaCl solutions elicited initial phasic and long-lasting tonic taste responses, and did not evoked saliva enough to wash away the stimuli from the oral cavity. Licking of rejectable quinine evoked only a small phasic taste response and was followed by taste rejection behavior, accompanied by maximum salivation which could wash out the stimuli. (3) When taste responses were compared under awake and anesthetized (the tongue adapted to water) condition, sucrose response was larger, while responses to other taste stimuli were smaller under the awake condition. Rise time of the phasic NaCl response was longer under the awake condition. These taste response alterations may reflect the effects of prolonged adaptation of the tongue to the mixture of submandibular saliva and body surface substances, and flow rate of licked taste stimuli on the tongue surface.     

Specificity of amiloride inhibition of hamster taste responses

Amiloride, a blocker of epithelial sodium channels, was found to have significant effects on electrophysiological and behavioral taste responses in the golden hamster (Mesocricetus auratus). Recordings from the whole chorda tympani nerve showed that amiloride rapidly, reversibly, and competitively inhibited responses to NaCl applied to the anterior tongue. The apparent dissociation constant for amiloride binding, extrapolated to zero NaCl concentration, was 10 nM, a value comparable to estimates for various transporting tight epithelia. Recordings from single chorda tympani nerve fibers showed that 10 microM amiloride completely inhibited responses of Na-selective N fibers but had minimal effect on responses of electrolyte-sensitive H fibers, even though both types of fibers responded well to 0.1 M NaCl. Sucrose responses were not affected by amiloride. Addition of 100 microM amiloride to 0.1 M NaCl consistently increased consumption of NaCl in two-bottle drinking tests. These data suggest that one mechanism by which the taste of NaCl is sensed, which does not require adsorption or a second messenger, involves entry of Na+ into taste bud cells through an amiloride-blockable sodium channel. Taste bud cells utilizing this mechanism exclusively activate N fibers, which are involved in the control of NaCl intake. A different mechanism for the detection of NaCl and other electrolytes is utilized by taste bud cells that activate H fibers.     

The contribution of gustatory nerve input to oral motor behavior and intake-based preference. I. Effects of chorda tympani or glossopharyngeal nerve section in the rat

Two-bottle intake tests and taste reactivity (TR) tests were used to reveal whether changes in ingestive behavior would follow bilateral section of either the chorda tympani (CT) or the glossopharyngeal (GP) nerve. Rats received two-bottle intake tests to compare 24-h ingestion of water to that of NaCl, MgCl2, quinine, or sucrose. Prior to each long-term intake test, rats received a 1 min, 1 ml intraoral infusion of the same chemical stimulus. Ingestive and aversive oral motor responses elicited by these 1 ml infusions were videotaped and subsequently analyzed. GP-section did not alter quinine or sucrose preference; overall, preference of MgCl2 and NaCl was also similar to controls. In contrast, TR tests in GP-sectioned rats revealed that most quinine, MgCl2 and NaCl stimuli elicited significantly fewer aversive oral motor responses. In addition, the latency of aversive responses to these 3 chemical stimuli was increased for these rats. Intake-based preference tests failed to show any difference between rats with CT nerve section and controls. In TR tests, however, CT-sectioned rats displayed significantly fewer ingestive oral motor responses to NaCl, MgCl2, and quinine than controls. Neither sucrose intake nor sucrose-elicited TR were altered by CT or GP nerve section. This report confirms the failure of long-term intake tests to uncover behavioral deficits following the section of gustatory nerves. In contrast, the use of a different behavioral test makes clear for the first time that gustatory nerve section has dramatic consequences on ingestive behavior. The examination of taste elicited oral motor behaviors reveals a coherent and nerve specific pattern of neurological deficit following peripheral nerve section.     

Gustatory receptor cell responses to the sweeteners, monellin and thaumatin

Mouse gustatory responses to sucrose, monellin and thaumatin were examined extracellularly from the chorda tympani nerve and intracellularly from individual taste receptor cells. Although monellin and thaumatin taste intensely sweet to humans and old-world monkeys, they do not appear to elicit chorda tympani nerve responses in rats and other mammals. However, there is considerable species variation in the taste responses of mammals, including differences in taste responses of different strains of mice. In the present study with Slc:ICR mice, we show that chorda tympani and taste receptor cell response profiles, and behavioral results for monellin and thaumatin, are similar to response profiles for sucrose.     

Elucidating coding of taste qualities with the taste modifier miraculin in the common marmoset

To investigate the relationships between the activity in different types of taste fibers and the gustatory behavior in marmosets, we used the taste modifier miraculin, which in humans adds a sweet taste quality to sour stimuli. In behavioral experiments, we measured marmosets' consumption of acids before and after tongue application of miraculin. In electrophysiological experiments responses of single taste fibers in chorda tympani and glossopharyngeal nerves were recorded before and after tongue application of miraculin. We found that after miraculin marmosets consumed acids more readily. Taste nerve recordings showed that after miraculin taste fibers which usually respond only to sweeteners, S fibers, became responsive to acids. These results further support our hypothesis that the activity in S fibers is translated into a hedonically positive behavioral response.     

Effects of inorganic constituents of saliva on taste responses of the rat chorda tympani nerve.

The effects of saliva on the taste responses of the chorda tympani nerve to the 4 standard chemical stimuli (sucrose, NaCl, HCl, and quinine hydrochloride) and water were investigated in anesthetized rats. When the tongue was adapted to pilocarpine-stimulated whole saliva (pH 8.7), the magnitude of neural response to sucrose was about 2 times that obtained when the tongue was adapted to distilled water. Under saliva-adapted conditions, the magnitude of responses to other taste stimuli was reduced by 10-30%, and the water response appeared. These changes were dependent on the concentration of electrolytes (Na+, K+, Cl-, and HCO3-) and on the pH of the saliva. When the tongue was adapted to 10-30 mM NaHCO3 (pH 8.4-8.6), taste and water responses were similar to those under saliva-adapted conditions. Single fiber analyses revealed that the enhancement of the sucrose response after adaptation to NaHCO3 was produced by an increased overall activity of sucrose-responsive fibers. The correlation coefficients of the magnitude of the taste responses between the 4 taste stimuli remained unchanged, but the water response showed a high correlation to HCl and quinine hydrochloride responses after adaptation. Possible mechanisms for the effects of saliva on taste and water responses were discussed.     

The effect of taste stimuli on histamine release in the anterior hypothalamus of rats

We studied the effect of gustatory stimulation on hypothalamic histamine release. Administering a four-basic taste mixture significantly increased histamine release, but not in the chorda tympani-transected rats. 0.1 M NaCl significantly increased histamine release, whereas 0.5 M sucrose, 0.02 M quinine HCl and 0.01 M HCl had no effect. However, when the concentration of HCl was increased to 0.03 M, a significant increase in histamine release was seen. These results suggest that taste information via the chorda tympani activates the histaminergic system.     

Convergence in mammalian nucleus of solitary tract during development and functional differentiation of salt taste circuits

To determine the type and extent of neural rearrangements that are made during functional differentiation of circuits for salt taste processing, we determined receptive field size and salt response characteristics of second-order taste cells in 3 age groups of sheep. Neurophysiological recordings were made from single cells in the nucleus of the solitary tract (NST) in fetal, perinatal, and postnatal sheep. Responses to NH4Cl, NaCl, and KCl were measured, and location and number of fungiform papillae in the receptive field were determined by stimulating individual papillae with anodal electrical current. The data are compared with previous, parallel measures from chorda tympani nerve afferent taste fibers to permit conclusions about convergence or divergence onto second-order cells. Receptive field size of second-order taste neurons increases during development, in contrast to the decrease in field size observed previously for chorda tympani nerve fibers during the same period. Furthermore, receptive fields of second-order cells are significantly larger than those of first-order fibers at perinatal and lamb ages, but not fetal. Thus, there is convergence of first-order taste afferents onto brain-stem neurons, and the convergence increases remarkably between fetal and perinatal periods. Associated with the increase in convergence are increased salt response frequencies relative to afferent fibers for NaCl in perinatal animals and lambs, and for KCl in lambs. The increase in frequencies occurs before NST neurons are functionally mature, as indicated by the rapid response adaptation of many cells in young animals. Convergence in NST during development apparently functions to maximize gain for processing neural responses to NaCl. In the periphery, response frequencies to NaCl are very low in fetuses, and increase progressively during development. In the NST, NaCl response frequencies are high even in fetuses, and remain high. The process of convergence onto second-order cells is accomplished with maintenance of order in afferent projections because receptive fields of NST neurons are composed of fungiform papillae that are clustered together, not dispersed over the tongue. Our quantification of taste receptive field size at 2 neural levels provides strong evidence for increasing convergence in the NST during development. Altering patterns of afferent neural input and geometry of second-order neurons may have a role in establishing convergence. The convergence has an apparently special function: to increase gain for NaCl taste sensation. Therefore, neural rearrangements during differentiation of salt taste pathways result in specific functional outcomes.     

Response properties of fibers in the hamster superior laryngeal nerve

The purpose of the present investigation was to record electrophysiological responses from single fibers in the hamster superior laryngeal nerve (SLN) that were responsive to chemical stimulation of the larynx. Twenty chemical solutions, commonly used in studies of mammalian gustatory physiology, were applied to taste buds on and around the epiglottis. These stimuli were dissolved in physiological saline. Responses were the number of impulses elicited over a 15-s period following stimulus onset, above or below the background activity elicited by a previous rinse with saline. Unlike fibers in the hamster chorda tympani or glossopharyngeal nerves, SLN units were not easily classifiable into response types. Excitatory stimuli were primarily acids and bitter-tasting stimuli, with the order of their effectiveness being urea tartaric acid > HCl > KCl > citric acid > caffeine > quinine hydrochloride > acetic acid. The sweet-tasting stimuli and most salts other than KCl were primarily inhibitory, with the order of inhibitory effectiveness being CaCl₂ > sucrose > fructose > LiCl > NaNO₃ > Li₂SO₄ > NaCl. A hierarchical cluster analysis of fibers yielded no distinct clusters, yet differing sensitivities across the fibers were suggested. SLN fibers are highly responsive to sour and bitter stimuli, although they are not sensitive to fine differences in taste quality, as are fibers in other gustatory nerves.     

Gustatory neural response in the mouse

Gustatory responses from the mouse chorda tympani nerve were tested with various chemical solutions. Magnitudes of integrated chorda tympani responses to the 4 basic taste stimuli were greater in the order of HCl, sucrose, NaCl and quinine-HCl. Sucrose was the most effective sugar tested, while NaCl was the least effective salt, but divalent chloride salts were prominently effective stimulants. Many of single chorda tympani fibers responded specifically to one or two of the 4 basic taste stimuli. Single fibers sampled were classified into the following 5 types: sweet-type, Na-type, CaMg-type, acid-type and quinine-type.     

Taste effects of 'umami' substances in hamsters as studied by electrophysiological and conditioned taste aversion techniques

Behavioral and electrophysiological experiments were performed to examine whether or not the taste of 'umami' substances such as monosodium glutamate (MSG), disodium 5'-inosinate (IMP), and disodium 5'-guanilate (GMP) is really unique in hamsters. When the animals were conditioned to avoid ingestion of MSG (or IMP) or their mixture by pairing its ingestion with an i.p. injection of LiCl, suppression of drinking generalized to IMP (or MSG), GMP, NaCl, and other sodium salts. Suppression of drinking after conditioning to NaCl generalized to MSG, IMP, GMP, and inorganic sodium salts. These learned aversions to umami substances and sodium salts were abolished by bilateral deafferentation of the chorda tympani, but were not affected by destruction of the bilateral glossopharyngeal nerves. The integrated whole-nerve responses of the chorda tympani to MSG, IMP, and NaCl were similar to each other, consisting of the initial dynamic phase and the following tonic phase. Synergism of chorda tympani responses to a mixture of MSG and IMP was not observed. Across-fiber response patterns of the chorda tympani for MSG, IMP, or their mixture were very similar to that for NaCl. Even the high concentrations of umami substances (0.3 M MSG, 0.3 M IMP, and the mixture) did not elicit any detectable responses in the glossopharyngeal nerve. These results suggest that the taste of umami substances is not unique in the hamster, but is similar to that of sodium salts, and is mediated exclusively via the chorda tympani.     

Cortical responses to electrical and gustatory stimuli in the rabbit.

The distribution of surface positive cortical potentials evoked by electrical stimulation of the chorda tympani, glossopharyngeal and lingual nerves which innervate the tongue was mapped in rabbits. All projections were bilateral. Judging from the extent of the cortical response area and the amplitude and latency of the responses, the major projection of the chorda tympani was ipsilateral, whereas that of the lingual and the glossopharyngeal nerves was contralateral. Both the chorda tympani and the glossopharyngeal nerve project to a confined area in the insular cortex and the lingual nerve projects to the appropriate part of the somatotopic pattern of somatic sensory area I. Further, a single unit study was undertaken to characterize the response of units in the cerebral cortex which was induced by gustatory stimulation of the anterior tongue, Twenty-four gustatory units were found in the insular cortex and the claustrum. The gustatory units were divided into an early response type (21 units) and a late response type (3 units) based on latency measurements. Gustatory units were also classified according to discharge patterns into excitation type (21 units) and inhibition type (4 units). Eleven units responded to 1 or 2 kinds of conventional taste stimuli, and 13 units responded to more than 3 different taste stimuli. Sensitivities of cortical units to the 4 conventional taste stimuli were found to be mutually independent and randomly distributed among cortical units. The frequency of discharges increased in the excitation type units and decreased in the inhibition type units monotonically with the excitation type units and decreased in the inhibition type units monotonically with an increse of NaCl concentration exfept at the highest concentrations.     

A novel method to analyse response patterns of taste neurons by artificial neural networks.

Despite several notions on the gustatory code proposed over three decades, investigators have not yet reached a consensus. This paper describes a new approach to analyse gustatory neural activities. Three-layer neural networks were trained by the back-propagation learning algorithm, to classify the neural response patterns to four basic taste qualities. The discrimination by the trained networks on taste qualities in the response patterns of rat chorda tympani fibres (CT) and cortical taste neurons (CN) was consistent both with the correlation analysis and with behavioural experiments. By examining the connection weights of each neuron, some input neurons representing CN were 'pruned' without deteriorating the ability of the network to discriminate taste. This characteristic of the network is contrary to a previous hypothesis, that taste neurons are of equal importance in the neural coding.     

Amiloride inhibition of responses of rat single chorda tympani fibers to chemical and electrical tongue stimulations

Amiloride inhibition of single fiber responses of the rat chorda tympani to ionic chemical and electrical tongue stimulations was studied. Amiloride reduced responses to both chemical and electrical stimulations with NaCl or LiCl in most of the single fibers. However, the magnitude of reduction of the response by amiloride varied among the fibers and was greater for chemical than electrical stimulation with NaCl in each fiber. Thirty-two single fibers were divided into two groups, such as 18 high (HAS) and 14 low amiloride-sensitive (LAS) fibers. Percent responses (control, 100%) of the former group to chemical stimulus with NaCl after amiloride ranged from 1.1 to 42.5%, while those of the latter from 72.8 to 108.0%. In HAS fibers, amiloride also reduced responses to KCl and CaCl2, but to a smaller degree than those to NaCl and LiCl. Fifteen out of 18 HAS fibers more strongly responded to a chemical stimulus with 0.1 M NaCl than 0.01 M HCl, while the opposite was true for 13 out of 14 LAS fibers, although the threshold concentration for NaCl was rather lower in LAS fibers than in HAS fibers. These results suggest that there exist at least two different receptor mechanisms for NaCl or LiCl which are amiloride-sensitive and -insensitive, and the observed differences in relative specificities to ionic taste stimuli and sensitivities to amiloride among rat chorda tympani fibers are possibly due to a disproportional distribution of these two receptors.     

The anion in salt taste: a possible role for paracellular pathways

It is well established from psychophysical and electrophysiological measurements that both Na and Cl contribute to the taste response to NaCl. The contribution of Na to the NaCl response can be studied using amiloride, a drug that inhibits Na transport in taste and other epithelial cells. The pathways involved in response to Cl are less well understood. We undertook a series of experiments in the rat to determine whether tonic chorda tympani responses to NaCl are inhibited by specific inhibitors of anion transport. Whole nerve responses to NaCl were unchanged by bathing the tongue in SITS, DIDS, bumetanide, furosemide, 9-anthracene carboxylic acid, or an antibody that blocks Cl conductance pathways in many epithelia. Thus, Cl co-transporters, exchangers, and channels (at least in the apical membrane of taste cells) are probably not involved in NaCl taste responses. When other anions (acetate, isethionate, methane sulfonate, gluconate, tartrate), which are generally impermeant in other Cl-selective pathways, were substituted for Cl, the dose-response curves for the chorda tympani response were shifted toward higher concentrations than the response to NaCl, but achieved the same maximum value at sufficiently high concentrations (1.0 M Na). For all the organic Na salts, the amiloride-insensitive portion of the response was substantially less than for NaCl. Experiments with Na acetate at different pHs showed that intracellular acidification is not responsible for the differences between NaCl and organic salts of Na. One possibility which remains is that apical stimulation with these other Na salts results in a taste cell membrane potential that is hyperpolarized with respect to the membrane potential in NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)     

Branched chorda tympani neurons and interactions among taste receptors

The preterminal branching pattern of nerve fibers from the chorda tympani nerve was examined in the tongues of rats using a normal silver stain. The lingual branch of the trigeminal nerve was sectioned unilaterally proximal to where it is joined by the chorda tympani and these fibers were allowed to degenerate from eight to ten days prior to sacrifice of the animals. Termination of the chorda tympani fibers in the anterior tongue was found to be limited to the taste bud region of the fungiform papillae. The control side of the tongue showed ubiquitous fiber terminations in the basal layers of the common epithelium of the filiform papillae as well as in the lateral walls of the fungiform papillae. These fibers were assumed to be of trigeminal origin. Seventy-nine percent of the fungiform papillae on the experimental side of the tongue received fibers from a branched nerve bundle. Branch points within the nerve bundles were located deep within the tongue 300–500 μ below the taste bud in 43% of the papillae, at the base of 36% of the fungiform papillae about 125 μ below the taste bud, and within the papilla. These data corroborate electrophysiological observations that single chorda tympani fibers receive input from more than one taste bud. Lateral inhibition observed among adjacent taste buds has been postulated to result from interaction of fiber inputs at branch points in the afferent fiber. From a theoretical consideration of the morphology of chorda tympani nerve fibers it is concluded that modification of the neural response may be feasible at branch points.