Maxwell M. Mozell

Chromatographic Separation of Odorants by the Nose: Retention Times Measured across in vivo Olfactory Mucosa

The column of a standard gas chromatograph was replaced with in vivo frog olfactory sac. The wide range of relative retention times as measured across the olfactory mucosa for 15 different odorants supports the concept of a chromatographic separation along the mucosa as a mechanism for distinguishing different odorants.

OLFACTORY DISCRIMINATION: ELECTROPHYSIOLOGICAL SPATIOTEMPORAL BASIS.

Simultaneous recordings were taken from two widely separated branches of the olfactory nerve of the frog. Odors from a variety of chemical substances were blown into the nares, and each elicited a different response magnitude ratio between the two branches. In addition, the time lapse between the two nerve responses differed for different chemicals. This suggests a time-space encoding of the mucosal response to odors.

MECHANISMS UNDERLYING THE ANALYSIS OF ORDORANT QUALITY AT THE LEVEL OF THE OLFACTORY MUCOSA I. SPATIOTEMPORAL SORPTION PATTERNS

In the early 1950s Adrian’sl-* electrophysiological recordings from the olfactory bulb of rabbits and cats led him to propose two possible mechanisms that might underlie olfactory discrimination at the level of the olfactory mucosa: 1) the receptors themselves may be selectively sensitive to different odorants; 2) the molecules of different odorants may spread differentially across the mucosa both in time and in space. In the first mechanism, each receptor signals the degree to which the incoming odorant molecules have matched its particular sensitivity. In the second mechanism, on the other hand, the receptors need not have any selective sensitivity of their own, but instead signal the degree to which their positions along the epithelium have been reached by the incoming molecules. It should be emphasized that these two mechanisms need not be mutually exclusive, and, as Adrian at one point suggested,s they could very well act in concert. Such a combined code would appear much more capable of representing the vast number of discriminable odorants than would either one alone. Of these two possible mechanisms, by far the greatest attention has been paid by most investigators to the selective sensitivity of the receptors themselves, and the following paper (Blank, this annal) will have more to say about this mechanism. This paper will instead pursue the possibility of a discrimination mechanism based upon the different space-time patterns by which the molecules of different chemicals spread across the mucosa. As noted above, this latter mechanism was originally based, not upon studies of the olfactory epithelium, but rather upon multiunit recordings taken from the olfactory bulb. Adrianl, 2 observed that at threshold each odorant elicited a response in a particular region of the bulb and in none other. I n addition to this spatial differentiation there was a temporal one: the discharges elicited by some odorants had shorter durations and were more abrupt in their growth and decay than were the discharges elicited by other odorants. That this apparent spatiotemporal encoding of odorants in the bulb also operated at suprathreshold stimulus intensities was shown by h,lozell and Pfaffmann5 and by Mozell.6 With the higher intensities, however, the spatial differentiation was observed to be relative rather than absolute. That is, although at the higher intensities a given chemical might elicit responses throughout the entire bulb rather than at one particular region as with threshold intensities, there was still one region at which it was most effective,

Distribution of butanol molecules along bullfrog olfactory mucosa.

ALTHOUGH electrophysiological1 and gas chromatographic2 studies have indicated that odorant molecules drawn into the intact frog olfactory sac establish a concentration gradient along the olfactory mucosa, conclusive evidence for such gradients requires a direct determination of how the molecules are actually distributed. This requirement is met here by labelling butanol molecules with tritium and directly mapping their sorption along the mucosa.

Initial mechanisms basic to olfactory perception

Animal experimentation has proposed three mechanisms at the olfactory mucosa that may underlie olfactory discrimination. First, the olfactory receptor cells appear selectively tuned to different odorants. Second, in a gas chromatographic-like process, the molecules of different odorants appear to be distributed in different sorption patterns across the mucosal surface. Third, different regions of the mucosa appear to have different selective sensitivities. These three mechanisms could complement each other by together generating a greater number of neural discharge patterns to encode the odorants passing over the mucosal surface. In this interplay, the mucosal distribution patterns could differentially limit the receptor cells and mucosal regions to which different odorants have access. The mucosal distribution pattern could thereby affect the odorant analyses made by these other mechanisms as well as contribute its own analysis. The mucosal distribution patterns appear fairly stable in the face of rather wide variations in the pertinent variables characterizing the nasal airflow (namely, odorant concentration, flow rate, volume, and duration). There are, however, limits to these variables beyond which significant shifts in the molecular distributions and neural discharge patterns can be produced. Thus, in humans any naturally occurring or surgically induced alteration in the nasal airflow which appreciably alters these variables may affect olfactory perception. Olfaction in a laryngectomized patient is discussed as an example.

Olfactory receptor response characteristics: A factor analysis

The responses to odor stimulation of 22 single olfactory units in the frog olfactory mucosa were recorded with metal filled micropipettes. Seven purified odorants matched in concentration and a pure air stimulus were administered. The change in firing frequency for each unit to each chemical was evaluated to determine: (1) whether the response patterns developed across all units are different from chemical to chemical and, (2) whether individual units can be grouped in terms of their similarity of their responses to all odorants. The results of a Bartlett Sphericity test suggest that each odorant produces an independent pattern of responses across units. To answer the second question, a factor analysis was employed. It examined the responses of each unit to all chemicals and yielded 7 out of a possible 22 independent factors, suggesting that there are 7 ways by which the units studied look at the odorant stimuli and an air control employed in this study. These analyses were evaluated and discussed in terms of previously published data suggesting receptor site specificity.

Processing of Olfactory Stimuli at Peripheral Levels

Based upon his electrophysiological work during the early 1950’s, Adrian (1950, 1951, 1953, 1954) has left a legacy concerning the mechanisms which might underlie olfactory discrimination at the level of the olfactory mucosa. One such mechanism involves the selective sensitivity of individual receptor cells to particular odorants or groups of odorants. Any given cell would not be equally sensitive to all odorants, but would instead be maximally excited by some odorants and excited less, or not at all, by others. Thus, each receptor cell would signal the degree to which its particular sensitivity is matched by the molecules of incoming odorants.

A technique to occlude the nasal chemoreceptors during lingual flavor stimulation

Abstract A method was devised to prevent stimulation of the nasal chemoreceptors when flavors are presented to the tongue. This was accomplished by bucking the otherwise normal movement of odorous molecules from mouth to nose through the nasopharynx with a stream of air flowing in the opposite direction.

The mixture symposium. Summary and perspectives.

I have never actively entered the field of trying to decipher the effects of chemical mixtures on chemoreceptive responses, although some years ago I became interested in a related topic, namely, looking at the relative contributions of the nose and tongue to the identification of flavors.’ However, a summary by someone not immediately involved in the area perhaps could give a more global view. Even if I do not come up with any ingenious insights, maybe I can at least be an irritative focus for stimulating discussion. Each of the contributors has demonstrated how, from his particular approach to the field, the response to a mixture of chemicals differs from the responses to the chemicals given alone; but in describing these approaches each contributor has drawn attention to the formidable task we face in trying to unravel how stimulus mixtures are processed by the chemical senses. As Price chidingly noted, even before we address how single stimuli and mixtures of stimuli are differentially processed, we must first give some thought to whether we have ever recorded electrophysiological discharges or observed behavioral responses to anything but mixtures. This is especially true in olfaction. Recall that even a slight contaminant can become major if its partitioning between the air phase and the mucosa phase markedly favors the mucosa phase. Even if absolutely pure on entering the nose, a single stimulant could possibly become a mixture of stimulants if it is metabolized. It may be, therefore, that we cannot compare mixture responses to single-stimulus responses because we have not yet seen any of the latter. Frijter’s discussion did not make me feel any easier. He emphasized the problems involved in trying to derive a psychophysical model for taste-taste and smell-smell mixtures. He pointed out that even the most successful models are not universally applicable, each having some significant limitation. This was further emphasized by Hornung and Enns who pointed out that the specifics of the model might even depend upon the semantics of the judgments required. That is, they proposed that when subjects are instructed to judge the “intensities, ” they give different estimates than when instructed to judge the “overaN intensities. ” This is all somewhat disappointing, but with all due respect to those working diligently in this field, the big disappointment for a nonparticipant like myself is that all the models for predicting the effect of