Relative excitability of antennal olfactory receptors in the stick insect,Carausius morosus L.: in search of a simple, concentration-independent odor-coding parameter

Abstract

1. Recordings of the electrical activity of single sensory hairs in a field of sensilla on the twelfth antennal segment of the stick insect,Carausius morosus L., have revealed three types of olfactory cells. Two of these (A andB) responded to the same six artificial mixtures of chemically pure substances and to sliced lemon, but only one of them (B) reacted to leaf odors of food plants. Neither responded to carboxylic acids, whereas the third did (C). 2. The distinction between Type A and Type B cells is confirmed by their differential responses to 78 single odor substances vaporized out of 10−2 mol/l mineral oil solutions (Table 1). Type B cells responded to most of these more strongly than Type A did, but with some substances the reverse was true: Type A cells responded more strongly than Type B. 3. This reversal of relative excitability of the two cell types for some substances provided a criterion by which to distinguish two groups of odor substances; this distinction was valid at all concentrations between 10−4 and 10−2 mol/l for the six substances tested throughout the range. Type A cells responded more strongly than Type B to one of them (citronellol) and less strongly than five others (pentanol, hexanol, heptanol, octanol, linalool). None of these five, however, could be distinguished either on the basis of the excitation level of a single cell (Figs. 5, 6) or on the basis of the difference in the levels of the two types, because of the effects of concentration. The ratio, on the other hand, of Type B to Type A cell responses permits the discrimination of hexanol from the other four when the ratio is formed from the mean responses of all cells of each type, though only at concentrations from 10−4 to slightly above 10−3 mol/l. But when single cells of the two types are paired selectively to form the ratio, all five substances are discriminable from 10−4 to 10−2 mol/l. On the average, 59% of the cell pairs on which any two of the five substances were tested were capable of discriminating them (Table 2) by the ratio of their responses. These ratios could serve as a concentration-independent, substance-specific coding parameter. 4. Levels of excitation ranged from spontaneous frequency (1–3 imp/s) to a maximum of 80 imp/s in Type A and 230 imp/s in Type B cells. The excitability of either type was affected by the interstimulus interval. When a single substance was presented at 1-min intervals, the second response of a successive pair was on the average 9 % lower than the first. With 4-min intervals recovery of excitability was much more complete (Fig. 2).