A comparison of the discriminatory ability and sensitivity of the trigeminal and olfactory systems to chemical stimuli in the tiger salamander

Abstract

Trigeminal receptors can respond to a wide variety of chemical stimuli, but it is unknown whether these receptors mediate discrimination between chemical stimuli matched for equal perceptual intensity. The present electrophysiological and behavioral experiments address this issue using tiger salamanders, Ambystoma tigrinum, and four compounds (amyl acetate, cyclohexanone, butanol, and d-limonene). In addition, the relative sensitivities of the trigeminal and olfactory systems to these compounds are compared. In electrophysiological cross-adaptation experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), there was complete cross adaptation such that only concentrations above the background (cross-adapting) stimulus concentration elicited responses, suggesting that chemical stimuli may stimulate trigeminal receptors nonspecifically. In behavioral experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), only animals with intact olfactory nerves could discriminate between perceptually equivalent concentrations, that is concentrations that elicited the same level of responding. Both electrophysiologically and behaviorally, the trigeminal system exhibited higher thresholds than the olfactory system. We conclude that trigeminal chemoreceptors, at least in salamanders, are unable to discriminate between these two pairs of compounds when matched for equal perceptual intensity, and that trigeminal chemoreceptors are less sensitive than olfactory receptors.

Qualitative Effects in Nasal Trigeminal Chemoreception

“…In contrast to the human psychophysical data reviewed above, animal behavioral data bearing on this question give scant support. Specifically, Silver et al 33 conditioned salamanders to avoid either amyl acetate or d ‐limonene but to tolerate cyclohexanone and butanol. After olfactory nerve sectioning, the animals could no longer discriminate between the paired odorants presented at concentrations that produced an equal (ensemble) electrophysiological response in the trigeminal nerve.…”

Ultrastructural studies on membrane, cytoskeletal, mucous, and protective compartments in olfaction

“…Many mammals possess two types of ciliated olfactory epithelia; in addition to the main olfactory organ, there is the ciliated septal olfactory organ which may have an alerting function (Marshall and Maruniak, 1986;Miragall et al, 1984; see also Adams, upcoming issue). Intranasal parts of the trigeminal nerve also serve a chemosensory function in vertebrates (Silver et al, 1988;Zielinski et al, 1989) as may be true for intranasal parts of the terminal nerve as well (Graziadei, 1977;Zheng and Jourdan, 1988) (see Getchell and Getchell, upcoming issue). Finally, among the less frequently occurring cell types, there are several kinds of microvillous cells in the main olfactory epithelium.…”