Figure 1.

Comparison of sensory signaling systems for vision, olfaction, hearing and balance, taste, and pain/thermosensation. The events underlying signal transduction are shown schematically for (A) rod and cone photoreception; (B) olfaction in the main olfactory epithelium; (C) salt (left) and sweet (right) taste; (D) hearing and balance; and (E) pain/thermosensation. Schematics A–E refer to vertebrates. The final step in olfactory signaling consists of the calcium-dependent opening of anion channel TMEM16B, although recent work suggests that the resulting anion current plays only a minor role in olfactory signal transduction (Billig et al. 2011). For pain/thermosensation mediated by TRPV1, the figure shows inflammatory agents (extracellular protons, bioactive lipids, peptides, and neurotrophins) acting to enhance channel opening either as direct allosteric modulators of TRPV1 or through second-messenger signaling pathways. In auditory and vestibular hair cell bundles, it is not known whether transduction channels reside at both ends or at only one end of the extracellular elastic elements (tip links); in panel D the channels are shown at both ends.