Key Concepts of Section 22.4

Sensing the Environment: Touch, Pain, Taste, and Smell

• Mechanoreceptors and pain receptors are gated Na⁺ or Na⁺/Ca²⁺ channels. They are present on specialized terminals at the end of sensory neurons, and convey afferent information to the spinal cord and brain stem.

• Touch sensitivity requires several cytoskeletal and extracellular matrix proteins as well as a gated Na⁺ channel (Figure 22-32).

• Piezo 1 and 2 are large channel proteins that convert mechanical stimulus directly into cation conductance (Figure 22-34).

• TRPV channels are nociceptors that are activated by a variety of stimuli, including heat and capsaicin. Their molecular structure is similar to the structure of voltage-gated ion channels, and was recently solved by single particle cryoEM (Figure 22-33).

• Five primary tastes are sensed by subsets of cells in each taste bud. Salty and sour tastes are detected by specific ion channel proteins, and G protein–coupled receptors detect sweetness, umami, and bitterness.

• In all cases, tastants lead to membrane depolarization and secretion of small molecules such as ATP that stimulate the adjacent neurons. Some tastant GPCRs are found in different homo- and heterodimeric combinations to detect different tastes (see Figure 22-35).

• Taste is represented in a topographic map in a part of the cortex called the insula. Activation of taste receptors by a specific type of taste (e.g., sweet or salty) activates neurons in specific, nonoverlapping regions of the insula.

• Odorant receptors, which are seven-transmembrane G protein–coupled receptors, are encoded by a very large set of genes. Any one olfactory receptor neuron expresses one and only one olfactory receptor gene, so a signal from that cell to the brain unambiguously conveys the nature of the chemical sensed.

• ORNs that express the same receptor gene send their axons to the same glomerulus, and projection nerves (mitral neurons in mammals) carry odorant-specific information from the glomeruli to the brain (see Figures 22-37, 22-38, and 22-39).