Piezo2, a channel protein, is currently believed to be responsible for the transduction of cutaneous mechanical rapidly adapting responses. This is because it forms a pore that allows the passage of nonselective cations in response to mechanical stimuli. The channel's activation and inactivation kinetics align with the generation of the rapidly adapting mechanoactivated current, and it can be blocked by certain agents like gadolinium and ruthenium red. Additionally, the introduction of small interfering RNA for Piezo2 inhibits the rapid mechanoactivated current in ganglion cells. Messenger RNA expression of Piezo2 has been detected in dorsal root ganglion cells.

Both low-threshold (light touch) and nociceptive-type ganglion cells express Piezo2, indicating its involvement in both innocuous and painful touch sensations. Piezo proteins (Piezo1 and Piezo2) are found in various organs and may also be involved in visceral sensation. Piezo2 has been identified in main proprioceptors such as muscle spindles and Golgi tendon organs, where it appears to function as the primary mechanotransducer protein.

The receptor that binds capsaicin, responsible for the spiciness of chili peppers, has been identified and is expressed in certain populations of dorsal root ganglion cells. These receptors belong to the transient receptor potential (TRP) protein family and are currently considered the most likely candidates for transducing thermal sensations.

While many ion channels and proteins are sensitive to temperature, TRP channels rely on temperature as the direct gating mechanism. Fig. 7.8 (bottom) indicates the temperatures at which specific TRP channels are activated, with arrows representing the direction of increased activation. The firing rates of several thermosensitive fibers are also plotted in Fig. 7.8 as a function of temperature. The response ranges of the afferents closely correspond to those of individual heat-sensitive channels. However, the firing range of cold fibers is wider than that of any single TRP channel. This suggests that dorsal root ganglion cells may express multiple classes of TRP channels, allowing them to respond to a broader range of physiological temperatures.
Currently, Piezo2 is thought to be channel protein underlying the transduction for cutaneous mechanical rapidly adapting responses, because it forms a nonselective cation pore that opens in response to mechanical stimuli. Moreover, the activation and inactivation kinetics of this channel are consistent with its causing the rapidly adapting mechanoactivated current, and it is blocked by agents (gadolinium, ruthenium red) that block these rapidly adapting currents. Furthermore, transfection of ganglion cells with small interfering (si)RNA for Piezo2 blocks the rapid mechanoactivated current in these cells. Lastly, messenger (m)RNA expression of Piezo2 has been shown in dorsal root ganglion cells.
Both low-threshold (light touch) and nociceptive-type ganglion cells have been found to express Piezo2, indicating that it plays a role in both innocuous and painful touch sensation. The Piezo proteins (Piezo1 and Piezo2) are both found in a variety of organs and thus may underlie visceral sensation as well. Piezo2 has been found in the main proprioceptors (muscle spindles and Golgi tendon organs [see ]) and appears to be the main mechanotransducer protein there as well.
The receptor that binds capsaicin (the molecule in chili peppers responsible for their spiciness) has been identified, and either it or one of a family of related proteins has been found to be expressed in populations of dorsal root ganglion cells. These proteins belong to the TRP (transient receptor potential) protein family and are currently the most likely candidates for being the transducers of thermal sensations.
It is important to note that many ion channels (and other proteins, e.g., enzymes) are sensitive to temperature; however, in the case of TRP channels, temperature is acting directly as the gating mechanism. The temperatures at which specific TRP channels are active are indicated by arrows in
Fig. 7.8 (bottom), in which the direction of each •Fig. 7.8 Temperaturedependenceoffiringratesindifferentthermo-sensitiveafferents.BelowthefiringratecurvesareshowntherangesoverwhichthedifferentTRPchannelsareactivated.Thedirectionofincreasingactivationisindicatedbyanarrowineachcase.Notehowinsomecasestherangeoverwhichanafferentisactive(topgraphs)correspondswelltotheactivationrangeofasingleTRPchannel,thussuggestingthattheafferentwouldneedtoexpressonlyasingletypeofchannel.InothercasestheactiverangeoftheafferentsuggeststhatmultipleTRPchannelswouldbeneededtounderliethecompleteresponsivenessoftheafferent.
arrow indicates which temperatures cause greater activation. For comparison,
Fig. 7.8 also plots the firing rates of several thermosensitive fibers as a function of temperature. Note how the response ranges of the afferents largely match up with those of individual heat-sensitive channels. The cold fibers, however, show firing over a wider range than any one TRP channel does. One possible explanation for this discrepancy is that dorsal root ganglion cells may express multiple classes of TRP, which would enable them to respond over a wider range of physiological temperatures.