While the chemical and mechanical details of vision, hearing, taste, and smell are fairly well understood—most people are passingly familiar with rods and cones, cochlear hair cells, and taste bud receptors—the details of touch have eluded our grasp. That may soon change, thanks in part to a pair of studies presented April 6 in Nature. According to these studies, a kind of dual-sensor system is responsible for touch. While the ends of nerve cells respond to cells, so do Merkel cells, which are found adjacent to nerve cells. Also, both nerve cells and Merkel cells seem to depend on a particular kind of protein, called Piezo2, to convey signals that are interpreted as gentle sensations.
In an article entitled “Epidermal Merkel cells are mechanosensory cells that tune mammalian touch receptors,” a research team led by Ellen Lumpkin, Ph.D., associate professor of somatosensory biology at Columbia University Medical Center, showed that Merkel cells actively participate in touch reception in mice. In particularly, the researchers showed that Merkel cells display fast, touch-evoked mechanotransduction currents.
The Columbia team used optogenetics, a technique that uses light to control neurons that have been genetically sensitized to light, to demonstrate that in intact skin, “Merkel cells are both necessary and sufficient for sustained action-potential firing in tactile afferents.” In addition, the team used mice engineered to lack Merkel cells to reveal touch-sensing deficits.
“These findings,” wrote the authors, “indicate that Merkel cells actively tune mechanosensory responses to facilitate high spatio-temporal acuity.” The authors also described a division of labor in the Merkel cell–neurite complex: “Merkel cells signal static stimuli, such as pressure, whereas sensory afferents transduce dynamic stimuli, such as moving gratings.”
The companion study led by the Scripps team was described in an article entitled “Piezo2 is required for Merkel-cell mechanotransduction.” These researchers engineered mice deficient in Piezo2 in the skin, but not in sensory neurons, and showed that Merkel-cell mechanosensitivity completely depends on Piezo2.
The discovery comes four years after the laboratory of Aredem Patapoutian, Ph.D., identified Piezo2 as a mechanically activated "ion channel" protein with a likely role in touch sensation. Piezo2 ion channels have been thought to respond to the stretching of the nerve membrane where they are embedded—a stretching caused by something that presses against the skin, for example. When activated in this way, the ion channels open to allow an inflow of sodium or other positively charged ions. Such a surge of electrical charge into a nerve can initiate a signal that travels up the nerve and to the brain via a relay of neurons along the spine.
In the earlier study, Dr. Patapoutian's team found evidence that Piezo2 proteins are made within touch-sensing neurons, including gentle-touch neurons that extend their nerves into the skin and against the mysterious Merkel cells. The team's current study, besides uncovering additional details about the mammalian sense of touch, could have relevance for certain pain syndromes in which touch sensations trigger pain.
"Touch and pain are very closely related," said Dr. Patapoutian, "and thus the characterization of these mechanisms of touch should help us to understand pain better, too."