Touch is normally active, resulting from self-motion. Merkel afferents (top right, green and magenta) “fired” action potentials (white) during both movement and touch as mice ran on a treadmill and moved their whiskers to explore. Merkel afferents were identified by genetically altering them to fire when exposed to blue light (bottom left, bolt), not just to movement and touch. Image courtesy of O’Connor and JHU School of Medicine.
Humans and animals normally navigate their world through active touch-that is, initiating body movements to feel for the location of an object or to assess its tactile characteristics. Humans and animals also require proprioception, which is the ability to sense a body part’s relative position in space. Research has suggested that the same populations of neurons might be responsible for both proprioception and touch, but whether this was true and which neurons accomplish this feat have been largely unknown until recently: Johns Hopkins University (JHU) researchers report they have identified a group of nerve cells in the skin responsible for active touch. The discovery of this basic sensory mechanism, described online April 20 in the journal Neuron, may inform the advancement of smart prostheses that provide the user with more natural sensory feedback to the brain during use.
“You can open up textbooks and read all about the different types of sensors or receptor cells in the skin,” said study leader Daniel O’Connor, PhD, assistant professor of neuroscience at the JHU School of Medicine. “However, almost everything we know is from experiments where tactile stimulation was applied to the stationary skin-in other words, passive touch.”
O’Connor and his team developed a system with mice that allowed them to record electrical signals from specific neurons located in the skin, during touch and motion. They accomplished this by working with members of a laboratory led by David Ginty, PhD, a former JHU faculty member, now a professor of neurobiology at Harvard Medical School, to develop genetically altered mice. In these animals, a type of sensory neuron in the skin called Merkel afferents were mutated so that they responded to touch-their native stimulus-and to blue light, which skin nerve cells don’t normally respond to.
The scientists trained the rodents to run on a mouse-sized treadmill that had a small pole attached to the front that was motorized to move to different locations. Before the mice started running, the researchers used their touch-and-light sensitized system to find a single Merkel afferent near each animal’s whiskers and used an electrode to measure the electrical signals from this neuron. As the animals began running on the treadmill, they moved their whiskers back and forth in a motion that researchers call exploratory whisking.
Using a high-speed camera focused on the animals’ whiskers, the researchers took video while the mice ran and whisked, and then used algorithms to separate the movements into three categories: when the rodents weren’t whisking or in contact with the pole, when they were whisking with no contact, or when they were whisking against the pole. The researchers connected each of these movements to the electrical signals coming from the animals’ blue light-sensitive Merkel afferents. The results show that the Merkel afferents produced electrical spikes that neurons use to communicate with each other and the brain when their associated whiskers contacted the pole.
That finding wasn’t particularly surprising, O’Connor said, because of these neurons’ well-established role in touch. However, he said, the Merkel afferents also responded robustly when they were moving in the air without touching the pole, and the action potentials precisely related to a whisker’s position in space. These findings suggest that Merkel afferents play a dual role in touch and proprioception, and in the sensory-motor integration necessary for active touch, O’Connor said.
Although these findings are particular to mouse whiskers, he cautioned, he and his colleagues believe that Merkel afferents in humans could serve a similar function.
Besides shedding light on a basic biological question, O’Connor said, his team’s research could also eventually improve prosthetic limbs and digits. By integrating signals similar to those produced by Merkel afferents, he explained, researchers might eventually be able to create prostheses that can send signals about touch and proprioception to the brain, allowing movements akin to native limbs.
Editor’s note: This story was adapted from materials provided by JHU School of Medicine.