Working with genetically engineered mice – and especially their whiskers – Johns Hopkins researchers report they have identified a group of nerve cells in the skin responsible for what they call “active touch,” a combination of motion and sensory feeling needed to navigate the external world. The discovery of this basic sensory mechanism, described online April 20 in the journal Neuron, advances the search for better “smart” prosthetics for people, ones that provide more natural sensory feedback to the brain during use.

Study leader Daniel O’Connor, PhD, assistant professor of neuroscience at the Johns Hopkins University School of Medicine, explains that over the past several decades, researchers have amassed a wealth of knowledge about the sense of touch.

While some research has suggested that the same populations of nerve cells, or neurons, might be responsible for sensing both proprioception and touch necessary for this sensory–motor integration, whether this was true and which neurons accomplish this feat have been largely unknown, O’Connor says.

To find out more, O’Connor and his team developed an experimental system with mice that allowed them to record electrical signals from specific neurons located in the skin, during both touch and motion.

The researchers accomplished this by working with members of a laboratory led by David Ginty, PhD, a former Johns Hopkins University faculty member, now 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 one long documented in previous research — but also 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.



Using a high–speed camera focused on the animals’ whiskers, the researchers took nearly 55,000,000 frames of video while the mice ran and whisked. They then used computer–learning algorithms to separate the movements into three different 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.

They then connected each of these movements – using video snapshots captured 500 times every second – to the electrical signals coming from the animals’ blue–light–sensitive Merkel afferents.

The results show that the Merkel afferents produced action potentials when their associated whiskers contacted the pole. That finding wasn’t particularly surprising, O’Connor says, because of these neurons’ well–established role in touch.

However, he says, the Merkel afferents also responded robustly when they were moving in the air without touching the pole. By delving into the specific electrical signals, the researchers discovered that 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 says.