Mind

Humans have special nerves that respond to gentle stroking.

By Seriously ScienceFeb 22, 2016 6:00 AM
Image: Flickr/Johan Larsson
Image: Flickr/Johan Larsson

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Sometimes there’s nothing more comforting than loved one’s caress. And, in fact, our bodies are built to respond to touch: we have special nerves in our skin (C-tactile afferents, or “CT”) that fire when we are gently stroked. In this study, the authors wanted to test how specific these CT nerves are. To do so, they measured how the nerves responded to strokes from robot hands at different temperatures and speeds. It turns out that CTs respond best to slow caresses, and, just like Goldilocks and the Three Bears, these nerves like it warm—if the hand is too hot or too cold, they don’t fire. The researchers conclude that the specificity of these nerves “provides a peripheral mechanism for signaling pleasant skin-to-skin contact in humans, which promotes interpersonal touch and affiliative behavior.” How touching!

Human C-tactile afferents are tuned to the temperature of a skin-stroking caress.

"Human C-tactile (CT) afferents respond vigorously to gentle skin stroking and have gained attention for their importance in social touch. Pharmacogenetic activation of the mouse CT equivalent has positively reinforcing, anxiolytic effects, suggesting a role in grooming and affiliative behavior. We recorded from single CT axons in human participants, using the technique of microneurography, and stimulated a unit's receptive field using a novel, computer-controlled moving probe, which stroked the skin of the forearm over five velocities (0.3, 1, 3, 10, and 30 cm s(-1)) at three temperatures (cool, 18 °C; neutral, 32 °C; warm, 42 °C). We show that CTs are unique among mechanoreceptive afferents: they discharged preferentially to slowly moving stimuli at a neutral (typical skin) temperature, rather than at the cooler or warmer stimulus temperatures. In contrast, myelinated hair mechanoreceptive afferents proportionally increased their firing frequency with stroking velocity and showed no temperature modulation. Furthermore, the CT firing frequency correlated with hedonic ratings to the same mechano-thermal stimulus only at the neutral stimulus temperature, where the stimuli were felt as pleasant at higher firing rates. We conclude that CT afferents are tuned to respond to tactile stimuli with the specific characteristics of a gentle caress delivered at typical skin temperature. This provides a peripheral mechanism for signaling pleasant skin-to-skin contact in humans, which promotes interpersonal touch and affiliative behavior."

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