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Our fingernails know us so well that they can sometimes even tell us about our health. But there's a surprising amount we don't know about them, such as their unexpectedly precise sensitivity to the slightest touch.
A study has found humans can identify where touch is applied to their fingernails almost as accurately as they can identify touches on their fingertip.
"It is not just that people can perceive touch applied to the distal extremity of the fingernail," writes University of London neuroscientist Matthew Longo in a new paper.
"People can tell precisely where on the fingernail a stimulus was applied."
To investigate the sensory capabilities of our fingernails, Longo recruited 38 participants between the ages of 18 and 60 to perform two experiments.
Half of the volunteers were asked to indicate where they felt (but couldn't see) a tactile stimulus received on their finger by marking the corresponding position on a photo of their fingernail or fingertip.
The other half were asked to do the same, but their fingers and fingernails were touched with a filament instead of a stick. This allowed a consistently small amount of pressure to be applied each time.
In order to generalize beyond one specific finger, the first experiment was conducted with the middle fingers and the second with the thumbs.
"Localization performance on the fingernail was above chance levels for all participants tested, and broadly comparable to performance on the fingertip itself," Longo found.
This is despite the fact that the ends of our fingernails lack touch receptors. Our skin contains mechanoreceptors, which respond to a variety of stimuli, including temperature and pressure.
Some pressure receptors are activated by the slightest pressure, whereas others require a stronger impact before they'll send a signal through nerves to alert them of immediate proximity to a physical presence.
Our skin's sensory system detects pressure, vibration, temperature, pain, and itching through different types of mechanoreceptors.
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But we are also capable of localizing a touch applied to a stick held in our hand, such as the canes used by blind people. So scientists suspect similar mechanisms may be involved with the far ends of our fingernails.
"In both cases, the precise location of a stimulus is perceived despite the absence of any tactile receptor within the stimulated surface itself," explains Longo.
Pacinian corpuscle mechanoreceptors lie deep within the second-last layer of our skin, the dermis. These nerve endings detect patterns of vibrations across widespread regions of our skin, allowing us to understand the tactile location of tools.
Longo believes it's these touch receptors that may be coding the sensory information from our fingernails too, despite not being found in our nail beds.
Why our fingernails are so sensitive is yet to be confirmed, but this sensorimotor function may play a role in the exceptional dexterity of our species, along with the extra touch sensitivity recently found around our hair follicles.
"It will also be interesting to determine whether this ability is specific to fingernails, or whether tactile stimuli can also be precisely located on toenails," suggests Longo.
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