Scientists reveal the secret of cuddles

Gaia Vince

Scientists have discovered why being cuddled feels so good - human skin has a special network of nerves that stimulate a pleasurable response to stroking.

The revelation came after doctors realised that a woman with no sense of touch still felt a "pleasant" sensation when her skin was caressed.

Normal touch is transmitted to the brain through a network of fast-conducting nerves, called myelinated fibres, which carry signals at 60 metres per second. But there is a second slow-conducting nerve network of unmyelinated fibres, called C-tactile (CT), the role of which was unknown. The CT network carries signals at just one metre per second.

"It must be used for unconscious aspects of touch because it is so slow," says Håkan Olausson, who led the study at the Department of Clinical Neurophysiology at Sahlgrenska University Hospital, Sweden. "It seems the CT network conveys emotions, or a sense of self."

"This study definitely helps our understanding of how touch systems work," says Brian Fiske, assistant editor at Nature Neuroscience. "The researchers were very fortunate to have found a patient who had lost the main touch receptors but still had the slow CT fibres."

Below the nose

Scientists have known for some time that myelinated nerve fibres transmit information about touch, such as its strength and position. But the function of CT fibres was a mystery. This was because it is impossible to distinguish the CT fibre signals from those of the continuously activated fast myelinated fibre.

The patient examined by the Swedish researchers had a disorder that left her with no myelinated touch fibres in her body below the level of her nose. But her CT fibres remained intact.

Olausson stroked the patient's arm and hand with a paintbrush. Although she could not feel touch, tickle or vibration, the patient said she experienced a "pleasant" pressure when her arm was caressed with a paintbrush.

MRI scans of her brain revealed that the stroking activated insular region of the cerebral cortex associated with emotional response.

Hairy skin

The researchers concluded that the CT system may be of important for emotional, hormonal and behavioural responses to tactile stimulation.

"They are the opposite to pain fibres and give the message that the touch is non- harmful," Olausson told New Scientist. "Stimulation of CT fibres is probably linked to the release of pleasure hormones, like oxytocin. Studies have shown that if you stroke infants, their levels of oxytocin increase."

Further research by the Swedish team suggests that CT fibres are only present in hairy skin - the patient showed no response to the palm of her hand being stroked.

Olausson speculates that because the hand is used for so many critical tasks, it needs to be very sensitive to touch and therefore has a greater density of faster- conducting nerves.

Journal reference: Nature Neuroscience (DOI: 10.1038/nn896)

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