Scientists discover why some parts of the body are more sensitive than others

When it comes to human body parts, some are more sensitive than others.

Lips and hands, for example, easily gather detailed information via sensory neurons. This is a vital survival function that enables us to safely navigate the world, from detecting a scalding cup of tea before taking a gulp, to noticing a bath is ice-cold before jumping in.

Thighs and forearms are at the other end of the spectrum of body parts, being much less sensitive.

Now, researchers have uncovered the mechanism that could explain why certain body parts – or ‘skin regions’ – are so sensitive, and others not so much.

Experts from Harvard Medical School have put the differences down to what they call an overrepresentation of sensitive skin surfaces in the brain.

They found that sensory neurons which exist in the most sensitive skin regions and relay information to the brain stem form a greater number of connections, and stronger ones, than neurons elsewhere in the body.

It is thought that this overrepresentation of sensitive skin surfaces in the brain allows different species to develop sensitivity in different areas. In humans, sensitive areas include the mouth and fingers, while for pigs it is their snout.

Crucially, they hope that the study could provide a stepping stone to further research that would lead to a better understanding of certain neurodevelopmental disorders.

“This study provides a mechanistic understanding of why more brain real estate is devoted to surfaces of the skin with high touch acuity,” said senior author David Ginty, a neurobiology professor at Harvard Medical School. “Basically, it's a mechanism that helps explain why one has greater sensory acuity in the parts of the body that require it.”

Scientists have long known that certain body parts are overrepresented in the brain, and previously it was thought this could be attributed to a higher density of neurons in those skin areas.

However, work by Mr Ginty’s lab revealed that while sensitive skin does contain more neurons, these extra neurons are not sufficient to account for the additional brain space.

“We noticed that there was a rather meagre number of neurons that were innervating the sensitive skin compared to what we’d expect,” said co-first author Brendan Lehnert, a research fellow in neurobiology, who led the study with Celine Santiago, also a research fellow at the lab.

To investigate, the researchers studied mice and found that between adolescence and adulthood, the sensitive, hairless skin on their paws became increasingly overrepresented in the brain, even though the density of neurons remained stable. This was not seen in less sensitive, hairy-paw skin.

They also found that these connections between neurons were stronger and more numerous for the sensitive, hairless skin than for less sensitive, hairy skin.

“This immediately told us that there’s something more going on than just the density of innervation of nerve cells in the skin to account for this overrepresentation in the brain,” Mr Ginty said.

Mr Lehnert added: “It was really unexpected to see changes over these postnatal developmental timepoints.

“This might be just one of many changes over postnatal development that are important for allowing us to represent the tactile world around us and helping us gain the ability to manipulate objects in the world through the sensory motor loop that touch is such a special part of.”

The study was carried out on mice but since the overrepresentation of sensitive skin regions in the brain is a feature across mammals, it is thought the conclusions can be applied to humans too.

The findings were published on Monday in the journal Cell.