David Ginty, Ph.D., professor of neuroscience at Johns Hopkins explains:
"You can deflect a single hair on your arm and feel it, but how can you tell the difference between a raindrop, a light breeze or a poke of a stick? Touch is not yes or no; it's very rich, and now we're starting to understand how all those inputs are processed."
In order to learn how touch-responsive nerve cells develop, Ginty and his team decided to develop new tools that allow them to examine individual nerve cells. In the skin there are over 20 broad classes of so-called mechanosensory nerve cells that sense everything from pain to temperature - six of these nerve cells account for light touch.
Until now, electrical recordings were the only way to distinguish one cell from another as each cell type produces a different current based on what is senses.
Ginty and his team genetically modified rodents in order to make a fluorescent protein in the C-type low-threshold mechanosensory receptor or C-LTMR (one type of nerve cell). Under a microscope cells containing fluorescent protein could be seen in their entirety. The scientists discovered that each C-LTMR cell sent projections to up to 30 different hair follicles.
Mice have three different types of hair:
- 1% of total of body hair is a thick, long guard hair
- 23% of body hair is a shorter hair called the awl/auchene
- while 76% is a fine hair called the zigzag
After similarly marking two other types of touch nerve cells the team discovered that each hair type has a different and specific set of nerve endings connected with it. Ginty said: "This makes every hair a unique mechanosensory organ."
Furthermore, the team discovered that each hair type is evenly spaced and patterned throughout the skin.
In order to determine how all the input from each hair is gathered and transmitted to the brain the team used a different staining method which allowed them to dye the other end of the cell, in the spinal cord.
They discovered that the nerves linking to each patch of skin containing one guard hair and other associated smaller hairs line up in columns in the spinal cord - nearby columns correspond to nearby patches of skin. The team estimates that there are approximately 3,000 to 5,000 columns in the spinal cord, with each column responsible for 100 to 150 hair follicles.
So how does the brain translate what each hair follicle experiences? Ginty explains: "How this happens is remarkable and we're fairly clueless about it." However, Ginty speculates that the organization of the columns is vital to how all the different inputs are processed before a message is transmitted to the brain.
Although, humans are not as hairy as mice, we share several of the same structures according to Ginty. This investigation together with the development of new cell-marking tools opens several doors for new research in understanding touch and other senses, said Ginty.