The complex system of nerve cells in hairy skin has been seen in detail for the first time, revealing that different types of hair follicle are tuned to sense different sorts of soft touch. The research provides the first full picture of how the nerve cells that carry signals from hair on the skin to the spinal cord are organised, and enhances our understanding of how sensory information is processed for the perception of delicate touches.
“For the first time we’re actually able to see the end terminals of the sensory fibres that innervate hair follicles,” says Jeff Woodbury, associate professor of zoology and physiology at the University of Wyoming in the U.S. and one of the study authors. “We’ve also been able to identify how these combinations of nerve cell respond to fine tactile stimuli, so we can really begin to tease apart the circuitry of touch sensation.”
Mice have several different types of hair follicles in their coat, each of which is linked to the central nervous system by low-threshold mechanoreceptors (LTMRs) – long wire-like nerve cells that stretch all the way to the spinal cord, where the myriad signals carried from the skin are integrated, processed and sent to the brain. It is this network of nerve endings in the skin of most hairy mammals that allows them to perceive fine tactile sensations, such as a mosquito landing on their back or the light brush of a leaf. Until now the organisation of this system has not been well understood, largely because there was no way to see how all of these different nerve cells were arranged – both in the skin and at the top of the spinal cord, where they end up.
In a study published in Cell this month, however, Woodbury and colleagues report that they have come up with a way to genetically label LTMRs in order to observe the paths of individual nerve fibres. First they identified specific genes only expressed in the LTMR cells they wanted to look at. Next they modified those genes to make them produce fluorescent proteins in mice, which allowed them to examine the patterns and paths of these specific nerve cells. They also used electrophysiological techniques to show that individual LTMR types have different functional characteristics.