Understanding Basis of Touch Offers Insights, Ginty Says
“The sense of touch is fundamental to our daily lives,” explained Dr. David Ginty at the NCCIH Integrative Medicine Research Lecture Series in Lipsett Amphitheater on Mar. 7.
It allows people to recognize objects in their hands, determine what something is based on how it feels, instructs them how to react to situations and allows them to emotionally bond with each other, said Ginty, the Edward R. and Anne G. Lefler professor of neurobiology at Harvard Medical School and an investigator at Howard Hughes Medical Institute.
In his lab, Ginty studies how neurons that send processes to the skin of mice can distinguish between different touch sensations and how they send this information to the spinal cord and brain. Findings from his work could one day lead to the treatment of spinal cord injuries.
The first step in the perception of touch, he explained, is activation of a group of neurons called low-threshold mechanoreceptors (LTMRs). These neurons, whose cell bodies are located near the spinal cord, send processes that are distributed throughout skin and terminate just underneath the skin’s surface.
Ginty said there are at least five types of LTMRs that respond to skin stimulation: Aβ SA1, Aβ RA, Aβ field, Aδ and C. Each has its own structure and function. Depending on the type, LTMRs can optimally detect hair deflection and pulling, skin indentation, stroking and skin stretching. Aβ RA LTMRs, for example, respond to air puffs acting on the skin and skin stroking while Aβ field LTMRs detect stroking but are unresponsive to hair deflection and skin indentation. Some types respond to multiple stimuli while others respond only to one. They are present in most if not all mammals, “everyone from mice to humans.” C LTMRs are the smallest and most common.
Some types of LTMRs, such as Aδ and Aβ subtypes, are covered in a fatty white substance called myelin. Like the plastic coating around an electrical wire, myelin insulates neurons, allowing nerve impulses to travel through the cell quickly. Other types, such as C LTMRs, are not insulated by myelin, so nerve impulses travel much slower.
“We think, collectively, the different tuning properties of these neurons explains how an animal perceives a large variety of tactile stimuli,” Ginty said. In other words, once different ensembles of LTMRs are activated, they send signals to the spinal cord and the brain.
Some LTMR subtypes wrap around the base of hair follicles. When a follicle moves, neurons send signals to a bundle of nerves in the dorsal horn, a structure that receives sensory information from the body. From there, the sensory information is sent to the brain.
Ginty is also studying whether or not LTMR dysfunction explains hypersensitivity to touch in mouse models of autism spectrum disorders and similar conditions such as Rett syndrome and fragile X syndrome. Many people with these conditions over-respond to touch.
“We thought we could provide some insight as to why this is the case and, perhaps, address the potential relationship between tactile hypersensitivity and some of the other behaviors [in these conditions],” Ginty said.
Recently, he located nerves in the spinal cord that underlie hypersensitivity to light puffs of air in mice. Currently, Ginty is conducting experiments to study whether or not inhibition of LTMRs indicates anxiety-like behavior in mice. Results of his research will be published soon.