Have you ever been touched in a way that excites you? Children want reassuring touches from their parents, adults long for pleasurable touches from their partners, and a depressed soul wants a genuine hug from a friend. Sometimes, after a long and stressful day, a simple massage could be enough to relax the brain and the whole body. Even pets get excited when touched by their owners. Everybody wants to be touched in some way because the brain loves it. 

No matter how good a touch is, it will not excite you if the brain doesn’t get any signal of it. For touch to be pleasurable, the brain must be triggered to release dopamine, the chemical released 

in the brain to make you feel good. 

A study recently conducted at the University of Columbia shows how messages are sent from the skin to the brain. It explains how “Mrgprb4-lineage touch neurons in the skin are required for sexual receptivity and dopamine release that makes this type of social touch rewarding.”

The study showed there is a route that transmits pleasurable touches from the neurons in the skin to the ventral tegmental area, the part of the brain associated with pleasure and reward. The report of the findings was published on January 23 in the journal Cell. 

Studying mice to know human behaviour

The study was conducted using a combination of mouse genetics, slice electrophysiology, circuit tracing and behavioural paradigms. The researchers, led by Dr Liah Elias, experimented with mice to examine how they react to soft touches. The team eliminated the Mrgprb4 cells from the mice, using genetic techniques. The purpose was to find out if mice would sexually respond to tactile stimulation. The result was a surprise decline in sexual receptivity. 

“The sexual receptivity just plummeted. We then knew for sure that these cells were important for social touch in natural encounters,” Dr Elias stated.

“We saw that by activating this understudied population of tactile sensory cells in the mouse’s back that the animals would lower their backs and take on this posture of dorsiflexion,” Dr Elias continued. Lowering their back in that manner only happens at a time when there is a touch from a suitor mouse. It is the primary sign of sexual receptivity. 

The result could only mean one thing. Unless there is the presence of the Mrgprb4 cells in the skin, the mice won’t find stroking exciting, as no pleasurable message will be passed on to the brain.  

To further establish how the skin of the animals connects to the brain, the team of researchers genetically engineered a line of mice so that the Mrgprb4 touch-sensitive cells would fire when illuminated with blue light. The dorsiflexion responses were shocking when the blue light was shined on the mice, which further confirmed the connection between the skin and their response to sexual stimulation. 

Interestingly, the research team later observed these animals voluntarily returning to the same spot where they had been initially illuminated with the bright blue light. Nothing better explains how excited the animals were to experience that feeling. They wanted a repeat of the action. 

The link between the skin and the brain

A study conducted at Harvard University in 2020 explains the route from the skin to the brain. In the study, the researchers confirmed that there are spinal cord cells, named GPR83 cells. These cells have neuron-to-neuron links with the same class of Mrgprb4 found in the mice.

“The anterolateral pathway consists of ascending spinal tracts that convey pain, temperature and touch information from the spinal cord to the brain,” the report stated. The pathway goes directly into the brainstem, and it is safe to conclude that the brain receives touch-related information directly from the skin. 

“That gave us the handle that these GPR83 neurons are probably a conduit connecting the skin all of the way to the brain,” Dr Abdus-Saboor, who contributed to the Harvard investigation, stated. 

Why the study is important

There are groups of people across the world that do not benefit from hugs and touches of affection. The first in this group are patients with autism. These people are said to have heightened sensory perception.

Since humans and animals share a similar pathway that carries information from the skin to the brain, researchers are wondering if the route could be modified to make it less sensitive for people with autism.

“A cardinal symptom for many people with autism is that they do not like to be touched. This begs the question of whether the pathway we’ve identified could be altered so people can benefit from touch that should be rewarding rather than aversive,” Dr Abdus-Saboor stated.

Should that happen, hugging, touching, and holding people with autism without them feeling pain could become a reality.

Dr Elias believes the study was important in knowing how the lack of physical contact could affect people, especially old people’s living in nursing homes.

“The pandemic made us all acutely aware of how devastating the lack of social and physical contact can be,” Dr Elias said.

“I think about how physical contact between parents and their newborns and young children is necessary for proper cognitive and social development. We don’t yet understand how these kinds of touch convey their benefits, whether acutely pleasurable or promoting long-term mental well-being. That’s why this work is so essential,” he added.