Our skin is an enormous sensory organ, studded with tiny touch receptors in different layers of the skin. They are dense in some areas and sparse in others. These touch receptors come in different flavors: some respond to light touch, some to deeper pressure, and some to changes in the position of the hairs on our skin. Some 50 years ago, study of these skin receptors in the cat revealed a receptor exquisitely sensitive to slow, gentle stroking of the skin with a bit of pressure - the kind of stroking we associate with massage, or comforting others, or simple gestures of affection. Now we know a bit more about them.
Mothers in many cultures routinely massage their babies with long slow strokes along the back and legs, or across their bellies. This calms the babies and teaches them what it feels like to be calm.
Think about comforting a friend, or a child. Long slow strokes across their back, affirming that they are not alone, and helping to calm the tension induced by fear or grief or hurt.
Or maybe you’ve had a rough week, so you decide to get a soothing massage.
The question is: how are we soothed or comforted or calmed by this kind of touch?
Neuroscience, of course.
First the information has to get from the skin to the brain, which is where it is interpreted as calming. Neurons that respond to touch have long processes extending in two directions. One end goes to the skin – the receptor end; the other to the spinal cord. In the pathway used to send information about gentle touch, sensory neurons make a connection to a second set of neurons in the spinal cord which send the information to a relay center called the thalamus, deep in the brain. From there, a connection is made to a third set of neurons that end in a region of the cortex called the insula. The information is then sent on to higher brain centers where it is combined with other inputs from the body and interpreted in the light of experience, memory and social and emotional context.
Back to the receptor endings in the skin, it turns out the sensory neurons highly sensitive to gentle stroking are responding to skin indentation. The skin endings of these neurons coil around the base of the hairs on the skin, but far enough away (mere 4 microns!) that they don’t respond to movement of the hairs.
Another characteristic of these receptor neurons makes them sensitive to long, slow stroking in particular. These neurons sometimes are called field neurons because each of them has many endings (up to 200 in a mouse), and those endings are spread out across a relatively large area of the skin (Bai et al., 2015). A slowly stroking hand will stimulate many of the endings during the time that the hand is moving over the neuron’s field. During a long stroke, say across the back, the moving hand will stimulate a series of these neurons as it moves along. Once the brain receives the information from these neurons, it produces a pleasant feeling that likely is an important part of social interactions. In the case of those long strokes bestowed by one human to another, it is calming or soothing or comforting - in short, emotionally powerful!
Bai et al. (2015) Cell 163: 1783–95
Author: Lynne Oland, PhD