Scientists believe they are a step closer to understanding why we like a gentle touch. A new study claims to have isolated the specific class of sensory cells in the skin that respond to a gentle caress. Social animals, from humans to cats, all seem to enjoy being stroked, but until now the neuronal circuitry underlying the sensation had been a mystery. Feels good: Scientists claim to have finally isolated the class of sensory cells in the skin that respond to a gentle caress, shedding light on why it is that humans and other animals find stroking so pleasurable . The skin is a human being's largest sensory organ, helping to distinguish between a pleasant contact, like a caress, and a negative sensation, like a pinch or a burn. Previous studies have shown that these sensations are carried to the brain by different types of sensory neurons that have nerve endings in the skin. Only a few of those neuron types have been identified, however, and most of those detect painful stimuli. Now biologists at the California Institute of Technology have identified in mice a specific class of skin sensory neurons that reacts to an apparently pleasurable stimulus. 'We've known a lot about the neurons that detect things that make us hurt or feel pain, but we've known much less about the identity of the neurons that make us feel good when they are stimulated,' said lead researcher David Anderson, Seymour Benzer Professor of Biology at Caltech. 'Generally it's a lot easier to study things that are painful because animals have evolved to become much more sensitive to things that hurt or are fearful than to things that feel good. 'Showing a positive influence of something on an animal model is not that easy.' The Caltech team did not set out to explain the pleasure of stroking. They were investigating a type of sensory cell discovered in 2007, the purpose of which had not yet been ascertained. To do this, the researchers bred genetically modified mice with fluorescent molecular markers attached to the neurons which lit up when the particular cells were activated. They also opened a tiny hole along the spine of the GM mice that they could peer through with a high-powered microscope to see when the cells lit up. 'We took advantage of the fact that these sensory neurons are bipolar in the sense that they send one branch into the skin that detects stimuli, and another branch into the spinal cord to relay the message detected in the skin to the brain,' Professor Anderson said. Painstaking: Researchers bred genetically modified mice with fluorescent markers that lit up when the neurons they were investigating were activated, then treated the animals to a range of different stimuli . Through a painstaking process of applying stimuli to one tiny area of the animal's body at a time, they found that these particular neurons lit up only when stroked. But the Caltech team could not be sure if the massage-like stimulus they were giving the mice was pleasurable to the mice in the way it would be to humans. 'As scientists we can't just assume that because something feels good to us, it has to also feel good to an animal,' said the professor. 'So we then had to design an experiment to show that artificially activating just these neurons — without actually stroking the mouse — felt good to the mouse.' To do this the researchers built a maze with left, right, and centre rooms which were different enough that a mouse could distinguish them through smell, sight, and touch. In the left room, the mouse received an injection of a drug that selectively activated the neurons shown to detect massage-like stroking. In the room on the right, the mouse received a control injection of saline. After a few sessions in each outer room, the animal was placed in the centre, with the doors open to see which room it preferred. It clearly favoured the room where the massage-sensitive neurons were activated. The findings: This graph shows the increases in fluorescence as a stroke stimulus was applied to the mice. The vertical yellow bars indicate the periods when the stimulus was applied . Professor Anderson said this was the first time anyone has used this type of conditioned place-preference experiment to show that activating certain neurons in the skin can actually make an animal feel good. He hopes his team's findings and the new methods they developed will allow researchers to trace the circuitry from those neurons up into the brain to ask why and how activating these neurons leads to pleasure. 'We are now most interested in how these neurons communicate to the brain through circuits,' he said. 'In other words, what part of the circuit in the brain is responsible for the good feeling that is apparently produced by activating these neurons? 'It may seem frivolous to be identifying massage neurons in a mouse, but it could be that some good might come out of this down the road.'
Social animals from humans to cats all seem to enjoy being stroked, but until now the neurobiology behind it had been a mystery .
Caltech team genetically modify mice so certain neurons fluoresced when activated then stroked, poked and prodded them .
They found a certain class that activated only in response to strokes and conducted a second experiment to determine it was indeed pleasurable .
Scientists believe they are a step closer to understanding why we like a gentle touch. A new study claims to have isolated the specific class of sensory cells in the skin that respond to a gentle caress. Social animals, from humans to cats, all seem to enjoy being stroked, but until now the neuronal circuitry underlying the sensation had been a mystery. Feels good: Scientists claim to have finally isolated the class of sensory cells in the skin that respond to a gentle caress, shedding light on why it is that humans and other animals find stroking so pleasurable . The skin is a human being's largest sensory organ, helping to distinguish between a pleasant contact, like a caress, and a negative sensation, like a pinch or a burn. Previous studies have shown that these sensations are carried to the brain by different types of sensory neurons that have nerve endings in the skin. Only a few of those neuron types have been identified, however, and most of those detect painful stimuli. Now biologists at the California Institute of Technology have identified in mice a specific class of skin sensory neurons that reacts to an apparently pleasurable stimulus. 'We've known a lot about the neurons that detect things that make us hurt or feel pain, but we've known much less about the identity of the neurons that make us feel good when they are stimulated,' said lead researcher David Anderson, Seymour Benzer Professor of Biology at Caltech. 'Generally it's a lot easier to study things that are painful because animals have evolved to become much more sensitive to things that hurt or are fearful than to things that feel good. 'Showing a positive influence of something on an animal model is not that easy.' The Caltech team did not set out to explain the pleasure of stroking. They were investigating a type of sensory cell discovered in 2007, the purpose of which had not yet been ascertained. To do this, the researchers bred genetically modified mice with fluorescent molecular markers attached to the neurons which lit up when the particular cells were activated. They also opened a tiny hole along the spine of the GM mice that they could peer through with a high-powered microscope to see when the cells lit up. 'We took advantage of the fact that these sensory neurons are bipolar in the sense that they send one branch into the skin that detects stimuli, and another branch into the spinal cord to relay the message detected in the skin to the brain,' Professor Anderson said. Painstaking: Researchers bred genetically modified mice with fluorescent markers that lit up when the neurons they were investigating were activated, then treated the animals to a range of different stimuli . Through a painstaking process of applying stimuli to one tiny area of the animal's body at a time, they found that these particular neurons lit up only when stroked. But the Caltech team could not be sure if the massage-like stimulus they were giving the mice was pleasurable to the mice in the way it would be to humans. 'As scientists we can't just assume that because something feels good to us, it has to also feel good to an animal,' said the professor. 'So we then had to design an experiment to show that artificially activating just these neurons — without actually stroking the mouse — felt good to the mouse.' To do this the researchers built a maze with left, right, and centre rooms which were different enough that a mouse could distinguish them through smell, sight, and touch. In the left room, the mouse received an injection of a drug that selectively activated the neurons shown to detect massage-like stroking. In the room on the right, the mouse received a control injection of saline. After a few sessions in each outer room, the animal was placed in the centre, with the doors open to see which room it preferred. It clearly favoured the room where the massage-sensitive neurons were activated. The findings: This graph shows the increases in fluorescence as a stroke stimulus was applied to the mice. The vertical yellow bars indicate the periods when the stimulus was applied . Professor Anderson said this was the first time anyone has used this type of conditioned place-preference experiment to show that activating certain neurons in the skin can actually make an animal feel good. He hopes his team's findings and the new methods they developed will allow researchers to trace the circuitry from those neurons up into the brain to ask why and how activating these neurons leads to pleasure. 'We are now most interested in how these neurons communicate to the brain through circuits,' he said. 'In other words, what part of the circuit in the brain is responsible for the good feeling that is apparently produced by activating these neurons? 'It may seem frivolous to be identifying massage neurons in a mouse, but it could be that some good might come out of this down the road.'
gentle caress. Social animals, from humans to cats, all seem to enjoy being stroked, but until sensation had been a mystery. Feels good: investigating were activated, then treated the when stroked. But the Caltech team could not be giving the mice was pleasurable to the mice in experiment to show that activating certain