Last night, late, this piece by Sabrina Richards, writing for New Scientist, came in: Pleasant to the Touch. (Which meant it was 1:32 AM before I could even think about going to bed!)
There were other great stories to read in New Scientist last night - there was one called A Nose for Touch, by Kenneth Catania about star-nosed moles, all about what incredibly sensorially-well-appointed little feeler-like noses these small mammals enjoy, with a great deal of information about sensory processing in general; Missing Touch by Meagan Scudellari is about the quest to give prostheses haptic capacity. Good Vibrations by Cristina Luiggi is about mechanoreception in all sorts of creatures. The Pliable Brain by Christina Karns is about how deaf peoples' auditory cortex might be used by them, outside awareness, to process mechanoreceptive information.
A bit outside the exteroceptive sensory processing ball park, but still about feeling, this time interoceptive processing of one's own stress response, is a piece by John Coates, A Story Biological: Coates is a former stock trader, and neuroscience investigator - he wrote a book. "The Hour Between Dog and Wolf: Risk Taking, Gut Feelings and the Biology of Boom and Bust." I happened to catch this CBC interview with him while driving home from CPA Congress in May. It was brilliant. He proposes that in men, a positive feedback loop between excitement and risk taking grows and grows until either victory or devastating defeat occurs. I.e., men just don't feel their growing stress response as anything with warning bells attached - instead they feed on it. (He thinks women and old guys should take care of the money, as they are way more risk aversive.)
But anyway, back to Kenneth Catania's piece about star-nosed moles...
"Exquisitely sensitive touchSo, thinking as a manual therapist, if we could make a human hand that contained the sensitivity of a single star nose, we'd need ... let's see.. a hand that was 6 times bigger and just as sensitive as it already is! If you think of visual data, that would be WAY too big and too fuzzy a "picture"! The pixel size needs to be dense to get such great resolution.
The star-nosed mole’s “nose” is not an olfactory organ, but a skin surface that mediates touch. Innervated by more than 100,000 sensory neurons, the star is probably the most sensitive and highly acute touch organ found on any mammal. Under a scanning electron microscope, the skin surface resolves into a cobbled landscape covered with tens of thousands of tiny epidermal domes. Each is about 60 µm in diameter, and each contains a circular disk in its center. Known as Eimer’s organs, these sensory protrusions cover the entire surface of the star’s 22 appendages. In total, a single star contains about 25,000 domed Eimer’s organs, each one served by four or so myelinated nerve fibers and probably about as many unmyelinated fibers.1 This adds up to many times more than the total number of touch fibers (17,000) found in the human hand—yet the entire star is smaller than a human fingertip."
We could die from all the pleasantness, maybe: (Pleasant to the Touch)
"Scientists hope an understanding of nerve fibers responsive only to gentle touch will give insight into the role the sense plays in social bonding."
"The C-Tactile Story
When Håkan Olausson and his colleagues at the University of Gothenburg began studying light touch perception in the early 1990s, most researchers in the field rejected the idea that humans might have slow-conducting nerve fibers sensitive to gentle pressure. “Nobody really thought they existed in humans,” Olausson explains. Textbooks at the time acknowledged that humans had slow-conducting nerves, but asserted that those nerves only responded to two types of stimuli: pain and temperature. Sensations of pressure and vibration were believed to travel only along myelinated, fast-signaling nerve fibers, which also give information about location. Experiments blocking nerve fibers supported this notion. Preventing fast fibers from firing (either by clamping the relevant nerve or by injecting the local anesthetic lidocaine) seemed to eliminate the sensation of pressure altogether, but blocking slow fibers only seemed to reduce sensitivity to warmth or a small painful shock.
In contrast to the work in humans, experiments in cats, rats, rabbits, and even monkeys found that unmyelinated, slow-conducting nerve fibers were indeed sensitive to light touch, but were found only in hairy skin. Some researchers speculated that humans had lost such fibers to evolution as they shed most of their body hair. While a few isolated studies suggested that facial skin retained the fibers, those studies were often dismissed as merely demonstrating the existence of a vestigial type of nerve fiber, says Olausson...
Then, in 1999, the group looked more closely at the characteristics of the slow fibers.They named these “low-threshold” nerves “C-tactile,” or CT, fibers, said Olausson, because of their “exquisite sensitivity” to slow, gentle tactile stimulation, but unresponsiveness to noxious stimuli like pinpricks... Unlike other types of sensory nerves, CT fibers could be found only in hairy human skin—such as the forearm and thigh... To address the question, Olausson’s group sought out a patient known as G.L. who had an unusual nerve defect. More than 2 decades earlier, she had developed numbness across many parts of her body after taking penicillin to treat a cough and fever. Testing showed that she had lost responsiveness to pressure, and a nerve biopsy confirmed that G.L.’s quick-conducting fibers were gone, resulting in an inability to sense any pokes, prods, or pinpricks below her nose. But she could still sense warmth, suggesting that her slow-conducting unmyelinated fibers were intact... G.L. also afforded scientists the opportunity to observe which areas of the brain respond to the gentle brushing. Sensations of touch stimulate two different brain areas, says Vaughan Macefield, a neuroscientist at the University of Western Sydney who researches how the brain processes pain. The somatosensory cortex registers the quick signals sent along myelinated nerve fibers and tells us where on our body the sensations originate. Slow, unmyelinated fibers send signals to the insular cortex—a section of the brain that processes taste and pain, as well as emotion. Most of our touch perception mingles information from both areas, says Macefield.
Olausson used functional MRI studies to examine which areas of the brain lit up when G.L.’s arm was gently brushed to activate CT fibers. In normal subjects, both the somatosensory and insular cortices were activated, but only the insular cortex was active when researchers brushed G.L.’s arm. This solidified the notion that CT fibers convey a more emotional quality of touch, rather than the conscious aspect that helps us describe what we are sensing. CT fibers, it seemed, specifically provide pleasurable sensations.Reading these studies while sitting on an airplane some 15 years ago,
Francis McGlone, whose research at the time focused on pain, had an epiphany. “I said, I know exactly what they’re for: grooming behaviors,” he explains.
McGlone had already begun hypothesizing that certain behaviors, like applying face creams, were motivated more by an underlying pleasant sensation than by any anti-aging benefits the creams might be providing. People repeat these behaviors, McGlone theorized, because they stimulate a subtle, positive, possibly unconscious sense of reward. CT fibers offered the perfect explanation of how this positive sensation was being transmitted to the brain.
These studies, taken together, led McGlone to think about how touch informs social interaction. In his view, it’s clear that pleasant touch is important during both infant development and adult social interaction. Although rigorous human studies have yet to be performed, anecdotal evidence in humans and studies on rats nurturing their pups supports the role of touch in brain development."
References under Richard's story:
- Å.B. Vallbo et al., “A system of unmyelinated afferents for innocuous mechanoreception in the human skin,” Brain Res, 628:301-04, 1993. ↩
- Å.B. Vallbo et al., “Unmyelinated afferents constitute a second system coding tactile stimuli of the human hairy skin,” J Neurophysiol, 8:2753-63, 1999. ↩
- H. Olausson et al., “Unmyelinated tactile afferents signal touch and project to insular cortex,”Nature Neurosci, 5:900-04, 2002. ↩
- L.S. Löken et al., “Pleasantness of touch in human glabrous and hairy skin: Order effects on affective ratings,” Brain Res, 1417:9-15, 2011. ↩
- I. Morrison et al., “Reduced C-afferent fibre density affects perceived pleasantness and empathy for touch,” Brain, 134:1116-26, 2011. ↩
Old blogposts re: Olausson:
1. Dermoneuromodulation: Ascending Pathways (March 2012)
2. Dermoneuromodulation: Ruffini Sensory Endings and Dorsolateral Prefrontal Cortex (March 2012)
3. More Insular Matters (May 2008)
4. "Somesthesis" (April 2008)
5. SomaSimple Pain Consensus (Jan 2008)
6. Manipulation and the Brain (Feb 2007)
Also, about grooming behaviours and how they relate to manual therapy:
OPERATOR / INTERACTOR MODELS OF MANUAL THERAPY