SKIN STRETCHING AND MOVEMENT ILLUSIONS

My take on why manual therapy "works", part 4.

OLDER POSTS IN THIS SERIES

Part 1: IS MANUAL THERAPY EVEN NECESSARY?

Part 2: NEUROTAGS! YOU'RE IT! 

Part 3: ABOUT "LEARNING" 

NEWER POSTS IN THIS SERIES

Part 5: TACTILE DIRECTION DISCRIMINATION IN THE DORSOLATERAL PREFRONTAL CORTEX

Part 6: MORE ABOUT DORSOLATERAL PREFRONTAL CORTEX

Part 7: TREATMENT CONTEXT, NON-SPECIFIC EFFECTS 

Part 8: SOME FINAL THOUGHTS ON NON-SPECIFIC EFFECTS

The 4th statement from the SomaSimple post is this:

4. Add a novel stimulus, e.g., skin stretching (see Gandevia and Collins 2005)(1), allow the brain to enjoy a movement illusion. "Illusory movements were evoked at the interphalangeal (IP) joints of the index finger, the elbow, and the knee by stimulation of populations of cutaneous and muscle spindle receptors, both separately and together."

SKIN INPUT AFFECTS MOTOR OUTPUT

Simon Gandevia has been at this whole neurophysiology business for decades. I tried to read his papers a number of years ago (but got distracted by something else, as usual..)
Anyway, I remember one paper in which he had difficulty separating all those annoying cutaneous receptors away from all the juicy muscle spindle receptors he was actually trying to get at and measure. I think it had to do with reflex testing - you know, tapping the patellar tendon to get the quads to jump a bit. In this paper, it looks like he succeeded in isolating muscle spindles but at the same time, Collins and he decided to study cutaneous receptors as well - their effects on movement illusion.

It's a great little paper. In my life it represents a bit of a watershed paper in that it pointed out a really important thing - that the brain takes cues from receptors in skin (and this from a muscle spindle researcher!), not just from muscle. You see, back in the day, my training was completely lopsided - the nervous system was only important for, and was taught as, a motor output system. There was no emphasis on anything afferent, except from muscle spindles. Nervous system as black box connected only to muscles, input or output. Everything else, inconsequential.
Unless you are an aspiring human primate social groomer - then it would have been really nice to have learned something about how the brain codes for skin-put.

Anyway, back to the Collins and Gandevia paper. Not only does the brain take cues from receptors in skin, it creates movement illusions based only on the information it gets from skin! Remarkable. Sort of a ++ for simple human primate social grooming. Aha! So, if you want the brain to think there is more space around a certain place, like a knee joint maybe, stretch the skin there and see if the knee will bend further after. (It will, most of the time. Unless there is an actual mesodermal blockage in there, and not just a critter brain, interoceptive pain illusion that feels like a restriction.)
Gandevia and Proske published another paper (2) a few years ago about kinesthesia.
Turns out Ruffini endings contribute to proprioception. What do you know? In fact they say skin receptors are more likely to play a role in joint position sense than joint receptors do. Oh, they still love their muscle spindles, of course, but look: 

"The cutaneous receptor most likely to subserve a kinaesthetic role is the skin stretch receptor, the slowly adapting Type II receptor served by Ruffini endings (Chambers et al.1972; Edin, 1992). For kinaesthesia at the forearm, stretch of skin over the elbow during elbow flexion can provide information about both position and movement. Movement illusions generated by stretch of skin of the hand and over more proximal joints, when combined with muscle vibration were greater than when either stimulus was applied on its own (Collins et al. 2005). The authors made the point that this was not just a matter of skin input facilitating the muscle input and that cutaneous input generated by skin stretch contributed to kinaesthesia in its own right."

Now, add this together with what Olausson and his colleagues have been doing in Sweden for decades(3), and I think we have a case for skin-put having an important role in the world. Not that we're going to ever stop doing it - human primate social groomers are just doing what our own vertebrate mammal primate nervous systems have always done, since whenever they started self-organizing then interacting half a billion years ago. 


YES-CICEPTIVE INPUT
I have already blogged my fingers to the bone about Olausson and his group in Sweden. Read all about it.

THE FINE OLD ART OF FINGER PULLING

• Mind tricks may help arthritic pain

• Illusion could halve the pain of osteoarthritis, scientists say

Don't be confused about this video - a lot of emphasis has been placed on the visual component of the illusion. I would argue that the kinesthetic component is equally if not more important. It wouldn't be very hard for some clever researcher type to design a test for that - blindfold one group of participants, pull their fingers in exactly the same way, in the machine. Have another group just look, don't touch them at all. Use the mirrors to make it look like their fingers lengthen all by themselves. A third group could have both illusions at the same time. 

I bet the last group would have the best results, but hey, if the first group had pretty good results, you wouldn't need a great big expensive machine to create a movement illusion, and ordinary human primate social grooming would be vindicated.

1. Collins D, Refshauge KM, Todd G, Gandevia SC; Cutaneous receptors contribute to kinesthesia at the index finger, elbow, and knee. J Neurophysiol. 2005 Sep;94(3): 1699-706 (full text pdf)

2. Proske U, Gandevia SC; The kinaesthetic senses. September 1, 2009 The Journal of Physiology, 587, 4139-4146. (full text)

3. Mind tricks may help arthritic pain

4. Illusion could halve the pain of osteoarthritis, scientists say