Touch and Sensation: A Deep Model

Barrett L. Dorko, P.T.


I’ve written recently about “deep models” used to explain the experiences we as therapists have in the clinic, and it’s only fair that I offer my own. By deep model I mean the description of those processes occurring beyond our vision or sensation that ultimately account for what we observe empirically. Deep models are essential if we are to understand both the failure and success of technique, and they should not violate physical law, as we currently understand it.

 Stretch Activated Ion Channels: How Touch Translates To Sensation

 The translation of mechanical deformation in various tissues to an electrical signal strong enough to be sensed and properly interpreted by the brain has been explained several different ways.

Sensation of any sort requires a difference in net charge across the neuronal membrane that exceeds its normal resting potential. Achieving this state normally requires the influx of sodium or calcium through "ion channels": cylinder-shaped protein molecules that permit chemical diffusion to occur without energy expenditure. When enough channels open, depolarization occurs and the experience of sensation is then possible.

 It has been known that ion channels may be ligand (chemical) or voltage (electrical) gated. Recently, ion channels that are activated via increased membrane tension have been discovered in many tissues. Mechanically sensitive tissues throughout the body possess stretch-activated (SA) ion channels capable of immediately responding to deformation of the tubes, rods, and strings that form the matrix of the cytoskeleton. These channels account for the propagation of sensation secondary to movement (proprioception) or internal stress (interoception).

Altering the SA ion channels in various sequences will eventually produce voltage gated ion propagation, which is very much like lighting a fuse, sending a quick message to the brain via the inter-neurons. The resting membrane tension of the cytoskeleton is directly related to the ease with which mechanical deformation may recruit enough SA ion channels necessary to eventually promote nervous depolarization. The higher the resting tension, the easier it is to open a sufficient number of channels.

If the resting tension in the epithelium and its intimately associated connective tissue is higher than normal, a chain reaction of channel opening will be facilitated. This is ultimately displayed by a report of sensation that knows no neurologic boundary. The absence of a typical dermatological reference is due to the fact that numerous tissues unrelated to nervous tissue possess SA channels. They may serve as a bridge from one dermatome to the next mediated by mechanical deformation of the connective tissue between nerves.

If distant sensation and change are common with Simple Contact, it’s reasonable to assume or propose the following:

Understanding the potential effect of Simple Contact in this way allows the therapist to appreciate how sensation and change in our patients can occur with remarkable variety and speed. Relinquishing the agenda usually present with manual care and choosing to be a companion to the patient and their unique processes reveals the intelligence and corrective power every person possess. This attitude arises from an understanding of the materials we handle, and if we deform the skin without a distinct knowledge of what happens next, and why, the potential for distant and profound change is lost.

Reference List

The Intimate Sense; Understanding the Mechanics of Touch by Frederick Sachs. "The Sciences" Jan/Feb., 1988 Published by The New York Academy of Sciences, Two East Sixty-Third Street, New York, NY 10021

Baroreceptor Mechanisms at the Cellular Level Frederick Sachs; 1986 Update In Cardiovascular Neurobiology; Federation Proceedings Vol. 46 No. 2 January 1987

Mechanotransducer Ion Channels in Chick Skeletal Muscle: The Effects of Extracellular PH Guharay and Sacks Journal of Physiology (1985) 363 pp. 119-134.

Single-Channel Currents from Acetylcholine Receptors in Embryonic Chick Muscle Averback and Sachs, Biophysics Journal Vol. 45, Jan.l984

Stretch-Activated Single Ion Channel Currents in Tissue-Cultured Embryonic Chick Skeletal Muscle Guharay and Sachs Journal of Physiology (1984) 352 pp. 685-701


The Canvas Tent Metaphor

A scientific explorer of the world has to decide when to reduce an issue to its material constituents and when to remain metaphorical.

Joel Achenbach in Captured by Aliens

The deep model above, in my experience, is insufficient when it comes to describing the consequences of touching the skin of another. Actually, you don’t actually have to touch the skin itself; deforming it through just about any clothing will work just as well.

Imagine you’re in a canvas tent in the rain. An experienced camper will know better than to touch the sides of the tent. This would orchestrate an opening in the fibers of the cloth large enough to allow leakage. The opening created is far too small to see, but it’s there nonetheless. If I didn’t understand anything about canvas, that is, if I didn’t have a deep model, this leaking would remain mysterious to me.

When we touch the skin, it leaks. The exchange of ions through the stretch-activated channels is enhanced by the membranous tension already present. More tension makes it easier to elicit reflexive reaction to touch. This makes scars, facilitated segments and areas of increased muscle tone better “doorways” into the nervous system.

You might notice that there is no way according to this model to elicit sensation without actually deforming the skin. Waving your hands near to the surface isn’t going to do anything other than what might be accomplished through some form of psychological appeal. The deep model used to explain that is well beyond the scope of my expertise.