These branches go everywhere, are multiply twigged, with a "leaf canopy" that ends up embedded in skin.
Skin is not “just” a 15-20 pound protective film wrapped round the outside of the body like living saran wrap. It has three dimensional structure. It is a sensitive raft floating on a sea of subcutaneous liquidy fat, tethered to a deeper layer of fascia by means of tough skin ligaments, hundreds of them, dense macroscopic structures that defy gravity (for a few decades anyway..). These ligaments permit movement in every direction, but not a lot of movement in any direction. Many of them are tubular, serving as conduits that convey vasculature and nerve to and from skin.
Skin is filled with varying proportions of different nerve endings, each with specific messaging capacity that can give the sensory cortex a layered perspective of how the skin is being contacted by the environment. The CNS can measure "threat" value, based on this incoming. Skin has palpable "behavior", all of it autonomic outflow in nature, enacted by smooth muscle effector cells. Through skin one can palpate tension in underlying structures. By interacting with skin, one can indirectly affect such tension toward improvement, greater overall relaxation of the organism, deeper breathing.
How is this possible? I maintain that if blind people can learn to "read" Braille, interpret a symbolic language based on a predictable system of raised dots on paper through senstive fingertips, allow their visual cortex to be taken over by kinesthetic interpretation, so can manual therapists use this same brain system to feel through someone else's physiological nervous system functioning by learning to "read" skin/motor response behavior. This has the added bonus of being an interactive form of "reading." Butler says, "Remember that just as you are sampling another's nervous system, their nervous system is sampling yours."
Skin performs many physiological functions, but from strictly a nervous system perspective, in light of what we want to do with it, we could view it as part of the brain’s sensor array, a “periscope” that completely surrounds the submarine of the brain, a window or door directly into the sensory cortex of the brain. To be invited all the way in, we must treat it respectfully and not barge our way right through it as if it did not exist as an organ of sensitivity in its own right. By attending to skin properly we can sway the patient’s nervous system to our intentions easily/effortlessly, and also, and more importantly, to the patient’s own non-conscious corrective mechanisms - correction away from a pain state. In the Melzack/Wall pain paradigm, you can minimize “threat” enormously, by handling skin properly.
It is useful to remember that both skin and brain and in fact all the nervous system, and the initial burst of immortal germ cells, all derive from ectoderm proper. In addition, early on, ectoderm gives rise to mesoderm, which makes everything our system uses to hold itself up and move along.. bones, muscles, blood, tendons, ligaments. Mesodermal structures are forever subservient to those of ectoderm, give ectoderm 'leverage' against the other forces in nature; by themselves, without a living tree existing throughout, they are just meat, matter, material.
Nerves themselves have their own "bark," or equivalent of "skin", a sensor array that protects them, called the nervi nervorum, intertwined with the feeding system to the nerve, called vasa nervorum. This signalling system lets the CNS know about potential problems before they develop, in particular any fall in oxygen levels (hypoxia). It is postulated by Butler and many others that a great deal of benign pain problems stem from simple local hypoxic conditions out in the "bark," about which the brain's alarm system has become sensitized.
Through skin, and by handling actual physical arms and legs and parts thereof, the living tree branches (neural structures) can be tugged, bowed, rolled, even slid through their mesodermal neural tunnels. This physical action changes internal juxtapositions of nerve container to nerve, and improves conditions sufficiently to allow new input to the CNS. The new input must be sufficient to allow the CNS to change its "output" (i.e. pain state) but not sufficient to cause it any threat, which would perpetuate and even compound the problem.
One of the perplexing attributes of the central nervous system is the way it has control of the volume knobs on pain; any little twig anywhere, trapped and hypoxic, can result in a pain state that seems way out of proportion, and which can be felt anywhere else. Furthermore, the CNS can create pain all by itself, as it does in cases of amputations and spinal cord injuries (phantom pain). One of the desireable attributes of a good manual therapist is that she or he knows this, will take a good history, and if the patient seems suitable, will comb through the neural tree, using his/her own developed kinesthetic senses to locate and help restore motion to anything that feels like it has less than adequate slide. S/he will use only enough force to get the job done and no more, all the while realizing that the CNS uses skin as a huge magnifying tool for all incoming, that it has its own perspectives on all incoming based on its own history of encounters with life outside itself. What if the patient is not a good candidate for manual therapy? A good manual therapist who is also happens to be a physiotherapist will have other hands-off ways of helping; listening, movement therapy, education, reframing, and encouragement.