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Indeed, the associations between the skin and the brain are extremely inti-"'. mate. This fact is the basis of the "lie detector test"; specific mental states directly influence the electrical properties of the skin, and in reguLfi ways that can readily be measured and correlated. And from blushing to hives, from goose bumps to shingles, the skin demonstrates a reflex expressiveness to scores of mental events. 1\ The Ectoderm This close association between the skin and the central nervous system could not have more concrete anatomical and physiological connections. All tissues and organs of the body develop from three primitive layers of cells that make up the early embryo: The endoderm produces the internal organs, the mesoderm produces the connective tissues, the bones, and the skeletal muscles, while the ectoderm produces both the skin and the nervous system. Skin and brain develop from exactly the same primitive cells. Depending upon how you look at it, the skin is the outer surface of the brain, or the brain is the deepest layer of the skin. Surface and innermost core spring from the same , mother tissue, and throughout the life of the organism they function as a single unit, divisible only by dissection or analytical abstraction. Every touch initiates 1 ECTODERM Fig. 2-15: The emergence of the three primitive germ layers - the ectoderm, the mesoderm, and the endoderm. The ectoderm gives rise to the nervous system and the skin, the mesoderm to the muscles and connective tissues, and the endoderm to the internal organs. ; 36 JOB'S BODY a variety of mental responses, and no where along the line can I draw a sharp Neun1 groove distinction betWeen a periphery which Notochordal ~--t-I-- plate purely responds as opposed to a central "'-'III;;;IT- fntra·embrto nic nervous system which purely thinks. mesoderm Head process My tactile experience is just as central .nO to my thought processes ~ are lan canal guage skills or categories of logic. Amnion Between the third and fourth week Primitlve node with opening of of the embryo'S life, the ectoderm amnio-enteric cana' begins to differentiate into skin tissues and neural tissues. The skin layer sepa Yolk sac mesode ... m rates from the central neural tube and Primitive strea){ and groove migrates outward, as the developing Ectoderm muscles, bones and organs push it far ther and farther away from the core. Fig. 2-16: The three germ layers in an 18-day-01d However, this is far from the end of embryo. the close chemical, structural, and functional affinities between the skin and the brain. In spite of the increasing distance that separates them, properties of the skin continue to playa material role in the development and organization of the central nervous system. amnio~ente..-ic L ~ ;.,.,..-----~ .. lacuna in Dilated gland Mouth of dilated gland syn_cytiotrophoblilst Slnu:!.oid Coa.gulum Entoderm Amnion Primary york sac Sinusoid Ectoderm lacuna in syncytiotrophoblast Primary Uterine mesoderm epithelium Fig. 2-T7: A drawing of a lz-day-old embryo, ~howing the three primitive germ layers. .. SKIN no- 37 Neural Mapping In order to accurately locate any stimulus on the body's surface, the brain relies upon a precise spatial arrangement of its circuitry, such that specific adja cent nerve endings on the skin transmit their signals through parallel neurons which terminate in specific adjacent cell bodies in the sensory cortex. The princi ple involved is like that used in modern fiber optics: If the glass fibers at the observer's end of the bundle are arranged as they are at the far end, he will receive a clear, unscrambled image of any object to,,:,{ard which the far end is ~~~ Weok stimulus - Model'llle stimulus Slronq stimulus Fig. 2-18: An area of skin, innervated by endings from several axons. The axons are gathered together into a nerve trunk, where they preserve their parallel arrangements throughout their full length. Thus, when three separate but simultaneous pin-pricks touch the skin, the. spatial relationship of the stimulated axODS in the trunk correspond to the arrangement of the pin-pricks on the surface of the skin. The principle is similar to fiber optics. The spatial relationships of the various parts of the periphery are, then, pro jected by their parallel nerve fibers and "mapped" onto corresponding areas ofthe cortex, where they are arranged as the familiar sensory homunculus. It is in this way that the brain separates functions and pinpoints locations throughout the body. For instance, in the dorsal column of the spinal cord (where all the sensory tracts are bundled together), the sensory fibers from the feet lie closest to the midline, and as we rise up the body and up the column, successively added sensory trunks add their fibers progressively toward the lateral sides of the dorsal column. This spatial organization is main tained with precision throughout the sensory pathway all the way to the somesthetic cortex. Likewise, the fiber tracts within the brain and those extending into motor nerves are spa tially oriented in the same way. 18 Fig, 2-19: The sensory cortex straddles the midsec tion of the upper surface of the brain. r r f ~ I'" 40 JOB'S BODY This same principle of parallel fiber arrangement produces a similar homun culus upon the motor cortex, and another upon the cortex of the cerebellum. These minianue maps of the body correspond not only to the peripheral ar rangement, but also to one another, so that parallel circuits not oniy link my actual hand to my sensory cortex "hand"; they also link this sensory "hand" t9 my motor cortex "hand" and to my cerebellar "hand" as well. EaclNnap 'corresponds point for point to all the others, and all are linked together by '--parallel circuits. At one time it was suggested by neural anatomists that such parallel pathways developed in the embryo before being assign~d to any specific functions, and that particular channels were later created by habitual usage, much like water wears a channel for a stream bed. Conflicting or extraneous paths would be eliminated by atrophy, and new channels could be re-routed in case of damage. Later it was postulated that these parallel circuits were established genetically in the developing brain and spinal cord, which then reached out from the central core with axons and nerve ends to contact the periphery. It was not clear just how the nerve ends knew exactly where to go; perhaps there was a good deal of randomness to their branching which the organism later sorted out by means of trial and error. More recently, it seems that neither of these earlier views may be the case. It~now appears that the organization of this parallel circuitry is actually initi aQ at the periphery. Local qualities in the skin, the joints, and the deep tissues "tag" the nerve ends which contact them with subtle chemical messages, and these chemical "tags" direct axons growing inward toward the appropriate con nections in the spinal cord and brain. The process is anything but random or '~'" trial and error. It is highly specific, and it is the periphery which helps to organize the connections in the central nervous system, not an organized central nervous system which reaches out to innervate the periphery. ,,' This more recent view that it is peripheral conditions which organize the '>~actual development of neural circuits and guide the process of mapping is of // central importance to bodywork. It suggests that the use of touch and sensation to modify our experience of peripheral conditions exerts an active influence upon the organization of reflexes and body image deep within the central nerv ous system. Such a view seems to be borne out by the following kinds of experi ments. If a patch of skin from the belly of a frog embryo is exchanged with a patch from his back, and the two patches are allowed to re-attach in their new locations and regenerate their nerve ends, a very curious thing happens. When I tickle the patch on the frog's belly, he rubs his back, and when I tickle the one on his back, he rubs his belly. In spite of the fact that the major sensory nerve trunks deep to the skin have not been disturbed, the frog confuses front and back when his transposed patches are stimulated. Nerve ends in the skin taken from the belly still somehow manage to excite the "belly" areas in the frog's sensory and motor cortexes, even though the signals must now go through circuits e, ~ ass t COl , I t & ~ f ( ho' shi car mv lim sta stir suI pn. nal "It W. the f } ( r I 1 SKIN n n. 1C associated with the back. The neural connections in the brain have some how shifted to accommodate the shifted ~reas of skin. These transplants can be repeated with many variations, involving various muscle sets, entire limbs and even the eyes. In every in stance, their results are similar stimulation of transplanted tissue re sults in reactions that would be appro priate if the tissue were still atits origi nal site, and not at the ~ new one. "It is necessary to conchicte," says R. W. Sperry, the author of a senes of these experiments, that the sensory fibers that made con nections with the grafted tissues must have been modified by the character of these tissues. . . . Growing freely into the nearest areas not yet in nervated the fibers established their peripheral terminals at random. Thereafter they must proceed to form 41 t To granule layer leading process of neuron Process of radial glial cell T railing process of neuron t From external granule layer Fig. 2-22: The growing sensory axon in the foetus follows the lead of a glial cell-its supportive and nutritive partner. Presumably, the glial cell carries chemical messengers from the skin which are matched against chemical tags al each ascending level up the spinal column and the brain, identifying the appropriate internal cOllnections. Each axon is then chemically coded to follow its particular glial cell. Fig. 2-23: An axon following its glial partner as they grow. Tbe nerve cell's nucleus is tbe large dark oblong shape. The glial cell is tbe grey band running from corner to corner diagonally, and the nerve's growing axon is next to it.