Diane
24-03-2004, 11:04 PM
Hi there,
On another forum just now there is a discussion going on about this topic:
"For many years, people who suffer a phantom lower limb have reported that when they urinate they can feel the sense of urination in the phantom foot. (Many probably kept it quiet for fear of people thinking they were mad). There are also reports of the sensation of orgasm being experienced in the foot...
How could this happen?"
The discussion continues, regarding homonculi, the 'smudging' thereof, the placement of parts in the representations of the brain. One participant wonders why the feet are located so close to the genitals in the brain map, when anatomically they are located at a long distance from each other.
Mysteries like this begin to make more sense when embryology is considered (I wish it was taught properly to PTs in PT school, as part of core cirriculum.) The premise is that an nerve, any nerve, has two ends, one in the body somewhere and one in the brain somewhere; if not the same actual continuous nerve, then some other nerve it can trust to get the message through. Skin = brain = nerves = ectoderm.
Please bear with me while I set the stage:
As the embryonic body unfolds, the anus is already in place. It has formed right from the start (proctodeum) along with the mouth (stomodeum). These two points of the body seem to function as poles; maybe the body has to have some areas act as 'beacons' so that other bits know where to migrate to.
Nothing much else happens midline bottom for a long long time. The embryo is very busy forming the rest of the gut tube, and from the same tissue layer form the lungs, and uterus/prostate among other surprises... the intestine is forming outside the body cavity in the umbilical cord, and will take its place inside when all is ready and large enough. The neural tube is forming and the top end is burgeoning and folding over into what will soon be a brain. Other systems are forming, morphing, changing, all in a beautiful and dazzling orchestration.
At the bottom of the body, the body thinks its work is done at about T12. But wait a minute, looks like this body will have to have an functional extension. More vertebrae coalese and the body lengthens out some more. The abs have to stretch downward, so do the ureters, and paired reproductive organs migrate down to the pelvis obliquely along some specialized connective tissue that later serve to support/suspend them. (The bladder forms later out of morphed, recycled umbilical cord parts.) Testicles have to hurry so they get out in time, before the ab wall/inguinal ligaments close off their gateway. Ovaries don't mind taking their own sweet time, they aren't interested in going outside. The external genitalia is being built around the same time as little limb buds, which later become feet, poke out of the body wall at the lower end.
Here's the pertinent part: The feet and external genitalia all start out with the same "skin"...(i.e.: siblings at the same time, in the same family of innervation.) Skin and brain both derive from ectoderm.
The feet emerge from the body wall, straight out to the sides, one foot on each side of midline, i.e.: externa genitalia, and then take off from the body, carrying their blood supply and nerve cables embedded around their soles like little stirrups. The cabling grows at equal speed, the bones and muscles form a bit later. The actual maturation of the legs takes a long time. Ever looked at a newborn? Their legs are still abducted out in a fetal position. It takes awhile before the legs internally rotate and extend and adduct into functional adult position.
Meanwhile, up in the brain end of the body, the skin outside is finding representation inside, on the sensory homonculus. Although what I am going to say next has yet to be established and irrefrutable, I would lay odds on the homoncular representations being like concentric circles, the innermost concentric circle being the foot and the successive concentric circles being the rest of the leg in segments, with the outermost circle being the hip. Perhaps the circles are quite elliptical, and have one edge where they all touch, and make contact with their old "sibling" the representation for the midline external genitalia. To me, this makes sense when I consider the top down path of the obturator nerve, all the branches it throws off inside the pelvis, and how far down the inside of the leg it travels.
The point of this little story is, that if one takes a bit of time to learn some small overview of embryology, a lot of seeming mysteries are a lot easier to understand.
Comments?
Cheers,
Diane
On another forum just now there is a discussion going on about this topic:
"For many years, people who suffer a phantom lower limb have reported that when they urinate they can feel the sense of urination in the phantom foot. (Many probably kept it quiet for fear of people thinking they were mad). There are also reports of the sensation of orgasm being experienced in the foot...
How could this happen?"
The discussion continues, regarding homonculi, the 'smudging' thereof, the placement of parts in the representations of the brain. One participant wonders why the feet are located so close to the genitals in the brain map, when anatomically they are located at a long distance from each other.
Mysteries like this begin to make more sense when embryology is considered (I wish it was taught properly to PTs in PT school, as part of core cirriculum.) The premise is that an nerve, any nerve, has two ends, one in the body somewhere and one in the brain somewhere; if not the same actual continuous nerve, then some other nerve it can trust to get the message through. Skin = brain = nerves = ectoderm.
Please bear with me while I set the stage:
As the embryonic body unfolds, the anus is already in place. It has formed right from the start (proctodeum) along with the mouth (stomodeum). These two points of the body seem to function as poles; maybe the body has to have some areas act as 'beacons' so that other bits know where to migrate to.
Nothing much else happens midline bottom for a long long time. The embryo is very busy forming the rest of the gut tube, and from the same tissue layer form the lungs, and uterus/prostate among other surprises... the intestine is forming outside the body cavity in the umbilical cord, and will take its place inside when all is ready and large enough. The neural tube is forming and the top end is burgeoning and folding over into what will soon be a brain. Other systems are forming, morphing, changing, all in a beautiful and dazzling orchestration.
At the bottom of the body, the body thinks its work is done at about T12. But wait a minute, looks like this body will have to have an functional extension. More vertebrae coalese and the body lengthens out some more. The abs have to stretch downward, so do the ureters, and paired reproductive organs migrate down to the pelvis obliquely along some specialized connective tissue that later serve to support/suspend them. (The bladder forms later out of morphed, recycled umbilical cord parts.) Testicles have to hurry so they get out in time, before the ab wall/inguinal ligaments close off their gateway. Ovaries don't mind taking their own sweet time, they aren't interested in going outside. The external genitalia is being built around the same time as little limb buds, which later become feet, poke out of the body wall at the lower end.
Here's the pertinent part: The feet and external genitalia all start out with the same "skin"...(i.e.: siblings at the same time, in the same family of innervation.) Skin and brain both derive from ectoderm.
The feet emerge from the body wall, straight out to the sides, one foot on each side of midline, i.e.: externa genitalia, and then take off from the body, carrying their blood supply and nerve cables embedded around their soles like little stirrups. The cabling grows at equal speed, the bones and muscles form a bit later. The actual maturation of the legs takes a long time. Ever looked at a newborn? Their legs are still abducted out in a fetal position. It takes awhile before the legs internally rotate and extend and adduct into functional adult position.
Meanwhile, up in the brain end of the body, the skin outside is finding representation inside, on the sensory homonculus. Although what I am going to say next has yet to be established and irrefrutable, I would lay odds on the homoncular representations being like concentric circles, the innermost concentric circle being the foot and the successive concentric circles being the rest of the leg in segments, with the outermost circle being the hip. Perhaps the circles are quite elliptical, and have one edge where they all touch, and make contact with their old "sibling" the representation for the midline external genitalia. To me, this makes sense when I consider the top down path of the obturator nerve, all the branches it throws off inside the pelvis, and how far down the inside of the leg it travels.
The point of this little story is, that if one takes a bit of time to learn some small overview of embryology, a lot of seeming mysteries are a lot easier to understand.
Comments?
Cheers,
Diane