The Self-Mending Net
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trical problems in spinal healing may be tackled sooner than in other
fields.
The public imagination has been captured by the computerized mus-
cle-stimulation techniques being developed by Jerrold Petrofsky, an en-
gineer at Wright State University in Dayton. The nationally televised
sight of his patient Nan Davis and other paraplegics taking tentative
steps and pedaling tricycles with their own muscle power was tremen-
dously exciting. But if we can get the body to do the same things by
itself, that will be even better. Any amount of regeneration would only
make other techniques more effective. Even restoring 10 percent of lost
function would be an unimaginable blessing to those who are now help-
less. I feel the electrical manipulation of spinal shock must be tested
vigorously now, for this is perhaps the one area where the barriers of
tragedy are closest to being broken.
The Brain
It might seem foolish to expect any regeneration in the most complex of
all biological structures, the brain, yet salamanders, some fish, and most
frogs in the tadpole stage can replace large parts of it, including the
optic lobes and the olfactory lobes, or forebrain, the part from which our
prized cerebral hemispheres developed in the course of evolution. Re-
placement depends on ingrowth of remaining sensory nerves, the olfac-
tory nerves in the case of the forebrain and the optic nerves for the optic
lobes. When these nerves grow back into the area where brain has been
destroyed, they stimulate the ependymal cells in the brain ventricles,
which proliferate outward into the damaged part and then differentiate
into new neurons and glial cells. If the animal's nose or eyes are removed
so that the injury zone receives no nerve input, no regeneration occurs.
Thus brain regrowth begins much like that of limbs, with the connec-
tion of nerve fibers to an epithelial tissue. The ependyma, remember, is
embryologically a close relative of the epidermis, and in fact can be
considered the central nervous system's "inner skin." Since the electrical
environment produced by the neuroepidermal junction is what stimu-
lates cells to dedifferentiate and divide in the salamander limb stump,
and since we started limb regeneration in the rat by crudely mimicking
this signal, it seems likely that a similar stratagem could induce brain
regeneration in animals normally lacking this ability.
A form of shock, called the spreading depression of Leao after its
discoverer, neurologist A. A. P. Leao, occurs after brain injuries. Start-