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The Body Electric
from the regulating influence of the brain. For the same reason, the
broken bones of paraplegics heal in half the normal time, whereas a bone
will heal very slowly or not at all if its peripheral nerve supply has been
cut. Only the communication between brain and spine is silenced in
paraplegia, and that makes all the difference.
Spinal fibers do reconnect in some animals, notably goldfish and, as
you might expect, salamanders. Their ability seems to decline dramati-
cally with age, however. Jerald Bernstein, a neurophysiologist now at
George Washington University Medical School who has studied goldfish
spinal regeneration extensively, has found that one-year-old fish heal al-
most all of the damage. This competence declines to about 70 percent at
two years and 50 percent at three. Since salamanders aren't raised in
biological supply houses but rather collected from the wild, any group is
likely to include young and old individuals, making comparisons diffi-
cult. In our lab we found that cord regeneration isn't uniform in sala-
manders, probably due to age differences.
Maturity may reduce the response of the ependymal cells, which are
responsible for the first step. They proliferate outward from the central
canal and bridge the gap in a few days. Marc Singer, in a recent study of
this process, concluded that the ependymal cells extend "arms" radiating
outward, which line up like the spokes of wheels stacked one atop an-
other, forming channels for the regrowing fibers to follow. The nerves
then reestablish their continuity within a few weeks.
SALAMANDER SPINAL-CORD REGENERATION
Bernstein also found that there's a critical period during which re-
growth must be completed or it will fail. After cutting the cords of
goldlfish, he inserted Teflonon spacers to block regeneration. The normal