206
The Body Electric
point of inhibiting seizures, the random spread of impulses in the brain.
They may also have an important part in memory, and they probably
conduct the direct currents so important to regeneration. They're essen-
tial to healing wherever it occurs in nerve tissue.
Peripheral Nerves
Peripheral nerve fibers can regrow—otherwise we'd lose sensation when-
ever we cut a finger—but neurons and their fibers in the CNS cannot.
The peripheral nerve's cell body survives, safe in the cord or brain, and
the cut end of the attached part of the fiber is sealed off. The outer,
severed part dies and degenerates; some of its Schwann cells digest it,
along with the now-useless myelinated membrane layers. The empty
Schwann tube remains, however, and begins to grow toward the prox-
imal fiber (the one nearer the center of the body), whose Schwann cells
are also growing across the chasm. When these cells meet, the nerve
fiber grows along its reconnected sheath and eventually makes contact
with the same terminals it originally served.
In salamanders this process is very efficient. The Schwann cells can
cross large gaps, and an experimenter who wishes to work with dener-
vated limbs must be diligent to keep the nerves from reentering. In
humans, the two ends of the tube usually can't find each other over a
distance of more than a centimeter. In that case the proximal sheath
with its intact nerve fiber hunts for its opposite number by growing in
an ever increasing spiral, apparently searching for some signal from the
distal end (the part farther from the body center). Since each nerve is
formed of many fibers, these spiraling tubes entangle each other in a
lump of nerve tissue called a neuroma. Neuromas are painfully sensitive
and often must be cut away. Occasionally a surgeon can move the two
ends of the nerve close enough for the Schwann cells to make contact. If
that's impossible, the gap may be closed with a piece of nerve grafted
from a less important peripheral nerve that can be sacrificed. Unfor-
tunately, nerve grafts don't take reliably, and other methods, such as
making artificial channels with tiny plastic tubules, are still in the ex-
perimental stage.
We don't know why the salamander's peripheral nerve regrowth is so
much more effective than ours, but I surmise that its more efficient DC
electrical system accounts for the difference. If the locator signal is elec-
trical, it should be possible to augment it in humans so as to grow nerve
fibers over longer distances. Beginning with a 1974 report from David