From: Max More (maxmore@primenet.com)
Date: Fri Jan 09 1998 - 01:22:42 MST
19. REGENERATION OF AXONS IN CENTRAL NERVOUS SYSTEM WHITE MATTER
When examining the gross anatomy of the mammalian brain and
spinal cord, a striking feature is the presence of large regions
with an opalescent ivory color. The color is due to myelin, the
substance that sheaths many nerve fibers in the central nervous
system. In the vertebrate central nervous system, the axons of
nerve cells involved in physiological functions that require
rapid signaling (for example, the neural control of voluntary
muscle) are wrapped in myelin with a special consequence. The
myelin sheath consists of concentric layers of electrically
insulating lipid material, but the sheath is periodically
interrupted, and at the points where the sheath is interrupted so
is the electrical insulation interrupted. The result, predictable
from the classical physics of electrical transmission lines and
the electrical parameters of nerve fibers, is that the propagat-
ion of an electrical pulse along such nerve fibers occurs at a
velocity much higher than that found in unmyelinated fibers.
Glial cells are cells of the central and peripheral nervous
system that metabolically support neurons and produce the
multiple membrane layers called myelin and enfold nerve cell
axons with it. The glial cells are found everywhere in the brain
and spinal cord, and one result of a localized injury to the
central nervous system is a local proliferation of glial cells to
form a scar matrix. Concerning brain and spinal cord injury, it
has always been a canon of neurobiology that adult central
nervous system neurons cannot regenerate after injury to re-
establish the connections to other cells necessary for proper
functioning. Davies et al (6 authors at 2 installations, US UK),
using microtransplantation techniques, now report that adult
central nervous system white matter can support long-distance
regeneration of adult axons provided the reactive glial
extracellular matrix at the site of the lesion can be bypassed.
The authors suggest this is the first time this glial barrier to
axon regeneration has been noted.
QY: Jerry Silver <jsx10@po.cwru.edu> (Nature 18/25 Dec 97)
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