Eleven
The Self-Mending Net
Spinal paralysis is the most devastating of injuries and also one of the
commonest; it afflicts over half a million Americans, including fifteen
thousand new sufferers every year. Until recently their outlook was abso-
lutely bleak, for the human central nervous system (CNS) had no known
regenerative capacity whatsoever. Only if part of the spinal cord re-
mained unsevered was some recovery possible with physical therapy.
Now, however, there's hope that we'll soon be able to coax nerve cells
into reestablishing the proper connections across the damaged section
and thus return the use of arms, legs, sexual and excretory organs, respi-
ratory muscles, and the sense of touch to quadriplegics and paraplegics.
In one way or another, this dream involves making human nerve cells
behave more like those in simpler animals.
The neuron is the basic unit of all nervous systems. It consists of a cell
body, containing the nucleus and metabolic organelles, surrounded by
dozens of filaments that carry messages in and out. The incoming den-
drites predominate in sensory neurons. There's usually only one motor
fiber, or axon, which carries the neuron's outgoing messages to dendrites
of other neurons or to receptors on muscle or gland cells. An axon, often
several feet long, is the principal fiber of a motor neuron, which relays
orders from the brain or spinal cord to the tissues and organs.
All neuron cell bodies reside in the brain and spinal cord. Only their
axons and dendrites extend outward, forming the peripheral nerves that
connect every part of the body with the CNS. Other fibers connect cer-
tain sensory and motor neurons within the spinal cord, creating reflex
arcs, like those that jerk our hands from hot stows without our having