392
UNIT 3
Regulation and Integration of the Body
11
mechanisms to distribute them. Axons quickly decay if cut or
severely damaged.
Transport Along the Axon
Because axons are oFen very long,
the task of moving molecules along their length might appear
difficult. However, through the cooperative efforts of motor
proteins and cytoskeletal elements (microtubules and actin
filaments), substances travel continuously along the axon both
away from and toward the cell body. Movement away from the
cell body is
anterograde movement
, and that in the opposite di-
rection is
retrograde movement
.
Substances moved in the anterograde direction include mi-
tochondria, cytoskeletal elements, membrane components used
to renew the axon plasma membrane, and enzymes needed to
synthesize certain neurotransmitters. (Some neurotransmitters
are synthesized in the cell body and then transported to the
axon terminals.)
Substances transported through the axon in the retro-
grade direction are mostly organelles returning to the cell
body to be degraded or recycled. Retrograde transport is also
an important means of intracellular communication to “ad-
vise” the cell body of conditions at the axon terminals, and
deliver to the cell body vesicles containing signal molecules
(like nerve growth factor, which activates certain nuclear
genes promoting growth).
One basic bidirectional transport mechanism appears to be
responsible for axonal transport. It uses ATP-dependent “mo-
tor” proteins (kinesin and dynein), depending on the direction
of transport. ±ese proteins propel cellular components along
the microtubules like trains along tracks at speeds up to 40 cm
(15 inches) per day.
Homeostatic Imbalance
11.1
Certain viruses and bacterial toxins that damage neural tissues
use retrograde axonal transport to reach the cell body. ±is
transport mechanism has been demonstrated for polio, rabies,
and herpes simplex viruses and for tetanus toxin. Researchers
are investigating using retrograde transport to treat genetic dis-
eases by introducing viruses containing “corrected” genes or
microRNA to suppress defective genes.
Myelin Sheath
Many nerve fibers, particularly those that
are long or large in diameter, are covered with a whitish, fatty
(protein-lipoid), segmented
myelin sheath
(mi
9
ĕ-lin). Myelin
protects and electrically insulates fibers, and it increases the
transmission speed of nerve impulses.
Myelinated fibers
(ax-
ons bearing a myelin sheath) conduct nerve impulses rapidly,
whereas
nonmyelinated fibers
conduct impulses more slowly.
Note that myelin sheaths are associated only with axons. Den-
drites are
always
nonmyelinated.
Myelination in the PNS
Myelin sheaths in the PNS are formed
by Schwann cells, which indent to receive an axon and then
wrap themselves around it in a jelly roll fashion
(Figure 11.5)
.
Initially the wrapping is loose, but the Schwann cell cytoplasm is
gradually squeezed from between the membrane layers.
When the wrapping process is complete, many concentric
layers of Schwann cell plasma membrane enclose the axon,
endoplasmic reticulum and a Golgi apparatus, the structures
involved with protein synthesis and packaging. Consequently,
an axon depends (1) on its cell body to renew the necessary pro-
teins and membrane components, and (2) on efficient transport
(a) Myelination of a nerve fiber (axon)
Schwann cell
cytoplasm
Axon
Schwann cell cytoplasm
Myelin
sheath
Schwann cell
nucleus
Schwann
cell plasma
membrane
1
2
3
A Schwann cell
envelops an axon.
The Schwann cell
then rotates around
the axon, wrapping
its plasma membrane
loosely around it in
successive layers.
The Schwann cell
cytoplasm is forced
from between the
membranes. The tight
membrane wrappings
surrounding the axon
form the myelin
sheath.
Myelin sheath
Outer collar
of perinuclear
cytoplasm
(of Schwann
cell)
Axon
(b) Cross-sectional view of a myelinated axon (electron
micrograph 24,000
m
)
Figure 11.5
Nerve fiber myelination by Schwann cells in
the PNS.
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