Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

channels and the electrical properties of membranes
625
l
~1 mm
mature myelin sheath
myelin
sheath
nodes of Ranvier
layers of
myelin
axon
nucleus
axon
(A)
axon
axon
glial cell
(C)
node of
Ranvier
myelin sheath
inactivate specific neurons. It is therefore now possible to transiently activate or
inhibit specific neurons in the brains of awake animals with remarkable spatial
and temporal precision. In this way, the rapidly expanding new field of optogenetics
is revolutionizing neurobiology, allowing neuroscientists to analyze the
neurons and circuits underlying even the most complex behaviors in experimental
animals, including nonhuman primates.
Myelination Increases the Speed and Efficiency of Action Potential
Propagation in Nerve Cells MBoC6 m11.32/11.34
The axons of many vertebrate neurons are insulated by a myelin sheath, which
greatly increases the rate at which an axon can conduct an action potential. The
importance of myelination is dramatically demonstrated by the demyelinating
disease multiple sclerosis, in which the immune system destroys myelin sheaths in
some regions of the central nervous system; in the affected regions, nerve impulse
propagation greatly slows or even fails, often with devastating neurological consequences.
Myelin is formed by specialized non-neuronal supporting cells called glial
cells. Schwann cells are the glial cells that myelinate axons in peripheral nerves,
and oligodendrocytes do so in the central nervous system. These myelinating
glial cells wrap layer upon layer of their own plasma membrane in a tight spiral
around the axon (Figure 11–33A and B), thereby insulating the axonal membrane
so that little current can leak across it. The myelin sheath is interrupted at regularly
spaced nodes of Ranvier, where almost all the Na + channels in the axon are concentrated
(Figure 11–33C). This arrangement allows an action potential to propagate
along a myelinated axon by jumping from node to node, a process called
saltatory conduction. This type of conduction has two main advantages: action
potentials travel very much faster, and metabolic energy is conserved because the
active excitation is confined to the small regions of axonal plasma membrane at
nodes of Ranvier.
(B)
1 µm
Figure 11–33 Myelination.
(A) A myelinated axon from a peripheral
nerve. Each Schwann cell wraps its
plasma membrane concentrically around
the axon to form a segment of myelin
sheath about 1 mm long. For clarity,
the membrane layers of the myelin are
shown less compacted than they are
in reality (see part B). (B) An electron
micrograph of a nerve in the leg of a
young rat. Two Schwann cells can
be seen: one near the bottom is just
beginning to myelinate its axon; the
one above it has formed an almost
mature myelin sheath. (C) Fluorescence
micrograph and diagram of individual
myelinated axons teased apart in a rat
optic nerve, showing the confinement
of the voltage-gated Na + channels
(green) in the axonal membrane at the
node of Ranvier. A protein called Caspr
(red) marks the junctions where the
myelinating glial cell plasma membrane
tightly abuts the axon on either side of
the node. Voltage-gated K + channels
(blue) localize to regions in the axon
plasma membrane well away from the
node. (B, from Cedric S. Raine, in Myelin
[P. Morell, ed.]. New York: Plenum, 1976;
C, from M.N. Rasband and P. Shrager,
J. Physiol. 525:63–73, 2000. With
permission from Blackwell Publishing.)