124
UNIT 2
Basic Tissues
Figure 7-7A.
Node of Ranvier between two adjacent Schwann cells.
EM, scale bar = 1.0
μ
m;
3
14,000
The
node of Ranvier
is a region
between two Schwann cells
where the axon membrane lacks a thick coating of insulating myelin.
This region is therefore able to carry out the complicated exchange of sodium and potassium ions across the membrane that is the
basis of the conduction of the action potential. Zones of
paranodal cytoplasm
are visible in this section. Each layer of the spiral
wrapping of myelin is associated with one of these zones, which provides metabolic support for the attached thin membranous layer
of myelin. Cytoplasm enclosed in the
incisures (clefts) of Schmidt-Lanterman
plays a similar metabolic support role to the myelin at
various points along the length of the myelin coating.
Exposed axon membrane
Exposed axon membrane
Axon
Axon
Node of Ranvier
Exposed axon membrane
Paranodal cytoplasm
Myelin
Axon
A
Major dense line
Schwann cell
cytoplasm
Schwann cell
nucleus
Microtubules
Myelin sheath
Position of inset
Axon
Neurofilaments
Unmyelinated axons
B
Figure 7-7B.
Myelinated and unmyelinated axons.
EM,
3
61,000
A medium-sized
myelinated axon
together with its associated
Schwann cell
and
myelin sheath
is illustrated in the center of this
photomicrograph. The
small rectangle
shows the position of the
inset
. The
higher power inset
shows the individual layers of the
myelin sheath, indicated by
major dense lines
.
Microtubules
are important in transporting neurotransmitters and other materials
from the cell body to the axon terminals (
anterograde transport
) and for transporting other materials (e.g., growth factors) from the
axon terminals back to the cell body (
retrograde transport
). Both microtubules and
neuro±
laments
are part of the cytoskeleton and
help maintain the structural integrity of the neuron. In the lower right-hand corner of the illustration, several small,
unmyelinated
axons
are lying in grooves in the body of a single Schwann cell.
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