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THE THIRD DIMENSION,
L AMINA-SPECIFIC TERMINATIO N
A
B
Normal
Trunk
Arm
Trunk
Face
Face
Many parts of the nervous system are layered struc-
tures like the tectum and the cortex, and innervating
axons must not only map to their correct topographic
position in two dimensions, but they must also find the
appropriate layer in which to synapse, making target-
ing a three-dimensional problem. Lamina-specific
targeting may involve a variety of different issues. In
many cases, a laminated target is composed of layers
of physiologically and molecularly distinct cells types,
and the innervating axons must therefore choose
between different cell types, possibly based on chem-
ical differences. In some cases, the layers are composed
of essentially similar cells, but the innervating axons,
through an activity-based competition, segregate them
into layers. This latter case can be considered an
example of the refinement of synaptic connections and
so will be dealt with in Chapter 9.
We have already encountered the first kind of the
laminated structure in the central projections of DRG
fibers in the spinal cord. These axons enter the spinal
cord and make synapses in various laminae of the
dorsal horn or ventral gray matter depending on their
modality. For instance, stretch receptors make monosyn-
aptic contact with motor neurons in the ventral horn.
In contrast, pain and temperature sensory fibers inner-
vate neurons in dorsal laminae of the spinal cord
(Figure 6.21). The result of this laminar arrangement by
different types of input is that somatosensory modali-
ties sort out in the spinal cord and so make a multilay-
ered registered map, such that a column of cells in the
spinal cord represents one area of the body with differ-
ent modalities at different depths. Multimodal, layered
maps are used in several places in the nervous system.
Why do only stretch receptors penetrate the more
ventral layers of the spinal neuropil? In the previous
chapter, we described the varying sensitivity of differ-
ent classes of neurons to the repulsive effects of sema-
phorin3A, which is expressed in the ventral layers only.
Semaphorin3A repulses pain receptors and thermore-
ceptors, which therefore map to dorsal layers, while
stretch receptors ignore Semaphorin3A and map to
ventral layers (Messersmith et al., 1995). In mice in
which the Semaphorin3A gene is knocked out, the ter-
minals of the pain and thermoreceptive axonal termi-
nals appear to extend into the ventral regions of the
spinal cord, similar to the stretch receptors and this
layer-specific targeting is abolished (Taniguchi et al.,
1997; Catalano et al., 1998) (Figure 6.21).
The cerebral cortex is another example of a highly
laminated structure, composed of different cell types in
Normal
Amputee
C
D
E
T
T
T
Sprouting
into arm
area of
cortex
A
Cortex
F
F
F
T
T
T
A
A
Thalamus
F
F
F
Arm area
of thalamus
degenerates
All connections
intact
Peripheral arm
amputation; DRG
neurons survive
Central amputation
or spinal injury;
DRG axons
degenerate
FIGURE 6.20 Large-scale plasticity in the somatosensory cortex.
A. The normal organization of the cortical topography in the
somatosensory system. B. Damage to the arm may result in large-
scale reorganization of the cortical map. C. There is an isomorphic
mapping of the somatosensory thalamus onto the primary cortex. D.
When the arm is damaged peripherally and the sensory neurons in
the DRG survive, the reorganization is cortical rather than thalamic.
E. When the damage is more central causing the DRG cells to degen-
erate, both the thalamus and cortex get reorganized. (After
Merzenich, 1998)
representation of the neighboring fingers expands
(Merzenich and Jenkins, 1993). These cortical changes
in the representation of somatotopy can be extremely
large even in normal animals, as was revealed by
an unusual experiment at the National Institutes of
Health. Antivivisectionists stole a set of experimental
monkeys after their somatosensory cortex was first
mapped, and they were not recovered until about 10
years later. When the scientists remapped their
somatosensory cortex, they found that the extent of the
rearrangement was dramatic, a matter of tens of mil-
limeters (Palca, 1991). Thus, minor reorganization of
the cortical somatosensory map is happening through-
out life, presumably influenced by experience and
activity. Even normal use can change topographic rep-
resentations in an impressive way. Monkeys trained to
use just one finger to feel textural differences for
a few hours a day over a period of months have a
hugely expanded cortical representation of that finger
(Recanzone et al., 1992).
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