Healthcare and Medicine Reference
In-Depth Information
A
A
Motor area
leg
1
A1
arm
A
2
5
3
4
B
5
a
C
D
E
B1
2
3
4
7
b
g
6
C1 2
8
3
5
8
face
1
4
7
D1 2
6
3
5
4
8
7
d
6
E1 2
5
3
4
B Extra whisker in row C
A1
2
3
4
5
a
B1
2
8
Extra barrel
3
4
7
b
g
6
C1 2
B
5
3
4
8
7
6
1
2 3 4 5 6 7 8
D1 2
3
5
4
8
7
d
6
E1 2
3
4
5
C Damaged whisker B3
Missing barrel
A1
2
3
4
5
a
B1
2
4
7
b
g
6
C1 2
3
5
8
4
7
1 2 3 4
D1 2
6
3
5
4
8
7
6
d
E1 2
5
3
4
D Two whiskers tied together in row D
A1
2
3
4
5
a
B1
2
3
4
7
b
g
6
C1 2
3
5
8
4
7
6
1
2 3 4 5 6 7 8
D1 2
3
5
4
8
7
FIGURE 6.18 Somatotopic representation in the cortex. A. The
motor area of the precentral gyrus of the cerebral cortex was stimu-
lated electrically in human patients during neurosurgery. B. A
“homunculus” of the body on the motor cortex illustrates the
sequence of representation as well as the disproportionate repre-
sentation given to the various muscles involved in skilled move-
ments. (After Penfield, 1954b)
d
6
E1 2
3
5
4
Fused barrel field
FIGURE 6.19 Plasticity of the mouse barrel field in the
somatosensory cortex. A. The correspondence between bristles and
barrel field in the cortex of a normal mouse. B. An extra whisker in
row C leads to the formation of an extra barrel in the appropriate
location in the cortex. C. Neonatal damage to the B3 whisker causes
the shrinkage of this barrel and the expansion of neighboring ones.
D. Tying two whiskers together causes their barrel field to coalesce.
(After Woolsey and Van der Loos, 1970)
shrinks, while the neighboring barrels expand into the
territory of the cortex originally devoted to the cauter-
ized whisker (Dietrich et al., 1981; Simons et al., 1984)
(Figure 6.19). When two whiskers are glued together,
their cortical barrels fuse. Perhaps the most surprising
finding is the case of a mouse that was born with an
extra whisker, as sometimes happens. This mouse had
an extra barrel in its cortex (Van der Loos et al., 1984).
From these results, it is clear that the neural represen-
tation of the body surface has flexibility in its structure.
The sensory fields themselves and their activity guide
this flexibility.
When an adult, through accident or medical inter-
vention, loses sensation in one area of the body as
happens when a peripheral nerve is cut, the cortical
representation of that area may be invaded by repre-
sentation from neighboring parts. This is thought to be
one reason why people who have lost a limb may
report sensations in the phantom limb, especially
when a part of the body is touched whose cortical
representation is adjacent to the missing limb
(Ramachandran and Rogers-Ramachandran, 2000). A
touch to the face in such a person can be experienced
as a touch on the missing hand. The explanation is that
nerve fibers that carry information about touch on the
face invade the neighboring cortical area that used to
receive such information from the lost limb. The rest
of the brain, however, has not yet “learned” the change
in the meaning of the input to this part of the cortex,
and still interprets it as a touch to the hand (Figure
6.20). Experiments with monkeys, in which a cuff of
TTX on the nerve temporarily paralyzes a single finger,
have shown that there is a rapid reorganization of the
somatosensory map in the cortex. Within days, the rep-
resentation of the insensitive finger shrinks, and the
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