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A
B
C
Control
Homotopic
Heterotopic
SCG
T5
SCG
C8
C8
C8
T1
T1
T1
T2
T2
T2
T3
T3
T3
T4
T4
T4
T5
T5
T5
T5
T6
T6
T6
T7
T8
T7
T8
T7
T8
spinal cord
spinal cord
spinal cord
D
5
4
SCG
T5
3
2
1
0
C8
T1
T2
T3
T4
T5
T6
Ventral
FIGURE 6.9 Topographic input into the sympathetic chain. A. Electrophysiological studies show
that the SCG receives input from many roots but primarily the more anterior ones. The ganglion at T5
receives its primary inputs from more posterior roots. B. When the SCG is removed and replaced with
another SCG, the axons that reinnervate it tend to be from more anterior roots. C. When a T5 ganglion
is put in place of the SCG, its neurons still tend to get innervated by more posterior roots even though
the ganglion is in an anterior position. D. This topographic specificity of reinnervation is reflected in
the shape of the histogram of EPSP amplitudes as a function of nerve root stimulation for the homo-
totopic and heterotopic transplants. (After Purves et al., 1981)
afferents from a limited number of spinal cord seg-
ments. Thus, the superior cervical ganglion (SCG) is
primarily innervated by preganglionic afferents from
thoracic segments T1-T4, whereas the more caudally
located fifth thoracic ganglion (T5) is primarily inner-
vated by afferents from T4-T7 (Nja and Purves, 1977).
In one experiment, a T5 ganglion was transplanted to
different locations along the sympathetic chain, expos-
ing this target to afferents from a large range of spinal
cord segments (Purves et al., 1981). Selective reinner-
vation was then assessed electrophysiologically. The
sympathetic chain ganglia were dissected out along
with the ventral nerve roots through which all pre-
ganglionic fibers course from the ventral spinal cord.
Stimulating electrodes were then placed on the ventral
roots from each spinal cord segment, and an intracel-
lular recording was obtained from the reinnervated T5
ganglion. The spinal segments that innervate each T5
neuron were thus recorded. The results clearly indi-
cated that T5 neurons were selectively reinnervated by
their original spinal segments (Figure 6.9). This was
not merely an artifact of the host transplantation site
because when the SCG was placed in the same loca-
tion, it, too, became reinnervated by its original set
of afferents. These experiments strongly suggest that
axons from different rostrocaudal levels can distin-
guish individual sympathetic ganglion cells, which
must also carry some label of their rostrocaudal origin.
C HEMOSPECIFICITY AND EPHRIN S
In the early 1940s, Roger Sperry cut the optic nerve
of a newt, rotated the detached eye 180° in its orbit, and
assayed the visuomotor behavior of the animal after its
nerve had regenerated. The newts, and in subsequent
studies, frogs, behaved as if their visual world were
back-to-front and upside-down: when a lure was pre-
sented in front of them, “they wheeled rapidly to the
rear instead of striking forward” and when the lure was
presented above “the animals struck downward in
front of them and got a mouthful of mud and moss”
(Sperry, 1943) (Figure 6.10). This led him to propose
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