Healthcare and Medicine Reference
In-Depth Information
showed that the crossed nerves, over the course of
time, had managed to uncross themselves and find
their original muscles again (Mark, 1969). Competition
experiments between original and foreign nerves for
the innervation of particular muscles show that the
original nerves always have an advantage (Dennis and
Yip, 1978). Thus axons, although they will innervate
inappropriate denervated targets when their own
targets are unavailable, seem to have a natural prefer-
ence for their own original targets.
late 1800s John Langley discovered that superior cervi-
cal ganglion (SCG) neurons mediate reflexes in a topo-
graphic manner (Langley, 1897, 1985). When Langley
stimulated the first or top thoracic root to the ganglion
in the rat, this activated ganglion cells that caused dila-
tion of the pupil. When he stimulated the fourth tho-
racic root to the ganglion, the blood vessels of the ear
constricted. This suggested that there was some sort
of topographical organization within the SCG. All
reflexes were immediately lost when the preganglionic
nerve to the SCG was cut, but the fibers reinnervated
the SCG in several weeks, as peripheral nerves often do
in mammals, and the autonomic reflexes recovered.
The surprising discovery was that the connections
reformed with such precision that all reflexes were
reestablished accurately (Figure 6.8). This result sug-
gested that individual SCG neurons have some mech-
anism that enables the regenerating preganglionic
fibers to distinguish one SCG neuron from another.
The sympathetic chain ganglia provide a simple
system in which to examine somatotopic specificity
because each ganglion is selectively innervated by
TOPOGRAPHIC MAPPING
In many targets there is a topographical relationship
between the position of the innervating neuron and the
position of its terminal arbor in the target field. A good
example is the visual system. RGCs in a particular
position in this retina are maximally stimulated from
a region of the visual world, and these cells send their
axons to a particular region in the tectum. Neurons in
neighboring retinal positions send their projections to
neighboring regions in the tectum. This orderly pro-
jection preserves visual topography in the brain and
provides a neuroanatomical basis for the contiguity of
perceived visual space. Similarly, most central audi-
tory nuclei have a representation of the cochlea's
frequency axis. Such maps may be referred to by the
anatomical substrates that they preserve (e.g., retino-
topic, cochleotopic) or by more perceptual terms (e.g.,
visuotopic or tonotopic). Even simple animals like the
nematode, with only 301 neurons, have ordered arrays
of sensory receptors that make somatopically organ-
ized central projections. These help them respond
appropriately to stimuli that strike the animal from
different directions. There is a second type of neural
map, a computational map, which can be revealed by
recording from neighboring single nerve cells in vivo.
What is represented in such maps is not obvious from
the anatomy of the connections, yet these maps may
also display orderly representations of a physical
parameter. For example, in the auditory system, we
find nuclei that display topography of sound source
location, even though the ear contains only a one-
dimensional array of spiral ganglion cells representing
sound frequency. Such maps are constructed from cells
that extract information and are referred to by the
functional characteristic that they encode (e.g., map of
auditory space) rather than a piece of tissue. There are
also stranger maps, such as maps of smell that we will
discuss below.
What is the developmental basis for the establishing
topographic projections in the nervous system? In the
Blood vessels
in ear
Vasoconstriction
A
B
C
SCG
Pupil dilation
SCG
S CG
cut
T1
regenerate original
connections and
behaviors
T2
Spinal
cord
Spinal
cord
T3
T4
FIGURE 6.8 Regeneration of topographic specificity. A.
Langley's classic study showed that stimulation of preganglionic
root T4 relayed through ganglion cells in the SCG that caused vaso-
constriction of the ear pinna vessels, whereas stimulation of root T1
excited other SCG neurons that caused dilation of the pupil. B. When
Langley cut the sympathetic tract above T1, all these sympathetic
reflexes were abolished, but with time they recovered. C. Shows the
specificity associated with this regeneration, such that the axons that
enter the chain at T4 reinnervate the SCG cells that cause ear vaso-
constriction, and the axons that enter at T1 reinnervate the cells that
cause pupil dilation. (After Langley, 1897, 1985)
Search Pocayo ::




Custom Search