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ination, or in other words decreased branch stability
(Schmidt and Buzzard, 1990).
The mechanisms by which activity may have such
effects on the fine-tuning of connections are discussed
in Chapter 9. Here, we would simply like to point out
how activity may affect topographic maps in the
nervous system. A very interesting example in this
regard is the somatosensory system. The discovery
of a somatosensory representation of the body, a
homunculus in the case of humans, was discovered by
the neurosurgeon, Wilder Penfield (Penfield, 1954a).
While performing operations to remove epileptic foci
in the brains of fully conscious patients, Penfield took
the opportunity to study the organization of the cortex
by locally stimulating different regions with an elec-
trode. When he stimulated points in the postcentral
gyrus, patients reported the sensation of touch in spe-
cific areas of their bodies. Stimulation of neighboring
points caused the patients to experience sensations in
neighboring parts of their body surface although there
were occasional jumps, such as between the hand and
the face. By mapping these sensations on the cortex of
different patients, Penfield was able to come up with
a consistent somatosensory homunculus and in the
precentral gyrus a matching topographic homunculus
where stimulation caused movements of specific
body parts (Figure 6.18). One striking feature of the
homunculus that Penfield noticed immediately is
the relative magnification of parts of the map. This
appears to be a consistent feature of many maps in the
CNS. The largest features of the human homunculus
are the lips, tongue, and tips of the fingers. In contrast,
the representation of the upper back is quite small.
In other animals, the somatosensory cortex has an
expanded representation of different body parts: the
hands of the raccoon, the snout of a star nose mole, and
the whiskers of the mouse, for example, are particu-
larly enlarged. The differential magnification of certain
body parts in the cortical representation of the body is
probably due to the density of peripheral innervation.
Thus, in humans, each fingertip has almost as many
sensory receptors as the whole of the upper back. In
mice, the vibrissae are most heavily innervated.
Central representations of the somatosensory
system are flexible and may depend on sensory stim-
ulation, especially during early life. In the mouse,
single cortical areas, called barrels, are devoted to each
vibrissa. The barrel fields of the cortex are almost equal
in size to the somatosensory cortex devoted to the rest
of the body (Woolsey and Van der Loos, 1970). There
are five rows of barrels that correspond to the five rows
of vibrissa. When a bristle is destroyed by cauteriza-
tion in early life, the cortical barrel that represents it
retinal growth
Nasal removed
FIGURE 6.17 Shifting connections. A. During the lifetime of a
frog or fish, its eye and brain continue to grow. The retina grows cir-
cumferentially like a tree, but the tectum grows in expanding pos-
terior crescents. As a result, new retina that is added temporally
must send axons to the anterior primordial tectum, while fibers from
the central primordial retina must shift posteriorly, and new nasal
fibers map to the new posterior tectum in order to keep the map in
topographic order. B. If half the tectum is removed from a fish, after
about a month the retinotopic map will regulate and compress,
mapping out evenly over the remaining half tectum. C. Similar reg-
ulation occurs when half the retina is removed. The remaining pro-
jection eventually expands over the whole tectum. (After Schmidt
and Easter, 1978; Gaze et al., 1979; Schmidt and Coen, 1995)
and fine-tuned. Part of this refinement may be based
on growth patterns or injury, as above, but refinement
also has an activity or experience-dependent aspect.
Without impulse activity, the retinotectal map of the
goldfish is topographic, but the sizes of the receptive
fields recorded in the tectum are larger and less precise
than normal. Analysis of individual retinal axonal
arbors shows that they are up to four times as large as
normal ones (Schmidt and Buzzard, 1990). Repeated
examination of single retinal arbors over time shows
the effects that activity has on branching and topogra-
phy. When retinal activity is abolished by tetrodotoxin
(TTX), the result is increased branch addition and elim-
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