Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

NEURAL DEVELOPMENT
1205
lips
nose
face
upper lip
lower lip
teeth, gums, and jaw
tongue
eye
pharynx
index
thumb
ring
middle
intra-abdominal
little
hand
wrist
forearm
elbow
arm
shoulder
head
neck
trunk
hip
leg
foot
toes
genitalia
Figure 21–76 A map of the body surface
in the human brain. The surface of the
body is mapped onto the somatosensory
region of the cerebral cortex by using an
orderly system of nerve cell connections
to pair body sites with the brain sites that
receive their sensory information. This
means that the map in the brain is largely
faithful to the topology of the body surface,
even though different body regions are
represented at different magnifications
according to their density of innervation.
The homunculus (the “little man” in the
brain) has big lips, for example, because
the lips are a particularly large and
important source of sensory information.
The map was determined by stimulating
different points in the cortex of conscious
patients during brain surgery and recording
what they said they felt. (After W. Penfield
and T. Rasmussen, The Cerebral Cortex of
Man. New York: Macmillan, 1950.)
system, neurons conveying information about touch map onto the cerebral cortex
so as to mark out a “homunculus”—a small, distorted, two-dimensional image of
the body surface (Figure 21–76).
The retinotopic map of visual space in the optic tectum is the best characterized
of all these maps. How does it arise? A famous experiment in the 1940s on frogs
provided an important clue. If the optic nerve of a frog is cut, it will regenerate. The
retinal axons grow back to the optic tectum, restoring normal vision. If, however,
the eye is in addition rotated in its socket at the time of cutting of the nerve, so as
to put originally ventral retinal cells in the position of dorsal retinal cells, vision is
still restored, but with an awkward flaw: the animal behaves as though it sees the
MBoC6 m22.105/22.77
world upside down and left–right inverted (Figure 21–77). If food is dangled in
front of it, for example, it will lunge perversely backward. This is because the misplaced
retinal cells make the connections appropriate to their original, not their
actual, positions. It seems that the retinal ganglion cells (RGCs) have positional
values—position-specific biochemical properties representing records of their
original location in the retina, assigned perhaps by earlier morphogen gradients,
and making RGCs on opposite sides of the retina intrinsically different.
Such nonequivalence among neurons is referred to as neuronal specificity.
It is this intrinsic characteristic that guides the retinal axons to their appropriate
target sites in the tectum. Those target sites themselves are distinguishable by the
retina
posterior
anterior
anterior
(A)
tectum
posterior posterior posterior
anterior anterior
(B)
Figure 21–77 Neurons in different
regions of the retina project axons
to different regions in the tectum.
(A) Neurons (RGCs) in the anterior retina
project axons to the posterior tectum (as
shown in Figure 21–75 for zebrafish).
(B) Regeneration experiments show
that retinal neurons have an intrinsic
preference for the part of the tectum they
normally connect to. If the eye is surgically
rotated when the optic nerve is cut, the
regenerating retinal axons connect to their
original targets, creating an inverted map.
(After E. Kandel et al., Principles of Neural
Science, 5th ed., New York: McGraw Hill
Medical, 2012.)