Healthcare and Medicine Reference
In-Depth Information
contralateral projection (Figure 9.18A, right panel).
Thus, isthmotectal axons can be induced to innervate
a part of the tectum that they would not ordinarily
contact (Udin and Keating, 1981). When nucleus
isthmus axons are labeled in normal animals they are
oriented very accurately along the rostro-caudal axis
of the tectum (Guo and Udin, 2000). In eye-rotated
animals, these isthmal axons appear disordered, often
terminating in two locations or meandering within the
tectum (Figure 9.18B).
In some cases, maps from two different sensory
systems are found within the same structure, and they
must come into alignment during development. In the
optic tectum of barn owls, there are maps of both the
visual and auditory world. The maps are aligned such
that neurons respond to acoustic and visual stimuli
from the same position in space. For example, neurons
that respond to visual stimuli directly in front of the
animal (0°) will also respond to a sound stimulus that
arrives at each ear simultaneously (i.e., 0 ms interaural
time difference). Neurons that respond to visual
stimuli at 20° to the right will also respond to a sound
stimulus that arrives first at the right ear and then at
the left (60 ms interaural time difference). Does the
alignment of these maps also depend on neural activ-
ity? To test this idea, owls were reared with prismatic
glasses that displace visual stimuli by 23° (Figure
9.19A). As in the retinotectal system, the physical con-
nections between eye and tectum are unaltered in
prism-reared animals. If no compensation were to
occur, then visual stimuli and auditory stimuli from
the same position in space would activate different loci
in the optic tectum. That is, maps of auditory space
and visual space would be out of alignment. In fact,
the auditory map adjusts to remain in register with the
visual map. When the prisms are removed, tectal
neurons that responded to visual stimuli directly in
front of the animal (0°) are now found to respond to
an auditory stimulus to one side. Therefore, auditory
connections must have changed in response to visual
activity (Brainard and Knudsen, 1993).
To determine how this happened, a tracer was
injected in the central nucleus of the inferior colliculus,
the structure that responds to interaural time differ-
ences (Figure 9.19B) (DeBello et al., 2001). In control
animals, tracer injected at a specific position labels
fibers that project to a unique location in a second
auditory nucleus, called ICX (Figure 9.19C, left). It is
the ICX neurons that will project to visually responsive
neurons in the optic tectum. In prism-reared animals,
the labeled ICC axons project to a different position
within ICX, permitting optic tectum neurons to inte-
grate a new auditory spatial position (Figure 9.19C,
right). Thus, the elimination and addition of synapses
depends on their activity pattern for some, but not all,
afferent pathways. Even in the visual cortex, genicu-
late afferents from the contralateral eye appear to
establish their innervation pattern first, and those from
the ipsilateral eye may be more dependent on activity
(Crair et al., 1998).
SPONTANEOUS ACTIVITY AND
AFFERENT SEGREGATION
Orientation and motion selectivity mature rapidly
from the onset of sight, and it is possible that normal
neuronal activity simply maintains a precise but unsta-
ble set of connections (Blakemore and Van Sluyters,
1975). Alternatively, it is possible that the innervation
patterns that underlay these coding properties may
not attain their mature pattern without the proper
stimulation. In simpler terms, when does activity
begin to influence neural development? In humans,
visual- and auditory-evoked brain activity can be
demonstrated in utero using the noninvasive brain
imaging techniques, MEG and fMRI (see BOX: Watch-
ing Neurons Think) (Schneider et al., 2001; Eswaran
et al., 2002; Fulford et al., 2003, 2004). Furthermore,
electrical activity is present in the nervous system even
in the absence of visual or auditory stimulation. This
“spontaneous” activity could have a profound influ-
ence on synapse formation and elimination.
We previously learned that stripes in Layer IV
formed in binocularly deprived cats (Figure 9.10). To
test whether spontaneous retinal activity might be
responsible for the segregation of thalamic afferents in
the cortex, retinal activity was completely eliminated
by injecting both eyes with TTX from about 2-6 weeks
postnatal (Figure 9.20). In TTX-reared cats, the LGN
afferents fail to segregate into stripes in Layer IV of the
cortex (Stryker and Harris, 1986). Similarly, suppres-
sion of spontaneous retinal activity prevents the elim-
ination of small retinal afferent sidebranches in the
inappropriate layer of LGN. However, the TTX injec-
tions were made after the thalamic afferents had begun
to segregate in the cortex, and it is possible that this
manipulation leads normally refined afferent projec-
tions to sprout, once again, into an inappropriate target
region (Chapman 2000).
Since spontaneous activity may influence synaptic
development, we should be able to record the amount
and the pattern. Retinal ganglion cells fire action poten-
tials before the system is activated by light. In the
embryonic rat, single retinal neurons discharge about
once every second, but occasionally fire short bursts of
almost 100/sec (Galli and Maffei, 1988). However, the
Search Pocayo ::




Custom Search