(NAS Colloquium) Vision From Photon to Perception (1999) / Chapter Skim
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Emergence of order in visual system development
Emergence of order in visual system development
Pages 46-52
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From page 46... ...
Some of the first evidence in favor of the idea that competitive interactions between retinal ganglion cell axons give rise to the eye-specific layers has come from studies in many species in which one eye is removed during development and the pattern of the retinogeniculate projection from the remaining eye is examined at later developmental times or in adulthood. The , Abbreviations: LGN, lateral geniculate nucleus; TTX, tetrodotoxin; En, embryonic day n; Pn, postnatal day n; HRP, horseradish peroxidase; EPSC, excitatory postsynaptic current.
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From page 47... ...
However, the effect cannot be entirely nonspecific since ganglion cell axons come to an abrupt halt at the LGN border, indicating that they can still respond to many cues even in the presence of TTX. In the context of all the evidence presented above, the most reasonable explanation for the alteration in axon arbor morphology following TTX treatment is that it has acted to block spontaneously generated action potentials and synjaptic transmission, which in turn are required for the formation of the normal specific patterns of axonal arborization.
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From page 48... ...
In contrast, if the two nerves are stimulated asynchronously, such that ganglion cell axons in one nerve never fire at the same time as those in the other nerve, then segregation to form the columns in layer 4 proceeds. This important experiment illustrates the fact that "cells that fire together wire together," in the sense that the timing of action potential activity is critical in determining which synaptic connections between LGN neurons and layer 4 cortical neurons are retained and strengthened and which are weakened and eliminated.
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From page 49... ...
during each wave (46~. Second, intracellular injections of neurobiotin, an agent known to cross gap junctions, reveals tracer coupling between retinal ganglion cells and amacrine cells from PI onwards in ferret retinas (47~.
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From page 50... ...
Two lines of preliminary experiments suggest that this is likely to be the case. To address the question of whether the bursts of action potentials generated by retinal ganglion cells are relayed to LGN neurons, an obvious experiment would be to record from the LGN neurons in vivo at the appropriate ages and see if they are driven to spike by the retinal inputs; however, the spontaneously generated retinal activity is sensitive to anesthetics (R.
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From page 51... ...
receptors, known to be present in LGN neurons at this age (56-58~. Blockade of NMDA receptors in vivo between P14 and P21 is also known to prevent the final refinement of retinal ganglion cell axons into sublayers receiving input from on-center or off-center ganglion cells (38~.
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From page 52... ...
608 Colloquium Paper: Shatz 12.
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