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Fig. 3. Schematic of how synaptic connections and densities of ephrin in the
tectum change continually according to the retinal induction model,
illustrated for the EphB/ephrinB interaction. A one-dimensional retina
innervates a one-dimensional tectum; not all connections are shown. (A)
Changing the mapping. Each synaptic strength (red) is continually modified
according to how closely the density of EphB (light blue) in the RGC resembles
the density of ephrinB (dark blue) in the tectal cell. (B) Changing the
tectal labels. An inductive signal is available at each tectal cell, made up
from contributions from the individual axons innervating the cell. The
magnitude of each contribution is in proportion to (1) the density of EphB in
the parent RGC and (2) the strength of the synapse. The ephrinB density in
each tectal cell is continually changed so as to reduce the difference between
the current density of ephrinB (dark blue) and the density of induced EphB
(light blue) at that cell. Note that the rules for the EphA/ephrinA system are
more complicated: synaptic strengths are changed according to how close the
product of the EphA density in the RGC with the ephrinA density in
the tectal cell is to unity; similarly tectal ephrinA densities are changed so
that the product of the induced EphA density and the actual tectal ephrinA
density will tend towards unity. See Table
1 for a mathematical description of this process.
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