Download Lecture 16 Topographic mapping Retinotopic mapping Frog optic

Topographic mapping
• Maps are orderly representations of the
physical world
Lecture 16
– Retinal photoreceptors receive light from a
particular part of the visual field
– Neighbouring photoreceptors in the retina
send axons to neighbouring targets in the
brain
Retinotopic mapping
Frog optic tectum
dorsal
• Representation of the retina in the brain
Frog optic tectum
Quadrants of the Retina
Dorsal
Ventral
Temporal
Nasal
ventral
D
N
T
V
eye
D
dorsal
T
V
ventral
1
Top View
eyes
• In optic tectum there is a multi-dimensional
representation of the retina
Nasal
Temporal
Anterior
Optic tectum
• How is this achieved?
Posterior
2
Anterior-Posterior Map
Eg. Mouse
Graded expression of:
1. EphA receptors by retinal cells
2. Ephrin A ligand by tectal / superior colliculus
cells
Ephrin A2
Ephrin A5
EphA5
N
T
Ant
Retina
• Retinal neurons with low receptors go to
areas of tectum with high ligand
• Retinal neurons with high receptors go to
areas of tectum with low ligand
high
low
high
low
Post
Superior Colliculus
• Test this idea make mutant mice
– Double mutant of EphA2 and EphA5
• Reduce ligand expression by tectum
• Expect?
– Reduced repulsion
• Supports the idea that graded repulsion
determines area of innervation
3
In Ephrin A2/A5
knockout observe:
• Temporal axons
project more
posterior
• Nasal axons project
more anterior!
• Yes, reduced repulsion for temporal
axons, but seems increased repulsion for
nasal axons
• Knockout studies only partially support the
role of Ephrin gradients
Dorsal
Retinal neurons
Dorsal - Ventral
control
Eph
D
ligand
Ephrin B
Ephrin B
Inactivated Eph
receptor
EphB
control
V
Retina
EphB
Tectum
Dorsal retinal neurons attracted to Eph (receptor) concentrations
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Neurons that express high levels of ligand
go to areas of the tectum with high levels
of receptor
• Ephrin acts as attractive cue!
• How?
– Depends on intracellular domain of ligand
Retinotopic mapping
Maps are dynamic
• Two axis of mapping: A/P & D/V
– A/P mapping by EphrinA
• Repulsion model partially explains map
– D/V mapping by EphrinB
• EphrinB mediates attractive cues with bidirectional
signaling
5
• Somatosensory maps representation of the body
surface
– Area on the map is proportional to amount of sensory
innervation
‘homunculus’
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