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Download Lecture 16 Topographic mapping Retinotopic mapping Frog optic
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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 4 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’ 6