Cours : Neural Circuits in the Visual System

Neural Circuits

• Neural circuits are complex pathways in the nervous system
• Maps are formed in the nervous system.
• The visual system forms topographic maps.
• Inputs from the retina form a topographic map in the tectum.
• A fly's location in the visual field correlates to a specific location in the tectum.
• The visual system preserves spatial information, while the olfactory system preserves only the quality of the stimulus.
• In mammals, visual projections from the retina are relayed in the lateral geniculate nucleus.
• The topographic map of the visual system maintains the exact positional information of stimuli.
• Axon targeting in the tectum is determined by repulsive cues.
• Gradients of Eph receptors and Ephrins control retinal axon targeting.
• Temporal axons are repelled by high concentrations of Ephrins.
• Eph receptors and Ephrins do not interact specifically.
• Axon-axon competition contributes to topographic map formation.
• Axons from olfactory sensory neurons that respond to the same odorant target the same glomerulus.
• The olfactory map is established in a stepwise manner.
• Timing and expression of repellants is crucial for axonal targeting.
• In contrast to the visual system, the repulsive ligand is not expressed in the target in the olfactory system.
• Sema3F is expressed by axons, not the target.
• Ligand-independent differences in receptor activity explain Nrp1/PlexinA1 expression gradients.
• Local wiring-segregation of glomeruli is regulated by sorting molecules.
• Olfactory receptors are involved in the regulation of different stages of olfactory system wiring.

Map Formation

• A map is a representation of information in the nervous system.
• Different types of maps exist, including topographic maps.
• Topographic maps preserve spatial information, while other types of maps may only preserve stimulus quality.
• The fundamental difference between maps lies in the information they preserve.