Questions and Answers
What occurs when the optic nerve is cut?
What type of visual field loss results from cutting the optic chiasm?
What is the effect of cutting the optic tract?
What initiates the process of photoreception in rods?
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What role does metarhodopsin II play in phototransduction?
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What is the effect of decreased cGMP levels in photoreceptors?
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What is a consequence of photoreceptor hyperpolarization?
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What vitamin is essential for regenerating 11-cis retinal?
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Study Notes
Effects of Lesions in the Optic Pathway
- Cutting the optic nerve results in blindness in the ipsilateral eye.
- Cutting the optic chiasm leads to heteronymous bitemporal hemianopia, affecting the outer half of the visual field in both eyes.
- Cutting the optic tract causes homonymous contralateral hemianopia, where both eyes share the same visual field loss.
- Cutting the geniculocalcarine tract results in homonymous hemianopia with macular sparing, meaning the central visual field remains unaffected.
Steps in Photoreception in the Rods
- Rhodopsin is the photosensitive element, consisting of opsin (a G-protein-coupled receptor protein) and retinal (an aldehyde of vitamin A).
- Photoisomerization occurs when light hits the retina, converting 11-cis retinal to all-trans retinal and forming intermediates, including metarhodopsin II.
- Vitamin A is essential for regenerating 11-cis retinal; deficiency leads to nyctalopia (night blindness).
- Metarhodopsin II activates transducin (Gt), a G protein, which then stimulates a phosphodiesterase.
- Phosphodiesterase converts cyclic guanosine monophosphate (cGMP) to 5'-GMP, causing a decrease in cGMP levels.
- Lower cGMP levels result in the closure of Na+ channels, reducing inward Na+ current and causing hyperpolarization of the photoreceptor cell membrane, increasing with light intensity.
- Hyperpolarization decreases the release of glutamate, an excitatory neurotransmitter.
- Two types of glutamate receptors on bipolar and horizontal cells dictate the excitation or inhibition of these cells.
- Excitatory ionotropic glutamate receptors lead to hyperpolarization (inhibition) due to decreased excitation from lowered glutamate release.
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Description
Test your understanding of the effects of lesions in the optic pathway and the steps involved in photoreception in the rods. This quiz covers important concepts related to vision, from optic nerve damage to the biochemical processes in phototransduction. Perfect for students of neuroscience and ophthalmology.
