Notes on Visual Pathways

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How it all happens:
• Photopigments in rods and cones absorb photons.
• Molecular structure of these pigments altered.
• Na channels close.
• Hyperpolarization.
• Less neurotransmitter released.
• Rods and cones release less neurotransmitter in light, and more neurotransmitter in dark.
• Rods and cones are connected to bipolar cells that are connected to ganglion cells.

Accommodation reflex:
• Ciliary muscle contracted.
• Pupillae muscle contracted.
• Convergence.

Lesions in visual pathways:
• Optic nerve: blindness and loss of sensory light reflex.
• Optic chiasm compression due to pituitary tumor or meningioma: loss of peripheral vision in both temporal fields, also known as bitemporal heteronymous hemianopia.

See diagram below:

Simple visual lesions

Pupillary light reflex:
• Light shone into one eye; impulses carried by optic nerve through to Edinger-Westphal nucleus, then distributed to both eyes.
• Edinger-Westphal nucleus = parasympathetic nucleus of oculomotor nerve.

Accommodation-convergence reaction:
• Occurs when the eye focuses on a nearby object after looking at a distant object.
• Involves: ciliary ganglion, preganglionic parasympathetic fibers, Edinger-Westphal nucleus, post-ganglionic parasympathetic fibers, ciliary muscle.

Additional Readings:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. The Limbic System
12. Sleep Disorders
13. Autonomic Nervous System
14. Cranial Nerves and Parasympathetic Ganglia
15. Cells of the Nervous System
16. Cerebrospinal fluid
17. Additional short notes on Cerebrum
18. Functions and Diseases of Cerebrum
19. Subcortical Grey Matter
20. Notes on The Spinal Cord
21. Regulation of Heart Rate by Autonomic Nervous System
22. Action Potentials, Axon Conduction, and Neuromuscular Junction

Related Topics

1. Nervous System Disorders
2. Histology of Nervous Tissue

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Page accessed on: July 29, 2010, 11:32 am.

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