Visual Pathway Lecture

Introduction

• Focus on visual pathway
• Explanation starts from the retina, through the optic nerve, to processing centers
• Previous knowledge of phototransduction cascade is necessary

Basics

• Two eyes: left and right
• Eyes detect different visual fields
• Nasal and temporal aspects of the visual fields explained
  • Nasal: closer to nose
  • Temporal: closer to temple

Visual Fields in Each Eye

• Right eye:
  • Right visual field
  • Left visual field
• Left eye:
  • Right visual field
  • Left visual field

Retina Components

• Temporal Hemi-Retina: Closest to temple, receives light from opposite visual field
• Nasal Hemi-Retina: Closest to nose, also receives light from opposite visual field
  • Example: Left visual field light hits right temporal hemi-retina

Optic Nerve Pathway

• Temporal hemi-retina fibers do not cross; stay ipsilateral
• Nasal hemi-retina fibers cross at the optic chiasm
• Optic chiasm: Right and left optic nerves meet at this structure near the pituitary gland
• Optic tract: Continuation of these fibers
  • Right optic tract
  • Left optic tract

Thalamic Relay - Lateral Geniculate Nucleus (LGN)

• LGN: Part of the thalamus
• Six layers (3 ipsilateral, 3 contralateral)

• Ipsilateral layers: 2, 3, 5
  • Contralateral layers: 1, 4, 6

• LGN to Occipital Lobe (Primary Visual Cortex)
  • Fibers project to the primary visual cortex (striate cortex) in the occipital lobe

Pathways Through Brain Lobes

• Temporal lobe pathway: Inferior retinal fibers (Meyer’s loop)
• Parietal lobe pathway: Superior retinal fibers (Baum’s loop)
• Optic radiations: Pathways combining from LGN to occipital lobe

Clinical Correlations (Lesions)

1. Optic nerve lesion
   • Leads to complete mon-ocular blindness (e.g., right side monocular blindness)
2. Optic chiasma lesion (pituitary tumor)
   • Results in bitemporal hemianopia
3. Bilateral lateral optic chiasma compression (aneurysm)
   • Causes binasal hemianopia
4. Optic tract lesion
   • Leads to homonymous hemianopia (e.g., left homonymous hemianopia)
5. Parietal lobe lesion (Baum's loop)
   • Results in contralateral quadrant-anopia (e.g., left inferior quadrant hemianopia)
6. Temporal lobe lesion (Meyer’s loop)
   • Results in contralateral superior quadrantanopia (e.g., left superior quadrant hemianopia)
7. Optic radiation lesion
   • Causes similar defect as optic tract lesion (e.g., left homonymous hemianopia)
8. Occipital lobe lesion affecting posterior cerebral artery
   • Results in homonymous hemianopia with macular sparing

Summary

• Detailed look at the visual field processing and pathway
• Regional damage leads to specific vision defects useful in clinical diagnosis