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Visual pathway anatomy
Anatomy & Optics
Key Points
- •Visual pathway: photoreceptors → bipolar cells → RGCs → optic nerve → chiasm (nasal fibres cross, temporal stay ipsilateral) → optic tract → LGN (6 layers: 1,2 magnocellular; 3-6 parvocellular; koniocellular interleaved) → optic radiation (Meyer's loop in temporal lobe for superior VF; parietal fibres for inferior VF) → V1 (area 17, calcarine sulcus, retinotopic)
- •LGN layers: layers 2,3,5 = ipsilateral eye; layers 1,4,6 = contralateral eye; magnocellular (1,2) = motion, flicker, achromatic; parvocellular (3-6) = colour (red-green), fine detail, form; koniocellular = blue-yellow colour; only ~10% of LGN input is retinal (rest is cortical feedback and brainstem modulation)
- •Visual field defects by site: optic nerve = ipsilateral monocular; chiasm = bitemporal hemianopia (pituitary adenoma); optic tract = incongruous contralateral homonymous hemianopia + contralateral RAPD; temporal lobe (Meyer's loop) = superior quadrantanopia ('pie in the sky'); parietal lobe = inferior quadrantanopia ('pie on the floor'); occipital cortex = congruous homonymous hemianopia with macular sparing
- •Pupillary light reflex: afferent: retina → optic nerve → chiasm → optic tract → pretectal nucleus (NOT LGN) → bilateral Edinger-Westphal nuclei → efferent: CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae; bilateral pretectal-EW projection = consensual response; RAPD = asymmetric afferent input (optic nerve disease, extensive retinal disease)
- •Light-near dissociation: pupils accommodate but do NOT react to light; lesion at pretectal nucleus/dorsal midbrain (the near pathway uses a DIFFERENT cortical afferent route); classic causes: Argyll Robertson pupil (neurosyphilis - bilateral, small, irregular), Parinaud syndrome (dorsal midbrain - + upgaze palsy + convergence-retraction nystagmus), Adie's tonic pupil (ciliary ganglion lesion)
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13 more sections are included with the Clinical plan.
- Definition
- Epidemiology
- Aetiology and causes
- Pathogenesis
- Classification
- Clinical features