Optic tract
Chiasm & tract · Incongruous homonymous Contralateral RAPD
The pathway beyond the chiasm: uncrossed temporal fibres of the ipsilateral eye joined by crossed nasal fibres of the contralateral eye, so each tract carries the whole contralateral hemifield. It winds around the cerebral peduncle to the lateral geniculate nucleus, giving off pupillary fibres to the pretectum through the brachium of the superior colliculus.
Traced from the start
Detail
- Contents
- Contralateral hemifield from both eyes (ipsilateral temporal plus contralateral nasal retina)
- Course
- Around the cerebral peduncle beneath the uncus, to the LGN
- Blood supply
- Anterior choroidal and posterior communicating arteries
- Leaving fibres
- Pupillary fibres to the pretectal nucleus and retinotectal fibres to the superior colliculus leave before the LGN
When it goes wrong
Lesion of one optic tract (uncal herniation, anterior choroidal artery infarct, tumour, demyelination)
Contralateral homonymous hemianopia that is incongruous (unequal in the two eyes), a mild RAPD in the contralateral eye because slightly more crossed fibres are lost, and later band ('bow-tie') atrophy of the contralateral disc
Practise this structure
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