Optic Tracts And Radiations

The optic tracts emerge from the posterolateral angles of the chiasm, travel posteriorly around the cerebral peduncles and terminate in the lateral geniculate bodies (LGB). Each tract contains visual (and pupillomotor) fibres, both crossed nasal retinal fibres from the contralateral eye and uncrossed temporal fibres from the ipsilateral eye.

The pupil fibres involved in the light reflex exit the optic tracts just before the LGB, enter the brain stem and synapse in the pretectal nuclei. These pretectal nuclei then project to both ipsilateral and contralateral Edinger-Westphal nuclei. Efferent parasympathetic papillary fibres then exit the midbrain with the oculomotor (3rd) nerve.

The lateral geniculate body is a small swelling on the under surface of the thalamus. It has a laminate structure consisting of 6 curved layers of cells orientated in a dome-shaped mound. The nerve fibres, which have travelled from the eye, terminate in the LGB by synapsing with visual neurons. The nasal nerve fibres that cross the midline terminate in layers 1,4 and 6, whilst the ipsilateral temporal fibres (that have not crossed in the chiasm) synapse in layers 2,3 and 5. Each LGB therefore receives visual information from both eyes and the structure and orientation of fibres is highly organised and predictable. Axons from the LGB visual neurons form the optic radiations and terminate in the visual cortex.