The Optic Nerve and Visual Pathway

Axons of the ON are derived from the retinal ganglion cells. Those from the nasal part of the retina merge straight to the optic disk, axons from the fovea form the papillomacular bundle and enter the temporal part of the optic disk, and axons from the remaining parts of the retina merge to the superior and inferior poles of the optic disk.^[6] The optic disk comprises the intraocular part of the ON and is divided into intraorbital, intracanalicular (within the optic canal), and intracranial portions. The two ONs merge into the optic chiasm, where axons derived from the nasal portion of the retina cross over. From the chiasm, each ON tract carries axons from the nasal retina of the contralateral eye and the temporal retina of the ipsilateral eye to the lateral geniculate nucleus (LGN) in the midbrain. Some axons continue through the LGN and terminate in the superior colliculus (for control of eye movement), other axons terminate in the pretectal region (for control of the pupillary light reflex), and still others continue directly to the peristriate and parastriate areas in the occipital cortex, next to the primary visual cortex.

From the LGN, postsynaptic axons form the optic radiation projecting the neurons to the visual cortex. Neurons carrying information from the inferior half of the retina pass in a loop (Meyer's loop) through the temporal lobe. Impulses from the upper half of the retina take a more direct course through the parieto-occipital cortex to the visual cortex. The visual cortex connects to areas in the occipital and parietotemporal lobes for more complicated visual perception.^[7]