Cranial Nerve II

The visual system is extremely important in human life, and a substantial percentage of CNS neurons is devoted to its function. Incoming visual stimuli are first processed in the eye, where photoreceptor cells in the retina, called rods and cones, transduce patterns of light into electrical signals that travel to the brain. The optic nerve begins at the back of the eye, where it is visible with an ophthalmoscope as the optic disk. Visual information is first transmitted via the optic nerve to the lateral geniculate nucleus of the thalamus, and then additional relays through the temporal and parietal lobes project to the primary visual cortex in the occipital lobe (Fig. 7). As it travels to the thalamus, the optic nerve splits into two components, one remaining on the same side as the eye from which it came and the other passing over to the other side. The importance of this anatomic feature is that each hemisphere receives visual input from the contralateral visual field; thus, the right hemisphere interprets visual input from the left side of vision and vice versa. This aspect of crossed function is typical of the organization of many brain systems and has important clinical and behavioral implications.

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