THE OPTIC NERVE

The optic nerves are responsible for transmitting all visual impulses form the retina to the brain, first crossing over at the optic chasm, and then penetrating the brain to form the optic tracts which terminate in the lateral geniculate nucleus of the thalamus. Specifically, beginning at the retina, the rods and cones project to horizontal and bipolar cells which in turn project to X, Y and W ganglion cells within the retina. Axons of the retinal ganglion cells run in parallel along the surface of the retina and converge at the optic disc (the blind spot) where they gather together and punch through the retina thereby forming the optic nerve.

The right and left optic nerves project into the cranial cavity via the optic foramina and then unite to form the optic chiasm where a decussation occurs, thereby forming the optic tracts. Subsequently, visual input from the left half of visual space projects to the right hemisphere, and input from the right half of visual space projects to the left cerebrum via the optic tracts which in turn project predominantly to the lateral geniculate nucleus of the thalamus, and to a much lesser extent, to the superior colliculus, and probably the hypothalamus, amygdala, and inferior temporal and superior parietal lobes. Because the optic nerve does not project directly to the brainstem, it is not a true cranial nerve -at least in mammals- although some fibers (via the optic tract) are received in the midbrain visual colliculi.

From the lateral geniculate nucleus these visual fibers form the optic radiations which project predominantly to the striate cortex within the occipital lobe, and give off collaterals to the inferior temporal and superior parietal lobe where the upper and lower visual fields are also represented.

Injury to the optic pathways necessarily produce visual defects and are referred to as homonymous if they are restricted to either the right or left visual field, or heteronymous if both visual fields are disrupted to some degree. Heteronymous defects suggest a lesion involving both cerebral hemispheres, or a lesion to retina or the optic nerve before it crosses over at the decussation.

If the visual defects are homonymous, then the lesion can be localized to one side of the optic tract or radiations and thus to the right or left hemisphere. Complete destruction of the optic tract or its terminal zones induces a homonymous hemianopsia to the left or right, whereas a partial injury may induce a quadratic homonymous defect: upper quadrant being associated with temporal lobe defects, and lower quadrant defects associated with the superior parietal lobe.

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