As demonstrated by their common embryological origin, the retinas and anterior visual pathways (optic nerves, optic chiasm, and optic tracts) are an integral part of the brain, providing a substantial proportion of total sensory input. They frequently give important diagnostic clues to central nervous system disorders. Intracranial disease frequently causes visual disturbances because of destruction of or pressure upon some portion of the optic pathways. Cranial nerves III, IV, and VI, which control ocular movements, may be involved, and nerves V and VII are also intimately associated with ocular function.
The Sensory Visual Pathway
Cranial nerve II subserves the special sense of vision. Light is detected by the rods and cones of the retina, which may be considered the special sensory end organ for vision. The cell bodies of these receptors extend processes that synapse with the bipolar cell, the second neuron in the visual pathway. The bipolar cells synapse, in turn, with the retinal ganglion cells. Ganglion cell axons comprise the nerve fiber layer of the retina and converge to form the optic nerve. The nerve emerges from the back of the globe and travels posteriorly within the muscle cone to enter the cranial cavity via the optic canal.
MRI of normal brain in sagittal section (upper left), coronal section (upper right), and axial section (lower left). The white arrows indicate the chiasm.
The optic pathway. The dotted lines represent nerve fibers that carry visual and pupillary afferent impulses from the left half of the visual field.
Intracranially, the two optic nerves join to form the optic chiasm (Figure 14–1). At the chiasm, more than half of the fibers (those from the nasal half of the retina) decussate and join the uncrossed temporal fibers of the opposite nerve to form the optic tracts. Each optic tract sweeps around the cerebral peduncle toward the lateral geniculate nucleus, where it will synapse. All of the fibers receiving impulses from the right hemifields of each eye thus make up the left optic tract and project to the left cerebral hemisphere. Similarly, the left hemifields project to the right cerebral hemisphere. Twenty percent of the fibers in the tract subserve pupillary function. These fibers leave the tract just anterior to the nucleus and pass via the brachium of the superior colliculus to the midbrain pretectal nucleus. The remaining fibers synapse in the lateral geniculate nucleus. The cell bodies of this structure give rise to the geniculocalcarine tract. This tract passes through the posterior limb of the internal capsule and then fans into the optic radiations that traverse parts of the temporal and parietal lobes en route to the occipital (calcarine, striate, or primary visual) ...