Optic Nerve

Cranial nerve II is composed of the distal parts of the axonal processes of retinal ganglion cells. These axons course caudally and medially from the retina to the optic chiasm, where some decussate (cross over) to the opposite side. The proximal parts of these same axons are then called the optic tract as they continue caudally and laterally from the chiasm to sites in the dience-phalon and midbrain.

The retinal ganglion cells receive input from retinal bipolar cells, which in turn receive input from the receptor cells of the retina (rods and cones). Rods are predominantly located in the peripheral parts of the retina, whereas cones are densely packed in the central part of the retina, particularly within the fovea. Rods transduce light stimuli of a broad range of wavelengths, whereas cones are of three types for color vision, each transducing a different part of the spectrum. The transduction process is a complex series of biochemical events initiated by the absorption of a photon by pigment within the receptor cells. The visual world topologically maps in precise order onto the retina, and this map is preserved throughout the system.

The retinal bipolar neurons correspond to the bipolar neurons that lie within the ganglia of other sensory cranial nerve components in terms of their relative position in the sensory pathway. The retinal ganglion cells are the first-order multipolar neurons of the pathway. At the optic chiasm, optic nerve fibers that arise from retinal ganglion cells in the nasal (medial) retina decussate, whereas axons from retinal ganglion cells in the temporal (lateral) retina do not. The net result is that the axons in the optic tract on the right side, for example, receive input that initiates from stimuli in the left half of the visual world. Thus, the right brain "sees" the left visual world and vice versa.

The optic tract projects to multiple sites in the diencephalon and midbrain. The major visual pathway for conscious vision is to neocortex via the dorsal lateral geniculate nucleus in the dorsal thalamus. This nucleus contains two large-celled (magnocellular) layers; they receive input relayed from rods via ganglion cells in the peripheral parts of the retina that conveys the general location of stimuli and their motion. It also contains four small-celled (parvicellu-lar) layers that receive fine form and color input from cones. The dorsal lateral geniculate nucleus projects to primary (striate) cortex, which lies in the caudal and medial part of the occipital lobe. The spatial information from retinal ganglion cells via the magnocellular layers ofthe dorsal lateral geniculate nucleus is relayed from striate cortex via multiple synapses predominantly to posterior parietal cortex, which is involved in spatial cortical functions, whereas the form and color information via the parvicellular geniculate layers is likewise relayed predominantly to inferotemporal cortex, which is involved in numerous complex functions including the visual recognition of objects and individuals.

Midbrain visual projections are to the superficial layers of the rostral part of the midbrain roof, the superior colliculus, in which visual information is mapped in register with similar maps of somatosen-sory and auditory space that are projected into its deeper layers. The superior colliculus visual input is relayed to part of the pulvinar in the dorsal thalamus, which in turn projects to extrastriate visual cortical areas, which border the primary visual cortex in the occipital lobe. The midbrain visual pathway is concerned with the spatial orientation of the visual world.

Damage to the retina or optic pathway causes loss of vision in part or all of visual space (the visual field)

depending on the location and extent of the lesion. Although damage to the retina or optic nerve results in blindness for the eye on the same side, damage located in the thalamocortical part of the pathway causes a deficit for the visual field on the opposite side. Damage to the central part of the optic chiasm, as can occur with a pituitary gland tumor in that region, causes "tunnel vision''—loss of the peripheral parts of the visual field on both sides.

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