Cranial Nerve II: Optic

Fig. 13.1 Optic nerves depicted on an axial CISS image. The nerves are hypointense surrounded by subarachnoid space which may not always be clearly visible. In this particular patient suffering from idiopathic hypertension, the spaces are enlarged as a result of increased intracranial pressure

Fig. 13.2 Axial CISS image showing optic tracts. The arrow points to the right optic tract which is retrochiasmatic

13.2 Function

The optic nerve is one of the five special senses, consisting of only SA fibers. CN II is important in providing the sense of vision. The nerve preserves a retinotopic map of the outside visual field such that the nasal area of each visual field projects to the temporal (or lateral) half of each retina and the peripheral or visual field projects to the nasal half of the retina. While the nasal retinal fibers decussate at the level of the optic nerve traveling into the opposite optic tract, the temporal retinal fibers course along CN II and into the optic tract without decussating. Thus, each optic tract contains information from both eyes (e.g., the right optic tract contains peripheral half of the visual field from the right eye and the medial visual field from the left eye). This information remains constant and well maintained at all levels including the lateral geniculate bodies, optic radiations and, in the cortex of the primary visual cortex. The optic radiations can be further divided into the parietal lobe optic radiations that subserve the inferior visual fields and the temporal lobe optic radiations that contain information from the superior visual fields.

Retinogeniculate pathway: the primary pathway for conscious vision.
Retinopretectal pathway: responsible for pupillary light reflex and for conjugate pupillary response.
Retinohypothalamic pathway: visual information reaches the suprachiasmatic nuclei of the hypothalamus promoting rhythmic circadian sleep-wake cycle and promoting neuroendocrine functions.
Retinocollicular pathway: allows conjugate eye movements and eye movement reflexes.