DESCRIPTION and FUNCTION: The optic nerve is really a brain tract because it develops as an outgrowth of the brain. The function is purely sensory and carries afferent impulses for vision.
ORIGIN and COURSE: Nerve fibers arise from the retina of the eye to form the optic nerve, which passes through the optic canal of the orbit (eye socket). The optic nerves converge to form the optic chiasma (Figure 2A) where fibers partially cross over, continue on as optic tracts, enter the thalamus, and synapse there. Thalamic fibers run to the occipital (visual) cortex, where visual interpretation occurs.
CLINICAL TESTING: Visual acuity* and visual field* are determined with eye charts and by testing the point at which the person first sees an object, such as a finger, moving into the visual field. Fundus of the eye is viewed with an ophthalmoscope* for routine examination of the optic disc and retinal blood vessels.
HOMEOSTATIC IMBALANCE: Damage to the optic nerve results in blindness in the eye served by the damaged nerve. Damage to the visual pathway beyond the optic chiasma results in partial visual losses. Visual defects, such as near and far sightedness, are called anopsias.
*Movies [QuickTime] from the NeuroLogic Exam and PediNeuroLogic Exam websites are used by permission of Paul D. Larsen, M.D., University of Nebraska Medical Center and Suzanne S. Stensaas, Ph.D., University of Utah School of Medicine. Additional materials were drawn from resources provided by Alejandro Stern, Stern Foundation, Buenos Aires, Argentina; Kathleen Digre, M.D., University of Utah; and Daniel Jacobson, M.D., Marshfield Clinic, Wisconsin. The movies are licensed under a Creative Commons Attribution-NonCommerical-ShareAlike 2.5 License.