The optic nerve is not a peripheral nerve in the strict sense since it represents the fiber pathway that unites the retina with the Brain. The receptors through which the visual impulses are perceived are the rods and cones which synapse with the inner nuclear or bipolar layer of the retina, the cells of which in turn synapse with those of the ganglion cells constitute the Optic nerves. The rods react to dim light, peripheral vision and night vision. They are absent over the macula, where only the cones are present in abundance. The cones are concerned with vision in bright light and color vision. The optic disc is devoid of rods and cones and hence in the visual field it projects an area known as the physiological blind spot.
The optic nerve extends from the retina to the optic chiasma through the optic foramina, where the fibers from both the nasal sides cross. Fibers from the lateral (temporal) half of the retina are situated in the temporal half of the nerve and they pass through the chiasma without crossing. posterior to the chiasma, the fibers constitute the optic tract which consists of the ipsilateral temporal fibers and contralateral nasal fibers. The optic tract ends in the lateral geniculate body through which the visual fibers enter. Neurones from the lateral geniculate body pass posteriorly as the optic radiations which terminate on the striate area (area 17) of the occipital cortex. The parastriate (area 18) and peristiate (area 19) cortex function in more complex visual functions such as visual association and color vision. The fibers subserving light reflex enter the brain stem at the superior colliculus.
Testing: Clinical examination for the optic nerve include test for visual acuity, visual fields and color vision. Funduscopy should always be done as a routine. The visual acuity is ideally tested using a snellen’s chart. However, at the bed side one could examine grossly any defect in the near and distant vision using typed pages at standard distances from each eye. Visual fields are ideally tested by using a perimeter for peripheral field and the Bjerrum’s screen for the central field. At the bedside, it can be tested by the confrontation method. Here the field under investigation is compared with the examiner’s visual field which is used as a standard. The examiner stands with one eye closed about one meter in front of the patient. The right eye of the patient is compared with the left eye of the examiner and vice versa. The patient is made to close one eye and instructed to fix the eye to be testes on the examiner’s nose. Each eye is tested individually. Either a pin with a white or colored head or even the examiner’s finger is used as a test object. The test object, held midway between the patient and the examiner, is brought into the field of vision through various meridians of the visual fields can be compared with those of the examiner and field defects could be identified. During the same sitting visual inattention or extinction can be tested by simultaneously presenting objects in the upper and lower temporal quadrants of vision on both sides.
Color vision is tested either by asking the patient to identify primary colors (red, green and blue) or by matching the colors of different objects in case he cannot name the colors.