The Visual Pathway
The Visual Pathway is known as the pathway of the Optic Nerve, which is the second of the Twelve Cranial Nerves. It consists of both afferent and efferent visual sensory fibers. The ability to understand the layout of the visual pathway and to recall where its fibers have been damaged by analysing an ophthalmological test or a patient’s description of their visual field is of paramount importance. That is why this knowledge, although theoretically within the subjects of Pathology and Pathophysiology, has been included with most Neuroanatomy courses.
It is suggested that a sound level of Atlas knowledge in the areas of the brain, the eye and especially the optic tract should be accomplished before reading this article for maximum comprehension, if nothing else.
In this instance, the visual field and correlating nerve damage possibilities will be listed here in an afferent order (from the periphery or the eye to the terminal synapse or the visual cortex):
- There is a left and right visual field in each eye. If all four visual fields are clear, then there is no visual pathway damage.
- Damage to the optic nerve would cause sight loss in one eye in both visual fields.
- Damage to one of the fibre bundles in the optic nerve (either the one that stays unilateral through the visual pathway or the one that crosses contra laterally at the optic chiasm); the corresponding visual field that that fibre bundle supplied would be lost.
- Damage at the optic chiasm would cause the lateral visual fields of both eyes to be lost.
- Damage at the sight of the optic tract would cause medial visual field loss in the eye on the same side and lateral sight loss in the eye on the opposite side.
- If there is damage done to the optic radiation, the result is the same as the damage done to the optic tract.
The site of damage determines the visual field loss due to the fibers that are included in that area and where the terminally synapse. The damage has the same outcome for both afferent and efferent fibres.