The Optic Nerve
The optic nerve is the second of the twelve cranial nerves. It consists of both afferent and efferent visual sensory fibers. The word afferent means toward the centre, as in from a peripheral area of a limb to the central nervous system. The word efferent is the opposite of afferent, meaning away from the centre and toward the periphery; when the stimulus is carried from the brain to a peripheral area.
Efferent Fibers Pathway
The pathway for the efferent fibers is the following (from the initiation in the brain to the termination in the periphery):
- In the occipital lobe just below the calcarine sulcus, the broadmann area number 17 is situated. This division of the brain is allocated to the visual cortex and is the terminal synapse for the efferent pathway of the optic tract.
- The efferent fibers pass through the optic radiation (lateral to the thalamus and the ventricles) in a radiated pattern (hence the name) and synapse on the lateral geniculate body.
- The lateral geniculate body is the stem of the optic nerve. Henceforth, the optic fibers travel within the optic nerve, known as the optic tract.
- At the optic chiasm, half of the fibers continue into the optic nerve proper, while the other half cross over and continue on the contralateral side and vice versa.
- Once the nerve fibers reach the eye they enter posteriorly through the optic disc, surpassing the lamina cribosa sclerae and surround the vitreous body, up until the ora serrata.
- The optic nerve fibers synapse on the ganglion cells.
- Ganglionic fibers then synapse on the bipolar cells with intermingled amakrine cells.
- The bipolar cells continue to pass along the fibers to the rods and cones with intermingled horizontal cells.
- Finally, the fibers treponate the pigmented epithelium and the choroidea to either the periphery or the macula, depending where the initial optic nerve fibers synapsed.
Afferent Fibers Pathway
The pathway for the afferent fibers is the following (from the stimulatory cells in the periphery to the final synapse in the brain): exactly the opposite of the efferent pathway.
Nerve Fiber Damage
Nerve fiber damage can alter the sight of a single eye, both eyes and certain areas of one eye and its scope of vision. This all depends on where the damage is within the optic tract. This knowledge is essential for anatomy students, even though theoretically it is knowledge within the subject of pathology and pathophysiology; understanding and being able to recall where there is fibre disturbance depending on a patient’s description or ophthalmic tests proves the individuals understanding of the correct functioning and anatomical layout of the optic tract.