The Oculomotor Nerve
The Oculomotor Nerve is the third 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 back to the brain from a peripheral area.
The pathway for the efferent fibers is the following (from the initiation in the brain to the termination in the periphery):
- The origin of the primary efferent oculomotor nerve fibers is the nucleus of the oculomotor nerve that exists within the superior colliculus of the encephalic tract or the brainstem as it is more commonly known.
- Secondary efferent oculomotor nerve fibers also contribute to the oculomotor nerve from the accessory nucleus of the oculomotor nerve also known as the Edinger-Westphal nucleus, which is situated adjacent to the primary nucleus mentioned above.
- The primary and secondary fibers bind together and exit the skull through the superior orbital fissure.
- The primary fibers innervate all the extraocular muscles except the superior oblique muscle and lateral rectus muscle.
- This nerve motorically innervates the previously mentioned eye muscles and allows the eye to move within the orbit.
- The secondary fibers branch off at the level of the ciliary ganglion and synapse there.
- They then continue on to innervate the ciliary body, the Iris, the sphincter pupillae muscle and the dilator pupillae muscle which contract and dilate the pupil respectively.
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.