Six of the 12 cranial nerves are involved in ocular physiology
and mechanics of vision. The second cranial nerve, the optic nerve,
allows afferent input for visual stimuli as well as pupillomotor
reactivity. Electrical impulses from the retina travel via the optic
nerve through the optic chiasm and the temporal and parietal lobes
of the brain, ultimately arriving at the occipital cortex to begin
the process of translation into vision. The third cranial nerve,
the oculomotor nerve, innervates the medial rectus (adduction),
superior rectus (elevation), inferior rectus (depression), and inferior
oblique (elevation, abduction, and extorsion) muscles, as well as
the levator palpebrae superioris muscle (elevation of the upper
eyelid). Additionally, the oculomotor nerve carries the parasympathetic
afferent input of the dilator muscle of the pupil. The fourth cranial
nerve, the trochlear nerve, innervates the superior oblique muscle
that depresses, abducts, and intorts the eyes. This is the most
likely nerve to be affected after trauma due to its long, unprotected
intracranial course over the dorsal aspect of the superior brainstem
to exit the cranial vault inferior to the brainstem. The fifth cranial
nerve, the trigeminal nerve, is responsible for sensory input from
the cornea and eyelids. In addition, the sympathetic nerve supply
to the eye, involved in vasomotor function and pupil constriction,
travels via the trigeminal nerve. The sixth cranial nerve, the abducens nerve,
provides innervation to the lateral rectus muscle (abduction). It
also has a long course beneath the brainstem along the floor of
the cranium, which makes it vulnerable to ischemic injury and palsy
caused by brain swelling. Lastly, the seventh cranial nerve, the
facial nerve, has both motor and sensory functions. This nerve carries
the afferent limb of the corneal sensory reflex and provides the
motor innervation to the eyelid orbicularis muscles, which close
the eyes.