Nerves of the Orbit
The nerves of the orbit aid in the various functions and movements of the eye, opening and closure of the eyelids, and allow for vision.
Multiple cranial nerves control the muscles of the orbit, and nerves of the oculus and extraocular region.
The optic nerve (CN II) is a paired nerve that transmits visual information from the retina to the brain, and is considered to be part of the central nervous system. The nerve is furthermore sheathed in all three meningeal layers (dura, arachnoid, and pia mater), and is composed of retinal ganglion cell axons and glial cells. The optic nerve leaves the orbit via the optic canal, which runs postero-medially towards the optic chiasm, where there is a partial decussation (crossing) of fibers from the temporal visual fields (the nasal hemi-retina) of both eyes. The optic nerve functions to transmit sensory information regarding brightness perception, red-green color perception, contrast (visual acuity), and visual fields to the brain for further processing. To note, the blind spot of the eye is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.
The oculomotor nerve (CN III) controls the following muscles:
- Superior rectus muscle (superior branch of CN III): responsible for elevation, incyclotorsion, and adduction of the eye.
- Inferior rectus muscle (inferior branch of CN III): responsible for depression, extorsion, adduction and abduction of the eye.
- Medial rectus muscle (inferior branch of CN III): responsible for adduction of the eye.
- Inferior rectus muscle (inferior branch of CN III): responsible for extorsion, elevation and abduction of the eye.
- Levator palpebrae superioris muscle: responsible for retracting and elevating the eyelid.
The trochlear nerve (CN IV) controls the superior oblique muscle, which is responsible for intorsion, depression and abduction of the eye.
The abducens nerve (CN VI) controls the lateral rectus muscle, which is responsible for abduction of the eye, and the retractor bulbi muscle.
The ophthalmic nerve (CN V1) is one of the three branches of the trigeminal nerve, the fifth cranial nerve. It has multiple branches: the nasociliary nerve (including the sensory root of ciliary ganglion, posterior ethmoidal nerve, long ciliary nerve, infratrochlear nerve, and anterior ethmoidal nerve), the lacrimal nerve, and the frontal nerve (including the supratrochlear nerve and supraorbital nerve).
The ophthalmic nerve carries only sensory fibers from the eyes, conjunctiva, and lacrimal gland; and it receives sensory branches from the nasal cavity, frontal sinus, ethmoidal cells, falx cerebri, dura mater in the anterior cranial fossa, superior parts of the tentorium cerebelli, upper eyelid, dorsum of the nose, and anterior part of the scalp. It supplies branches to the cornea, ciliary body, and iris; to the lacrimal gland and conjunctiva; to the part of the mucous membrane of the nasal cavity; and to the skin of the eyelids, eyebrow, forehead and nose.
The frontal branch of the ophthalmic nerve passes through the orbit superiorly, then re-enters the frontal bone prior to exiting above the orbit through the supraorbital foramen and the supratrochlear notch to provide sensory innervation for the skin of the forehead and scalp. The lacrimal nerve passes through the orbit superiorly to innervate the lacrimal gland. The nasociliary branch gives off several sensory branches to the orbit and then continues out through the anterior ethmoidal foramen, where it enters the nasal cavity and provides innervation for much of the anterior nasal mucosa. It also gives off a branch that exits through the nasal bones to form the external nasal branch.
The ophthalmic nerve is joined by filaments from the cavernous plexus of the sympathetic, and communicates with the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves. It also gives off a recurrent (meningeal) filament, which passes between the layers of the tentorium.