Nerves of the orbit

Optic nerve

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 to the brain for further processing. This sensory information consists of:

• Brightness perception
• Red-green color perception
• Contrast (visual acuity)
• Visual fields

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.

Oculomotor nerve

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 oblique 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.

Trochlear nerve

The trochlear nerve (CN IV) controls the superior oblique muscle, which is responsible for intorsion, depression and abduction of the eye.

Abducens nerve

The abducens nerve (CN VI) controls the lateral rectus muscle, which is responsible for abduction of the eye, and the retractor bulbi muscle.

Ophthalmic nerve

The ophthalmic nerve (CN V1) is one of the three branches of the trigeminal nerve, the fifth cranial nerve. It has multiple branches, such as the:

• Nasociliary nerve (including the sensory root of ciliary ganglion, posterior ethmoidal nerve, long ciliary nerve, infratrochlear nerve, and anterior ethmoidal nerve) - After giving off several sensory branches to the orbit, this branch 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.
• Lacrimal nerve - It passes through the orbit superiorly to innervate the lacrimal gland.
• Frontal nerve (including the supratrochlear nerve and supraorbital nerve) - This 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 ophthalmic nerve carries only sensory fibers from the eyes, conjunctiva, and lacrimal gland. 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
• Anterior part of the scalp 

It supplies branches to the:

• Cornea 
• Ciliary body
• Iris
• Lacrimal gland
• Conjunctiva
• Part of the mucous membrane of the nasal cavity
• Skin of the eyelids, eyebrow, forehead and nose

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.