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. This article will provide the details about the important nerves found in the orbit, namely the:
- optic nerve
- oculomotor nerve
- trochlear nerve
- abducens nerve
- opthalmic 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.
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.
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.
- 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:
- ciliary body
- lacrimal gland
- 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.
Clinical Eye Exam
The clinical eye exam is designed to check the functions of the eyes listed above. The exam includes testing of:
- visual acuity
- color vision
- visual fields
- pupillary response to light (tests both cranial nerves II and III, plus sympathetic nerve fibers)
Optic Nerve Damage
The list of common clinical eye disorders, diseases, and conditions is rather extensive due to the importance of the eyes to everyday life. There are a multitude of things that can go awry with the eyes, and the field of ophthalmology exists for this reason. For example, damage to the optic nerve typically causes permanent and potentially severe loss of vision, as well as an abnormal pupillary reflex, which is diagnostically important, especially if we are to consider the possibility of brain injury following a head trauma or concussion. Additionally, the type of visual field loss will depend on which regions of the optic nerve are damaged. Inflammation of the optic nerve causes optic neuritis, which can also affect vision.
Perhaps one of the most common conditions is glaucoma, which widely describes the group of ocular disorders that results in optic nerve damage or loss to the field of vision. In many patients it is caused by pressure buildup of fluid in the eye (intraocular pressure-associated optic neuropathy), and involves loss of retinal ganglion cells, which causes optic neuropathy in a pattern of peripheral vision loss, initially sparing central vision. Disorders are roughly divided between "open-angle" and "closed-angle" glaucoma. The angle in this case refers to the region between the iris and cornea, where fluid must flow through the trabecular meshwork to escape.
Open-angle, chronic glaucoma tends to progress at a slower rate and patients may not notice they have lost vision until the disease has progressed significantly, unlike in the more acute closed-angle glaucoma. Closed-angle glaucoma can appear suddenly and is often painful; and while visual loss can progress quickly, oftentimes patients seek medical attention due to the pain before permanent damage occurs.
Any damage or impingement on the cranial nerves of the orbit can affect whether or not eyes are able to move properly, and whether or not the eyelids can function normally.