The Nerve Supply of the Eye

The nerve supply to the eye and its accessory structures is derived from no fewer than six cranial nerves (denoted here by roman numerals). Most of them enter the orbital cone, but some reach accessory structures directly.

The optic nerve (II) enters the orbit through the optic foramen and contains axons arising from the innermost layer of the retina. It is rather slack in order to allow for the movements of the eye and is covered by meninges that it acquired during development as the stalk of the optic cup.

Though the name of the oculomotor nerve (III) implies that it controls movement of the eyeball, it does not innervate all the ocular muscles. It enters the orbit through the orbital foramen (fissure; foramen orbitorotundum in ruminants and the pig) and sends branches to the levator palpebrae; the dorsal, medial, and ventral recti; the ventral oblique; and part of the retractor bulbi muscles.

The trochlear nerve (IV) accompanies the third nerve and innervates the dorsal oblique muscle.

The ophthalmic and maxillary divisions of the trigeminal nerve (V) send branches to the eye. The ophthalmic nerve passes through the orbital foramen and supplies the following sensory branches: long ciliary nerves to the eyeball, especially the cornea; a lacrimal nerve to the eyelids and conjunctiva of the lateral angle; a supraorbital nerve that accompanies the supraorbital artery through the supraorbital foramen to supply the upper eyelid and skin medial to the orbit; an infratrochlear nerve (not present in all species) sensory to structures near the medial angle of the eye; and an ethmoidal nerve that follows the ethmoidal artery to innervate the caudal part of the nasal cavity. The maxillary nerve has only one relevant branch, the zygomatic nerve, which supplies the lateroventral segment of the eyelids and conjunctiva via a zygomaticofacial branch and the skin caudal to the orbit via a zygomaticotemporal branch.

The abducent nerve (VI) enters through the orbital foramen. It innervates most of the retractor bulbi and the lateral rectus muscles.

The auriculopalpebral branch of the facial nerve (VII) passes between the eye and ear and thus approaches the eyelids from a caudal aspect. It innervates the orbicularis oculi. It may be blocked to immobilize the lids or to relieve the "pressure" that tension in the muscle may exert on a painful globe. The levator palpebrae is not immobilized by this block.

Sympathetic postganglionic nerve fibers arise from the cranial cervical ganglion and follow arteries or the ophthalmic nerve to the orbit, where they innervate the orbital muscle and the dilator of the pupil. Tension in the orbital muscle keeps the eyeball protruded, the third eyelid retracted, and the palpebral fissure open. Active dilation of the pupil (mydriasis) is initiated by fear, excitement, or pain.

Parasympathetic preganglionic nerve fibers enter the orbit within the oculomotor nerve. They synapse in the ciliary ganglion, and the postsynaptic fibers, forming the short ciliary nerves, innervate the ciliary muscle and the constrictor of the pupil. They control both the accommodation of the lens and the constriction of the pupil (miosis) in response to light.

The axons of the long ciliary nerve, a branch of the ophthalmic nerve, form the afferent limb of the corneal reflex, which is the closure of the lids when the cornea is touched. The efferent limb of this reflex is the auriculopalpebral branch of the facial nerve. This reflex is used clinically when monitoring deep anesthesia.

Both the orbital muscle and the dilator muscle of the pupil receive sympathetic input. Loss of sympathetic innervation results in a sunken eye, protrusion of the third eyelid, and constriction of the pupil (miosis), a collection of clinical signs known as Horner's syndrome.