Innervation: Trigeminal Nerve

The eye and orbit are richly supplied by branches of the trigeminal nerve (n. trigeminus), or cranial nerve V. Although the nerve contains both general somatic afferent and somatic efferent axons, all of the axons distributed to the eye are general somatic afferent in nature.

The trigeminal nerve is the largest of the cranial nerves. The nerve leaves the brain at the juncture of the pons and trapezoid body. It passes through a canal in the rostral part of the petrous portion of the temporal bone. Here the trigeminal ganglion rests in the cavum trigeminale of the dura mater. The ganglion contains the cell bodies of the general somatic afferent neurons of the trigeminal nerve. Immediately distal to the ganglion the nerve divides into its three major divisions, the ophthalmic, the maxillary, and the mandibular nerves. The ophthalmic nerve is the principal sensory innervation of the eye and orbit. The ophthalmic nerve also mediates the oculorespiratory cardiac reflex in response to manual pressure on the eye (Joffe & Gay, 1966). Branches of the maxillary nerve innervate a part of the superficial structure of the eyelids. The mandibular nerve innervates structures of the ventral face and does not play a role in the innervation of ocular structures.

Ophthalmic Nerve

The ophthalmic nerve (n. ophthalmicus) arises from the rostromedial aspect of the trigeminal ganglion and arcs rostromedially into the orbital fissure to join cranial nerves III, IV, and VI. Sympathetic postganglionic axons from the internal carotid plexus join the ophthalmic nerve at its origin. Within, or immediately rostral to, the orbital fissure, the ophthalmic nerve divides into three branches, the frontal, lacrimal, and nasociliary nerves.

The frontal nerve (n. frontalis) is a small nerve that is sensory to most of the skin of the superior eyelid and medially to the dorsal midline. From the orbital fissure it passes rostrodorsally between the periorbita and dorsal rectus muscles to the superior lid.

The lacrimal nerve (n. lacrimalis) is a very small branch of the ophthalmic nerve that travels along the lateral edge of the dorsal rectus to innervate the lacrimal gland. Diesem (1975) found the lacrimal nerve occasionally originating from the maxillary nerve. He was also able to trace branches of the lacrimal, presumably sensory, to the lateral portion of the superior eyelid. The lacrimal nerve receives orbital branches from the pterygopalatine ganglion and supplies these parasympathetic postganglionic axons to the lacrimal gland.

The nasociliary nerve (n. nasociliaris) continues the ophthalmic into the orbit. It passes rostromedially between the dorsal and ventral rami of the oculomotor nerve to the dorsal surface of the optic nerve. Here it divides into the long ciliary nerves and the infratrochlear and ethmoidal nerves.

The long ciliary nerves (nn. ciliares longi) continue rostrally closely applied to the optic nerve. There are variable communications with the short ciliary nerves and a communicating branch to the ciliary ganglion is usually present (ramus communicans cum ganglio ciliari). The long and short ciliary nerves enter the globe adjacent to the optic nerve. According to Prince et al. (1960), the ciliary nerves continue anteriorly in the suprachoroidea, supplying sensory innervation to the choroid, ciliary body, iris, cornea, and bulbar conjunctiva. Axons enter the cornea at the limbus throughout its circumference and run toward the center, branching dichotomously. They may be visualized in the live dog with the biomicroscope. The sensory innervation of the cornea apparently exerts a trophic influence essential to its normal state (see Cornea). Sympathetic postganglionic axons in the long ciliary nerves are motor to the dilator muscle of the pupil.

The infratrochlear nerve (n. infratrochlearis) passes rostrodorsally along the medial edge of the dorsal rectus. The nerve passes ventral to the trochlea, as its name implies, and ramifies in the tissues and skin associated with the medial commissure of the lids.

The ethmoidal nerve (n. ethmoidalis) accompanies the external ethmoidal artery as it curves rostrally and medially dorsal to the extraocular muscles to leave the orbit through an ethmoidal foramen. It innervates part of the nasal mucosa and skin of the muzzle.

Maxillary Nerve

The zygomatic nerve (n. zygomaticus), a branch of the maxillary nerve, may enter the orbit either through the rostral alar foramen with the maxillary nerve or through a separate foramen. The nerve enters the periorbita at its apex and divides into the zygomaticofacial and zygomaticotemporal nerves. The zygomaticotemporal nerve courses rostrodorsally deep to the lateral aspect of the periorbita to the region of the orbital ligament, where it ramifies in the lateral superior eyelid and skin over the temporalis muscle to reach the dorsal midline. As the nerve passes the lacrimal gland, branches may communicate with the lacrimal nerve, possibly supplying it with parasympathetic innervation derived from the pterygopalatine ganglion.

The zygomaticofacial nerve is ventral to the zygomaticotemporal and parallel to it over most of its course within the periorbita. Near the orbital margin it turns ventrally and ramifies in the inferior eyelid and the skin overlying the zygomatic arch.