Innervation

The eye and its adnexa are innervated by cranial nerves II, III, IV, V, VI, and VII.

Innervation: Optic Nerve

Oculomotor Nerve

The oculomotor nerve (n. oculomotorius), or cranial nerve III, as its name implies, is the primary general somatic efferent innervation to the muscles of the eye. The oculomotor nerve is motor to the dorsal, medial, and ventral rectus muscles, the ventral oblique, and the levator palpebrae superioris muscle. It also contains parasympathetic preganglionic general visceral efferent axons that synapse in the ciliary ganglion. The mean diameter of myelinated and unmyelinated fibers in the oculomotor nerve (N = 6) is 10.23 (±0.68) µm and 0.43 (±0.21) µm, respectively.

Postganglionic axons from the ciliary ganglion innervate the m. sphincter pupillae of the iris and the ciliary muscle. The parasympathetic axons are located medially at the origin of the nerve and, as such, are vulnerable to pressure damage when there is compression by a neoplasm in the middle cranial fossa. This causes a dilated pupil (mydriasis) prior to strabismus or ptosis. The oculomotor nerve first becomes visible grossly where it leaves the ventral aspect of the mesencephalon on the medial aspect of the crus cerebri. The nerve runs laterally a short distance, then turns rostrally to pass beside the hypophysial stalk on the dorsal surface of the cavernous sinus, which it follows rostrally a short distance. It leaves the cranial cavity through the orbital fissure. Within the orbit the nerve divides into a small dorsal and larger ventral branch. The dorsal branch innervates the dorsal rectus and levator palpebrae muscles. The ventral ramus travels rostrally deep to the lateral rectus, sending branches to the medial and ventral rectus and to the ventral oblique. A short branch containing the parasympathetic axons enters the ciliary ganglion. The ganglion is located midway between the eyeball and the orbital fissure, closely applied to the ventrolateral surface of the optic nerve. Short ciliary nerves (nn. ciliares breves) leave the rostral surface of the ganglion and are distributed to the eyeball. The ciliary nerves demonstrate intense cholinergic activity. A communicating branch from the ganglion often joins the nasociliary nerve or long ciliary nerve. Lesions of the oculomotor nerve result in a ventrolateral strabismus because of the unopposed tension in the lateral rectus. The pupil is dilated and there is ptosis caused by paralysis of the levator palpebrae superioris muscle.

Trochlear Nerve

The trochlear nerve (n. trochlearis), or cranial nerve IV, is unique among the cranial nerves in that it leaves the dorsal surface of the brainstem. It is the smallest of the cranial nerves and difficult to preserve in anatomic preparations. The mean diameter of myelinated and unmyelinated fibers in the trochlear nerve (N = 6) is 10.53 (±0.55) µm and 0.33 (±0.04) µm, respectively. Trochlear nerve axons course dorsocaudally from their nuclear neuronal cell bodies to leave the dorsal surface of the mesencephalon immediately caudal to the caudal colliculus. The axons cross in the rostral medullary velum. It is the only nerve in the body in which all axons supply muscles on the contralateral side of the body (i.e., axons from cell bodies on the left side decussate in the rostral medullary velum and supply motor units on the right side of the body). The nerves arc ventrally between the cerebrum and cerebellum along the tentorium cerebelli and spine of the petrous part of the temporal bone. The nerve reaches the orbit by passing through the orbital fissure lateral to the oculomotor nerve. On emerging from the fissure, the nerve turns dorsomedially to enter the dorsal oblique muscle, the only structure it innervates. The strabismus caused by a lesion in the trochlear nerve would not be visible on external examination of the eyeball.

Ophthalmoscopic examination would show a lateral deviation of the superior retinal vein indicating extorsion of the eyeball from the unopposed tension in the ventral oblique muscle.

Innervation: Trigeminal Nerve

Abducent Nerve

The abducent nerve (n. abducens), or cranial nerve VI, supplies general somatic efferent axons to the lateral rectus and retractor bulbi muscles. The neuronal cell bodies are located in the rostrodorsal medulla adjacent to the midline. The abducent axons pass ventral to emerge through the trapezoid body immediately lateral to the pyramids of the medulla. The mean diameter of myelinated and unmyelinated fibers in the abducent nerve (N = 6) is and 10.45 (±1.27) µm and 0.47 (±0.09) µm, respectively. The nerve runs rostrally in the subarachnoid space medial to the trigeminal ganglion and leaves the cranial cavity through the orbital fissure, closely applied to its medial wall. One to two centimeters rostral to the orbital fissure, the abducent nerve gives a branch to the retractor bulbi, which further divides to supply the four fasciculi of the muscle. The abducent continues rostrally and laterally to reach the dorsal surface of the lateral rectus muscle. Lesions of the abducent nucleus or nerve result in medial strabismus because of paralysis of the lateral rectus muscle. The resultant decrease in eyeball retraction is difficult to assess.

Facial Nerve

The facial nerve (n. facialis), or cranial nerve VII, supplies somatic efferent innervation to the muscles of the eyelids and parasympathetic innervation to the lacrimal gland by way of the major petrosal nerve and pterygopalatine ganglion. It is also sensory to the concave surface of the auricle.

The facial nerve emerges from the medulla through the lateral aspect of the trapezoid body. The nerve courses laterally through the internal acoustic meatus with the vestibulocochlear nerve. The facial nerve enters the facial canal and turns caudally, forming the geniculum n. facialis. The cell bodies of the afferent axons of the facial nerve form the geniculate ganglion (ganglion geniculi), where the nerve makes its turn.

Parasympathetic preganglionic axons leave the facial nerve just distal to the genu as the major petrosal nerve. This joins the deep petrosal nerve to form the nerve of the pterygoid canal, which traverses a small canal in the basisphenoid bone. The nerve ends at the pterygopalatine ganglion, which lies ventral to the periorbita on the dorsal surface of the medial pterygoid muscle. Postganglionic axons from the pterygopalatine ganglion innervate the lacrimal gland via the lacrimal or zygomaticotemporal nerve or both.

The facial nerve leaves the facial canal and emerges on the caudolateral aspect of the skull through the stylomastoid foramen, caudal to the external acoustic meatus. The facial nerve gives rise to the caudal auricular nerves, the digastricus branch, the internal auricular branch, and the stylohyoid branch. The continuation of the facial nerve curves ventrally around the anular cartilage of the external acoustic meatus, gives origin to the dorsal and ventral buccal branches, and continues as the auriculopalpebral nerve (n. auriculopalpebralis). The auriculopalpebral nerve turns dorsally along the rostral aspect of the cartilage of the external acoustic meatus. Throughout this course the facial nerve is especially liable to damage in surgical manipulations for diseases of the external ear canal.

At the level of the origin of the zygomatic process from the temporal bone, the auriculopalpebral nerve divides to form the rostral auricular branches and the zygomatic branch. The zygomatic branch (ramus zygomaticus) curves rostrally along the dorsal margin of the zygomatic arch. Numerous branches join in the formation of the extensive rostral auricular plexus. Dorsocaudal to the lateral commissure, the zygomatic branch gives rise to the dorsal and ventral palpebral branches (rami palpebrales), which innervate the dorsal and ventral portions of the orbicularis oculi muscle. The retractor anguli oculi lateralis and the levator anguli oculi medialis are also supplied by rami of the zygomatic and palpebral branches. The dorsal palpebral branch continues medial to the eyelids to innervate the levator nasolabialis.

Injury to the facial nerve or to its palpebral branches paralyzes the orbicularis oculi muscle and is manifested as an inability to close the palpebral fissure. Such paralysis prevents distribution of the tear film that normally occurs during blinking and results in desiccation of the cornea. Severing only the ventral palpebral branch, as in lateral exploration of the orbit, does not interfere with normal eyelid function.

Roberts et al. (1974) have described a technique for blocking nerve transmission in the auriculopalpebral nerve, where it crosses the zygomatic arch, to paralyze the orbicularis oculi muscle and facilitate ocular examination or the replacement of a proptosed eyeball. It is important to remember that this does not interfere with any sensory innervation of the eye.