The orbital structures are innervated by cranial nerves (CNs) II, III, IV, V, VI, and VII (Table 12-1). Motor functions of the striated muscles are controlled by CN III, the oculomotor nerve; CN IV, the trochlear nerve; CN VI, the abducens nerve; and CN VII, the facial nerve. CN V, the trigeminal nerve, carries the sensory supply from the orbital structures. CN II, the optic nerve, carries visual information and is discussed in Chapter 13. This chapter discusses sensory and motor innervation of the orbit, including pathways, functions, and presenting signs of dysfunction.

T H E N E R V O U S S Y S T E M

Information comes into the central nervous system (CNS) via afferent fibers. Afferent sensory fibers usually have specialized nerve endings that respond to such sensations as touch, pressure, temperature, and pain.

Information processing occurs within the brain or spinal cord and involves communication between different areas of the CNS through fiber tracts. A fiber tract also may be called a fasciculus, a peduncle, or a brachium. The portion of the cranial nerve from the cell body in the nucleus to the exit from the brain stem is the fascicular part of the nerve.

Efferent fibers, either somatic or autonomic, carry information from the CNS to the target structures: muscles, organs, or glands. The efferent pathway in the somatic system generally consists of a fiber that runs the distance from the CNS to the target muscle. The autonomic pathway generally has a synapse within its efferent pathway (see Chapter 14).

A F F E R E N T P A T H W A Y : O R B I T A L S E N S O R Y

I N N E R V A T I O N

The eye is richly supplied with sensory nerves that carry sensations of touch, pressure, warmth, cold, and pain. Sensations from the cornea, iris, conjunctiva, and sclera consist primarily of pain; even light touching of the cornea is registered as irritation or pain.1

TRIGEMINAL NERVE

The fibers of the trigeminal nerve (CN V) serving ocular structures are sensory and originate in the innervated structures. The description of the pathways of these nerves begins at the involved structures and follows the nerves as they join to become larger nerves, come together in the ganglion of the fifth cranial nerve, and then exit the ganglion and enter the pons. It is hoped that this presentation, although unconventional, will enable the reader to keep in mind the actual direction of the action potential, and thus the information flow, in these fibers. Figure 12-1 shows the major branches and paths of the trigeminal nerve within the orbit.

Ophthalmic Division of Trigeminal Nerve

Nasociliary Nerve

Sensory fibers from the structures of the medial canthal area—caruncle, canaliculi, lacrimal sac, medial aspect of the eyelids, and skin at the side of the nose —join to form the infratrochlear nerve. This nerve penetrates the orbital septum, enters the orbit below the trochlea, and runs along the upper border of the medial rectus muscle, becoming the nasociliary nerve as other branches join it (see Figure 12-1).

Sensory fibers from the skin along the center of the nose, the nasal mucosa, and the ethmoid sinuses form the anterior ethmoid nerve; fibers from the ethmoid sinuses and the sphenoid sinus form the posterior ethmoid nerve. The ethmoid nerves enter the orbit with their companion arteries through foramina within the frontoethmoid suture.2 Both nerves join the nasociliary nerve as it runs along the medial aspect of the orbit (see Figure 12-1).

Corneal sensory innervation is dense, estimated to be 400 times as dense as other epithelial tissue innervation.3 Three networks of nerves are formed. One is located in the corneal epithelium, another (the subepithelial plexus) is in the anterior stroma, and the third, the stromal plexus, is in the middle of the stroma4 ­(Figure 12-2). No nerves are found in posterior stroma, ­Descemet’s membrane, or endothelium. The fibers

Supratrochlear

nerve

Infratrochlear

nerve

Anterior ethmoid nerve

Long ciliary nerve

Posterior ethmoid nerve

Nasociliary nerve

Optic nerve

Supraorbital nerves

Zygomaticotemporal nerve

Zygomaticofacial nerve

Short ciliary nerves

Ciliary ganglion

Long ciliary nerve

Sensory root to the

ciliary ganglion Frontal nerve

Lacrimal nerve

Zygomatic nerve

Ophthalmic nerve

Infraorbital nerve

Maxillary nerve

Trigeminal ganglion

Mandibular nerve

FIGURE 12-1

Orbit viewed from above showing branches of ­ophthalmic nerve.

A

B

C

FIGURE 12-2

Innervation of limbus and cornea.  Long ciliary nerve (A) supplies limbal region, then sends branches into cornea. Nerves also supply trabecular meshwork (B) and region of Schlemm canal. Note paucity of nerves in deep cornea (C) and their absence in region of Descemet membrane. (From Hogan MJ, Alvarado JA, Weddell JE, editors: Histology of the human eye, Philadelphia, 1971, Saunders.)

from these plexus come together in peripheral stroma and radiate out into the limbus as 70 to 80 branches; they become myelinated in the last 2 mm of the cornea.5-7

Some of these branches join with nerves from other anterior segment structures to form two long ciliary nerves. These long ciliary nerves, one on the lateral side and one on the medial side of the globe, course between the choroid and sclera to the back of the eye, where they leave the globe at points approximately 3 mm on each side of the optic nerve (Figure 12-3). (In addition to afferent fibers, the long ciliary nerves transmit sympathetic fibers to the dilator muscle of the iris.) The two long ciliary nerves then join the nasociliary nerve.

Superior rectus

FIGURE 12-3

Posterior sclera.  Posterior portion of globe showing optic nerve passing through posterior scleral foramen; long and short ciliary arteries and nerves passing through posterior apertures; and vortex veins passing through middle apertures.

Clinical Comment: (Scleral) Nerve

Loops (of Axenfeld)

A slight variation can occur in the pathway of the long ciliary nerve in which the fibers loop into the sclera from the suprachoroidal space, forming a dome-shaped elevation about 2 mm from the limbus on either the nasal or the temporal side. Often this raised area

is pigmented, usually blue or black, and should be differentiated from a melanoma.8 The nerve loop may be painful when touched, a characteristic that should aid in its diagnosis.5

The other branches radiating from the cornea into the limbus join other sensory nerves from the anterior segment; they enter the choroid, join with the choroidal nerves, then course to the back of the eye, where they leave as 6 to 10 short ciliary nerves (see Figure 12-3). The short ciliary nerves exit the sclera in a ring around the optic nerve in company with the short posterior ciliary arteries and enter the ciliary ganglion (see Figure 12-1). The sensory fibers do not synapse but pass through the ganglion, leaving as the sensory root of the ciliary ganglion, which then joins the nasociliary nerve. (The short ciliary nerves carry sympathetic­ and parasympathetic fibers in addition to sensory fibers.)