Superior

division

Iris sphincter muscle

Lacrimal

gland

Parasympathetic

Sympathetic

Sensory

PARASYMPATHETIC PATHWAY TO OCULAR STRUCTURES

The preganglionic neuron in the parasympathetic pathway to the intrinsic ocular muscles is located in the midbrain in the parasympathetic accessory third-nerve nucleus, also called the Edinger-Westphal nucleus. The preganglionic fibers leave the nucleus with the motor fibers of the oculomotor nerve and follow the inferior division of

that nerve into the orbit.19 The parasympathetic fibers leave the inferior division and enter the ciliary ganglion as the parasympathetic root13,20-22 (Figure 14-2).

The ciliary ganglion is a small, somewhat flat structure, 2 mm long and 1 mm high, located within the muscle cone between the lateral rectus muscle and the optic nerve, approximately 1 cm anterior to the optic canal.9,13,23 Three roots are located at the posterior edge of the ganglion: the parasympathetic root, mentioned

previously; the sensory root, which carries sensory fibers from the globe and joins with the nasociliary nerve; and the sympathetic root, which supplies the blood vessels. Only the parasympathetic fibers synapse in the ciliary ganglion; the sensory and sympathetic fibers pass through without synapsing (see Figure 14-2).

The short ciliary nerves, located at the anterior edge of the ciliary ganglion, carry sensory, sympathetic, and parasympathetic fibers. The postganglionic parasympathetic fibers, which are myelinated,20 exit the ganglion in the short ciliary nerves, enter the globe, and travel to the anterior segment of the eye to innervate the sphincter and ciliary muscles. Most of the fibers innervate the ciliary body; only approximately 3% supply the iris sphincter.20,21 The two groups of neurons likely share some characteristics and differ in others, but specifics have not been identified.24

Parasympathetic stimulation causes pupillary constriction, thus decreasing retinal illumination and reducing chromatic and spherical aberrations. It also causes contraction of the ciliary muscle, enabling the eye to focus on near objects in accommodation.

Clinical Comment: Iris Equilibrium

The iris contains muscles innervated by both autonomic systems. The parasympathetic system innervates the sphincter, and the sympathetic system innervates the dilator. The parasympathetic and sympathetic nerves are in some state of balance in the normal, healthy, awake individual, and the size of the pupil changes constantly and rhythmically, reflecting this balance. This physiologic pupillary unrest is called hippus and is independent of changes in illumination. During sleep the pupils are small because the sympathetic system shuts down and the parasympathetic system predominates.

Clinical Comment: Inhibition

of Ciliary Muscle

Parasympathetic activation causes contraction of the ciliary muscle in accommodation. Many investigators, using pharmacologic,25,26 electrophysiologic,27 and anatomic20,28,29 evidence, have demonstrated the presence of both sympathetic receptors and fibers in animals and humans.30,31 The sympathetic effect on the ciliary muscle appears to be a small, slow inhibition that is a function of the level of parasympathetic activity.1-5

AUTONOMIC INNERVATION

TO LACRIMAL GLAND

The efferent autonomic pathway to the lacrimal gland follows a complex route. Fibers controlling the parasympathetic innervation originate in the pons in an area within the nucleus for cranial nerve VII designated as the lacrimal nucleus. These preganglionic fibers exit the

pons with the motor fibers of the facial nerve, enter the internal auditory canal, and pass through the geniculate ganglion of the facial nerve without synapsing. They leave the ganglion as the greater petrosal nerve, which exits the petrous portion of the temporal bone.32 The greater petrosal nerve is joined by the deep petrosal nerve, composed of sympathetic postganglionic fibers from the carotid plexus. The greater petrosal and the deep petrosal nerves together form the vidian nerve (nerve of the pterygoid canal) (see Figures 14-1 and 14-2).

The vidian nerve enters the pterygopalatine ganglion, where the parasympathetic fibers synapse. The pterygopalatine ganglion (also called the sphenopalatine ganglion) lies in the upper portion of the pterygopalatine fossa (see Figure 12-5). It is a parasympathetic ganglion because it contains parasympathetic cell bodies and synapses; sympathetic fibers pass through without synapsing.

The autonomic fibers (all of which are now postganglionic) leave the ganglion, join with the maxillary branch of the trigeminal nerve, pass into the zygomatic nerve, and then form a communicating branch to the lacrimal nerve (see Figures 14-1 and 14-2). An alternate pathway bypasses the zygomatic nerve and travels from the ganglion directly to the gland.33 The parasympathetic fibers that innervate the lacrimal gland are of the secretomotor type and thus cause increased secretion. The sympathetic fibers innervate the blood vessels of the gland and might indirectly cause decreased production of lacrimal gland secretion by restricting blood flow.14 Parasympathetic stimulation causes increased lacrimation. Figure 14-3 provides a flow chart of the common autonomic nerve pathways to orbital structures. Sympathetic fibers from the zygomatic nerve also branch into the lower eyelid to innervate Müller’s muscle of the lower lid.34

Parasympathetic innervation to the choroidal blood vessels is believed to emanate directly from the sphenopalatine ganglion through a network of fine nerves, the rami oculares.35 Parasympathetic activation presumably causes vasodilation, which might raise intraocular pressure.33,36

Irritation of any branch of the trigeminal nerve activates a reflex afferent pathway, precipitating increased lacrimation.7,37

Clinical Comment: Corneal Reflex

Corneal touch initiates the three-part corneal reflex: lacrimation, miosis, and a protective blink (Figure 14-4). The pain sensation elicited by the touch travels to the trigeminal ganglion and then into the pons as the trigeminal nerve. Communication from the trigeminal nucleus to the Edinger-Westphal nucleus causes activation of the sphincter muscle. Communication to the facial nerve nucleus activates the motor pathway to the orbicularis muscle, causing the blink, and communication to the lacrimal nucleus and the parasympathetic pathway to the lacrimal gland stimulates increased lacrimation.

Deep petrosal nerve

Vidian nerve

Pterygopalatine ganglion

(no synapse)

Maxillary nerve

Zygomatic nerve

Communicating

branch

Lacrimal nerve

Lacrimal-gland

blood vessels

Vasoconstriction

Preganglionic

neuron

Preganglionic

fiber

Ganglion (synapse occurs)

Postganglionic

fiber

Structure

Action

B

Edinger-Westphal nucleus in midbrain

Oculomotor nerve

Inferior division

Parasympathetic

root

Ciliary ganglion

Short ciliary nerves

Miosis Accommodation

Lacrimal nucleus in pons

Facial nerve

Greater petrosal nerve

Vidian nerve

Pterygopalatine

ganglion

Maxillary nerve

Zygomatic nerve

Communicating

branch

Lacrimal nerve

Lacrimal gland

Lacrimation

P H A R M A C O L O G I C R E S P O N S E S O F

I N T R I N S I C M U S C L E S

Pharmacologic agents can alter autonomic responses. Topical ophthalmic drugs, which readily pass through the cornea, can be used to activate or inhibit the intrinsic ocular muscles.

After a brief discussion of neurotransmitters and drug types relative to iris musculature, this section presents specific drugs that induce mydriasis or miosis,

as well as drugs used in the differential diagnosis of certain pupillary abnormalities. The reader is encouraged to review a text on pharmacology for detailed information.

NEUROTRANSMITTERS

When an action potential reaches the terminal end of an axon, a neurotransmitter is released that activates either the next fiber in the pathway or the target structure, the effector. In the sympathetic pathway the