Ocular motor nerves

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23 Ocular motor nerves

Study Guidelines

Because of the immense diagnostic and therapeutic importance of ocular innervation, and because of its inherent complexity, neuro-ophthalmology has become a branch of medicine in its own right.

It is especially important to appreciate the way in which premotor centers are able to operate bilaterally in order to keep the gaze on target, even when the head is moving.

1 In addition to nerves III, IV, and VI, the sympathetic supply to the eye is reviewed here because it counterbalances the parasympathetic innervation contained in nerve III.

2 The nerve supply to the six muscles that move the eyeball is straightforward. However, nerve III also supplies the elevator of the upper eyelid.

3 The autonomic supply to the eye is straightforward. For the sympathetic, think of someone being startled awake at night. For the parasympathetic, think of someone eyeing food on a fork.

The Nerves

The ocular motor nerves comprise the oculomotor (III cranial), trochlear (IV cranial), and abducens (VI cranial) nerves. They provide the motor nerve supply to the four recti and two oblique muscles controlling movements of the eyeball on each side (Figure 23.1). The oculomotor nerve contains two additional sets of neurons: one to supply the levator of the upper eyelid, the other to control the sphincter of the pupil and the ciliary muscle.

Figure 23.1 Extrinsic ocular muscles.

The nuclei serving the extraocular muscles (extrinsic muscles of the eye) belong to the somatic efferent cell column of the brainstem, in line with the nucleus of the hypoglossal nerve. The oculomotor nucleus has an additional, parasympathetic nucleus which belongs to the general visceral efferent cell column.

Oculomotor nerve

The nucleus of the third nerve is at the level of the superior colliculus. It is partly embedded in the peri-aqueductal gray matter (Figure 23.2A). It is composed of five individual nuclei for the supply of striated muscles, and one parasympathetic nucleus.

Figure 23.2A–C Transverse sections of the brainstem showing the origins of the ocular motor nerves.

The nerve passes through the tegmentum of the midbrain and emerges into the interpeduncular cistern. It crosses the apex of the petrous temporal bone, pierces the dural roof of the cavernous sinus, runs in the lateral wall of the sinus, and breaks into upper and lower divisions within the superior orbital fissure. The upper division supplies the superior rectus and the levator palpebrae superioris; the lower division supplies the inferior and medial recti and the inferior oblique.

The parasympathetic fibers originate in the Edinger–Westphal nucleus. They accompany the main nerve as far as the orbit, then leave the branch to the inferior oblique and synapse in the ciliary ganglion. Postganglionic fibers emerge from the ganglion in the short ciliary nerves, which pierce the lamina cribrosa (’sieve-like layer’) of the sclera and supply the ciliaris and sphincter pupillae muscles.

Trochlear nerve

The nucleus of the fourth nerve is at the level of the inferior colliculus. The nerve itself is unique in two respects (Figure 23.2B): it is the only nerve to emerge from the back of the brainstem; and it decussates with its opposite number.

The IV nerve winds around the crus of the midbrain and travels through the cavernous sinus in company with the III nerve (Figure 23.3). It passes through the superior orbital fissure and supplies the superior oblique muscle.

Figure 23.3 (A) Middle cranial fossa with cavernous sinuses removed. (B) Coronal section in the plane of the hypophysis with the cavernous sinuses in place. III, Oculomotor nerve; IV, trochlear nerve; VI, abducens nerve; Vi, Vii, Viii, ophthalmic, maxillary, mandibular divisions of trigeminal nerve.

Abducens nerve

The nucleus of the sixth nerve, in the floor of the fourth ventricle, is at the level of the facial colliculus, in the lower pons (Figure 23.2C). The nerve descends, to emerge at the lower border of the pons, and runs up the pontine subarachnoid cistern beside the basilar artery. It angles over the apex of the petrous temporal bone and passes through the cavernous sinus beside the internal carotid artery (Figure 23.3). It enters the orbit through the superior orbital fissure and supplies the lateral rectus muscle, which abducts the eye.

Nerve Endings

Motor endings

All of the ocular motor units are small, containing 5 to 10 muscle fibers apiece (compared with 1000 or more in the tibialis anterior).

Type A fibers produce the fast twitches required for saccadic movements. Type B are slow-twitch and may be used for smooth pursuit. Type C show only local contractions beneath the individual plates. Type C fibers may be involved in keeping the visual axes of the two eyes parallel with one another. Since the visual axes diverge following administration of muscle relaxants, keeping them parallel must require continuous muscle action, even during sleep.

Sensory endings

In addition to neuromuscular spindles of standard type, numerous palisade endings exist in the form of nerve spirals around individual muscle fibers.

The extraocular muscle proprioceptors are the peripheral terminals of neurons in the mesencephalic nucleus of the trigeminal nerve. In monkeys, some of the central processes of these neurons reach as far caudally as the accessory cuneate nucleus in the medulla oblongata. This nucleus also receives proprioceptive terminals from the neck muscles, and it projects both to the ipsilateral cerebellum and to the contralateral superior colliculus. The conjunction of ocular and cervical proprioceptive information presumably assists in the coordination of simultaneous movements of the eyes and head.

Pupillary Light Reflex (Figure 23.4)

Constriction of the pupils in response to light involves four sets of neurons, as follows:

1 The afferent limb commences in the ganglionic layer of the retina, which gives rise to the optic nerve. Fibers leaving the chiasma enter both optic tracts and terminate in the pretectal nuclei, situated just rostral to the superior colliculus on each side (Figure 17.19).

2 Each pretectal nucleus is linked by internuncial neurons to both Edinger–Westphal (parasympathetic) nuclei; the contralateral nucleus is reached by way of the posterior commissure.

3 Preganglionic parasympathetic fibers enter the oculomotor nerve, leave the branch to the inferior oblique, and synapse in the ciliary ganglion.

4 Postganglionic fibers run in the short ciliary nerves and enter the iris to supply the sphincter (constrictor) pupillae.

Figure 23.4 Pupillary light reflex. For numbers, see text.

Accommodation

The near response

When the eyes view an object close up, the ciliary muscle contracts reflexly, thereby relaxing the suspensory ligament of the lens (Figure 23.5). Since the lens at rest is somewhat compressed (flattened) by tension exerted on the lens capsule by the suspensory ligament, the lens bulges passively when the ciliary muscle contracts. The thicker lens has the greater refractive power required to bring close-up objects into focus on the retina. The response of the lens is one of accommodation.

Figure 23.5 Intrinsic muscles of the eye.

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