there is a resting rate of action potential discharge in the primary vestibular afferents (center hair cell). Depolarization occurs when the stereocilia are deflected toward the kinocilium (represented by the longest cilium, with beaded end). Hyperpolarization occurs when the stereocilia are deflected away from the kinocilium. This movement of the stereocilia modulates the discharge rate in the vestibular nerve neuron. Bottom: The action potential generated by the shearing forces on the hair cell. Depolarization (left side of graph) causes an increase in the frequency of the action potential. Hyperpolarization causes a decrease in the frequency of action potential. (Reproduced with permission from Leigh RJ, Zee DS. The

Neurology of Eye Movements. 4th ed. New York: Oxford University Press; 2006.)

For information on clinical disorders of vestibular ocular function, see Clinical Disorders of the Ocular Motor Systems in Chapter 7.

Cerebellum

The other major connection in the ocular motor system is to the vestibulocerebellum. The structures in this area, largely through the brachium conjunctivum, are responsible for adjusting the gain of all ocular movements. Gain may be defined as the output divided by the input. For example, keeping the eyes stable in space while the head rotates requires the eyes to move in a direction opposite that of head rotation at the same velocity and distance; this would be considered a gain of 1. The cerebellum is involved in gain adjustment to allow compensation after peripheral lesions (eg, vestibular nerve dysfunction such as vestibular neuritis). Disease processes directly affecting the cerebellum may affect the vestibular ocular reflex.

Efference copy information (regarding the position of the eyes) is supplied directly from the ocular motor pathways (possibly through cell groups of the paramedian tracts within the vestibular nuclei), whereas afferent signal error information arrives at the cerebellum via the climbing fibers from the inferior olivary nucleus. Additional cerebellar inputs include mossy fiber input from the vestibular nuclei and the NPH. Purkinje cells within the paraflocculus discharge during smooth pursuit. The dorsal vermis may play a role in initiating pursuit and saccades. The fastigial nucleus is responsible for overcoming a natural imbalance in the input from the vertically oriented semicircular canals. Thus, loss of fastigial function may be associated with development of downbeat nystagmus, as the imbalance in the vertical information causes constant updrift.

See Chapter 9 for further discussion of nystagmus and other disordered eye movements.

Ocular Motor Cranial Nerves

Without neural activity, the visual axes are usually mildly to moderately divergent. The major tonic input to ocular motility is supplied by 3 pairs of ocular motor cranial nerves—CNs III, IV, and VI— that innervate the 6 EOMs controlling ocular movement (Fig 1-31). In addition, CN III innervates the levator palpebrae superioris and the pupillary sphincter muscles.

Figure 1-31 Lateral view of the course of CNs III, IV, and VI. (Illustration b y Dave Peace.)

Except for the inferior oblique muscle, the innervation to each of the EOMs occurs approximately one-third the distance from the apex. The inferior oblique muscle receives its innervation at approximately its midpoint from a neurovascular bundle running parallel to the lateral aspect of the inferior rectus muscle. All 6 EOMs receive their innervation on the inside surface, except for the superior oblique, where branches of CN IV terminate on the upper (outer) surface of the muscle.

See Chapter 8 for information on the clinical presentation of disorders due to infranuclear, fascicular, and peripheral ocular motor cranial nerve lesions.

Abducens nerve (CN VI)

CN VI originates in the dorsal caudal pons just beneath the fourth ventricle. Its nucleus is surrounded by the looping fibers (genu) of CN VII and is adjacent to the PPRF and the MLF (Fig 1-32). The CN VI nucleus contains primary motoneurons and interneurons that cross to the contralateral MLF to reach the CN III nucleus. Thus, pathology affecting the CN VI nucleus produces an ipsilateral gaze palsy. The motor axons exiting the CN VI nucleus (approximately 4,000–6,000 axons) travel ventrally and slightly laterally, medial to the superior olivary nucleus, to exit on the ventral surface of the caudal pons. As the fascicles pass through the brainstem, they lie adjacent to the spinal tract of CN V and traverse the corticobulbar tracts. Exiting the brainstem, the nerve runs rostrally within the subarachnoid space on the surface of the clivus from the area of the cerebellopontine angle to the posterior superior portion of the posterior fossa. The nerve pierces the dura approximately 1 cm below the petrous apex and travels beneath the petroclinoid ligament (Gruber ligament, which connects the petrous pyramid to the posterior clinoid) to enter the canal of Dorello. Within the canal, CN VI travels with the inferior petrosal sinus. Once it becomes extradural, the nerve is within the cavernous sinus (the only cranial nerve within the substance of the cavernous sinus), where it runs parallel to the horizontal segment of the carotid artery and V1. It is also joined for a short segment by branches of the sympathetic chain lying within the wall of the intrapetrous carotid artery. Reaching the

anterior portion of the cavernous sinus, CN VI traverses the superior orbital fissure (Fig 1-33) through the annulus of Zinn (Fig 1-2C) to enter the medial surface of the lateral rectus muscle.

Figure 1-32 Intra-axial course of the ocular motor nerves at the level of the pons (below) and midbrain (above). Note the relationship to the surrounding cerebellum and CNs V and VII.

Figure 1-33 A, Subarachnoid course of the ocular motor nerves. Note the relationship to the surrounding dural structures, particularly the tentorium and the dura of the clivus. The nerves enter dural canals at the posterior aspect of the cavernous sinus for CN III, at the tentorial edge for CN IV, and along the clivus for CN VI. B, Major blood vessels and their relationships to the ocular motor nerves. Note the passage of CN III between the superior cerebellar artery below and posterior cerebral artery above. CN VI runs by the anterior inferior cerebellar artery, which is a major branch off the basilar artery. C, Intracavernous course of the ocular motor nerves. CN III and CN IV run in the lateral wall of the cavernous sinus along with CN V divisions V1 and V2. CN VI runs in close approximation to the carotid artery within the cavernous sinus itself. As the

nerves course toward the anterior aspect of the cavernous sinus and the superior orbital fissure, V1 (ophthalmic) divides

into 3 branches: the lacrimal, frontal, and nasociliary nerves. ACoA = anterior communicating artery; ACP = anterior clinoid process; ICA = internal carotid artery; Inf. Br. = inferior branch; Prox = proximal; Sup. Br. = superior branch. (Illustrations b y

Craig A. Luce.)

Trochlear nerve (CN IV)

The trochlear nerve (CN IV) nucleus lies within the gray matter in the dorsal aspect of the caudal midbrain just below the aqueduct, directly contiguous with the more rostral third nerve nucleus (see Fig 1-32). The intra-axial portion (fascicle) of CN IV is very short, running dorsally around the periaqueductal gray to cross within the anterior medullary vellum just caudal to the inferior colliculi and below the pineal gland. CN IV is the only cranial nerve exiting on the dorsal surface of the brainstem and has the longest unprotected intracranial course (which is probably responsible for its frequent involvement in closed-head trauma). Within the subarachnoid space, CN IV (containing approximately 2,000 fibers) swings around the midbrain, paralleling the tentorium just under the tentorial edge (where it is easily damaged during neurosurgical procedures that involve the tentorium).

Just below the anterior tentorial insertion, CN IV enters the posterior lateral aspect of the cavernous sinus just underneath CN III. Covered by a variable sheath, CN IV runs forward within the lateral wall of the cavernous sinus. Anteriorly, CN IV crosses over CN III to enter the superior orbital fissure outside and superior to the annulus of Zinn. CN IV crosses over the optic nerve to enter the superior oblique muscle within the superior medial orbit.

Oculomotor nerve (CN III)

The nucleus of the oculomotor nerve (CN III) is located dorsally within the midbrain beneath the aqueduct connecting the third and fourth ventricles (see Fig 1-33). The nuclear complex itself represents a collection of subnuclei that have specific identifiable functions (Fig 1-34). The fibers destined to innervate the levator palpebrae superioris, medial rectus, inferior rectus, pupil sphincter, and ciliary body muscles exit ventrally ipsilateral to the individual nuclei from which they originate. In contrast, the fibers from the superior rectus subnucleus, which lies along the midline, cross before exiting the brainstem to innervate the superior rectus muscle. Within the midbrain, CN III is topographically organized into a superior division (supplying the superior rectus and levator palpebrae superioris muscles) and an inferior division (supplying the medial and inferior rectus, inferior oblique, pupillary sphincter, and ciliary body muscles), but the true anatomical division into 2 branches occurs at the level of the anterior cavernous sinus/superior orbital fissure.

Figure 1-34 Oculomotor nucleus complex. Note that all extraocular muscles served by CN III are innervated by their respective ipsilateral nuclei except the superior rectus muscle. Parasympathetic fibers traveling to the pupillary sphincter muscle synapse in the ciliary ganglion in the orbit (see Fig 1-40). (Illustration b y Christine Gralapp.)

The fascicles of CN III traverse the ventral midbrain tegmentum, passing near and possibly through the red nucleus, the substantia nigra, and the corticospinal tracts within the cerebral peduncle. Numerous fascicles, totaling approximately 15,000 fibers, exit on the ventral surface of the peduncles. Although seen as a single structure within the subarachnoid space—as is the case in the midbrain—the nerve and its various fibers are topographically organized. Within the subarachnoid space, the nerve passes between the SCA below and the PCA above. The nerve runs slightly oblique to the tentorial edge, parallel and lateral to the PCoA. The pupillary fibers are usually found on the