CN III, IV, VI

CRANIAL NERVE III: OCULOMOTOR NERVE

contains axons that arise in the oculomotor nucleus (which innervates

all of the oculomotor muscles except the superior oblique and lateral rectus)

Edinger–Westphal nucleus (which sends preganglionic parasympathetic axons to the ciliary ganglion).

leaves the brain on the medial side of the cerebral peduncle, behind the posterior cerebral artery and in front of the superior cerebellar artery.

then passes anteriorly, parallel to the internal carotid artery in the lateral wall of the cavernous sinus, leaving the cranial cavity by way of the superior orbital fissure.

The somatic efferent portion of the nerve innervates the levator palpebrae superioris muscle; the superior, medial, and inferior rectus muscles; and the inferior oblique muscle

The visceral efferent portion innervates two smooth intraocular muscles: the ciliary and the constrictor pupillae.

CLINICAL CORRELATIONS FOR NERVES III, IV, AND VI AND THEIR CONNECTIONS

Strabismus (squint) is the deviation of one or both eyes.

In internal strabismus, the visual axes cross each other

in external strabismus, the visual axes diverge from each other

Diplopia (double vision) is a subjective phenomenon reported to be present when the patient is, usually, looking with both eyes

caused by misalignment of the visual axes

Ptosis (lid drop) is caused by weakness or paralysis of the levator palpebrae superioris muscle

seen with lesions of nerve III and sometimes in patients with myasthenia gravis.

OCULOMOTOR (NERVE III) PARALYSIS

External ophthalmoplegia is characterized by divergent strabismus, diplopia, and ptosis.

The eye deviates downward and outward.

This corresponds to the weaknesses of the medial, superior, and inferior recti and the inferior oblique muscles.

position of the eye is described by the mnemonic "down and out."

Internal ophthalmoplegia is characterized by a dilated pupil and loss of light and accommodation reflexes.

There may be paralysis of individual muscles of nerve III

Isolated involvement of nerve III (often with a dilated pupil) occurs as an early sign in uncal herniation because of expanding hemispheric mass lesions that compress the nerve against the tentorium

Nerve III crosses the internal carotid, where it joins the posterior communicating artery; aneurysms of the posterior communicating artery thus can compress the nerve

Isolated nerve III palsy also occurs in diabetes, presumably because of ischemic damage, and when caused by diabetes, often spares the pupil

CRANIAL NERVE IV: TROCHLEAR NERVE

the only crossed cranial nerve originates from the trochlear nucleus,

which is a group of specialized motor neurons located just caudal to the CN III nucleus within the lower midbrain.

Axons cross within the midbrain, and then emerge contralaterally on the dorsal surface of the brain stem.

nerve then curves ventrally between the posterior cerebral and superior cerebellar arteries (lateral to CN III).

continues anteriorly in the lateral wall of the cavernous sinus and enters the orbit via the superior orbital fissure.

innervates the superior oblique muscle

TROCHLEAR (NERVE IV) PARALYSIS

rare condition slight convergent strabismus and

diplopia on looking downward patient cannot look downward and

inward -has difficulty in descending stairs.

head is tilted as a compensatory adjustment; this may be the first indication of a trochlear lesion.

CRANIAL NERVE VI: ABDUCENS NERVE

arises from neurons of the abducens nucleus located within the dorsomedial tegmentum within the caudal pons.

emerges from the pontomedullary fissure, passes through the cavernous sinus close to the internal carotid, and exits from the cranial cavity via the superior orbital fissure

long intracranial course makes it vulnerable to pathologic processes in the posterior and middle cranial fossae.

innervates the lateral rectus muscle

few proprioceptive fibers from the muscles of the eye are present in nerves III, IV, and VI and in some other nerves that innervate striated muscles.

central termination of these fibers is in the mesencephalic nucleus of V

ABDUCENS (NERVE VI) PARALYSIS

most common owing to the long course of nerve VI.

weakness of eye abduction convergent strabismus and diplopia. affected eye deviates medially, i.e., in

the direction of the opposing muscle