The Nervous System - Basic Version / The Cranial Nerves

Cranial nerves can be thought of as modified spinal nerves, since the “general” functional fiber types found in spinal nerves also are found in cranial nerves but are supplemented by “special” afferent or efferent fibers. fibers conveying olfaction (in cranial nerve I) and taste (in cranial nerves VII, IX, and X) are classified as special visceral afferent, while the designation of special somatic afferent is applied to fibers conveying vision (cranial nerve II) and equilibrium and hearing (cranial nerve VIII). Skeletal muscles that arise from the branchial (pharyngeal) arches are innervated by fibers of cranial nerves V, VII, IX, and X; these are classified as special visceral efferent fibers.

The 12 pairs of cranial nerves are commonly identified either by name or by Roman or Arabic numeral.

Oculomotor nerve (CN III or 3)

The oculomotor nerves arise from two nuclei in the rostral midbrain. These are (1) the oculomotor nucleus, the source of general somatic efferent fibers to superior, medial, and inferior recti muscles, to the inferior oblique muscle, and to the levator palpebrae superious muscle; and (2) the Edinger-Westphal nucleus, which projects general visceral efferent preganglionic fibers to the ciliary ganglion.

The oculomotor nerve exits the ventral midbrain, pierces the dura, courses through the lateral wall of the cavernous sinus, and exits the cranial cavity via the superior orbital fissure. Within the orbit it branches into a superior ramus (to the superior rectus and levator muscles) and an inferior ramus (to the medial and inferior rectus muscles, the inferior oblique muscles, and the ciliary ganglion). Postganglionic fibers from the ciliary ganglion innervate the sphincter pupillae muscle of the iris as well as the ciliary muscle.

With the exception of the levator palpebrae superioris muscle, which is innervated bilaterally, oculomotor neurons project primarily to orbital muscles on the same side of the head. A lesion of the oculomotor nerve will result in paralysis of the three rectus muscles and the inferior oblique muscle (causing the eye to rotate downward and slightly outward), paralysis of the levator palpebrae superious muscle (drooping of the eyelids), and paralysis of the sphincter pupillae and ciliary muscles (so that the iris will remain dilated and the lens will not accommodate).

Trochlear nerve (CN IV or 4)

The fourth cranial nerve is unique for three reasons. First, it is the only cranial nerve to exit the dorsal side of the brain stem. Second, fibers from the trochlear nucleus cross in the midbrain before they exit, so that trochlear neurons innervate the contralateral (opposite side) superior oblique muscle of the eye. Third, trochlear fibers have a long intracranial course before piercing the dura.

The trochlear nucleus is located in the caudal midbrain; the functional component of these cells is general somatic efferent. After exiting at the dorsal side of the midbrain, the trochlear nerve loops around the midbrain, pierces the dura, and passes through the lateral wall of the cavernous sinus. It then enters the orbit through the superior orbital fissure and innervates only the superior oblique muscle, which rotates the eye downward and slightly outward. Damage to the trochlear nerve will result in a loss of this eye movement and may produce double vision (diplopia).

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