Innervates four of the six extrinsic muscles that move the eye + the muscle that elevates the upper eyelid (skeletal motor).
Innervates smooth muscle in the eyeball (parasympathetic).
Skull opening
Superior orbital fissure.
Attachment to brainstem
Midbrain.
Nucleus of origin/destination in CNS
Skeletal motor fibers originate from the oculomotor nucleus in the midbrain.
Pre-ganglionic parasympathetic fibers originate from Edinger-Westphal nucleus in the midbrain.
Details
Cranial nerve III is so named because it is the chief motor nerve to the ocular muscles—it supplies all the extrinsic muscles that move the eyeball EXCEPT for the superior oblique and the lateral rectus muscles. It also supplies the levator palpebrae superioris—the muscle that elevates the upper eyelid.
The oculomotor nucleus is in the midbrain. CN III emerges from the anterior surface of the midbrain and passes anteriorly between the posterior cerebral and superior cerebellar arteries. It pierces the dura mater and passes through the cavernous sinus (where it is bathed by venous blood!). As it leaves the cavernous sinus it divides into a superior and an inferior division. The divisions pass through the superior orbital fissure and enter the orbit.
The superior division supplies the levator palpebrae superioris and the superior rectus muscles. The inferior division supplies the medial rectus, inferior rectus, and inferior oblique muscles.
Preganglionic parasympathetic neuron cell bodies are in the nucleus of Edinger-Westphal in the midbrain. Preganglionic fibers follow the oculomotor nerve—then the inferior division—to synapse in the ciliary ganglion within the orbit. Postganglionic fibers reach the eyeball through the short ciliary nerves. These fibers innervate the ciliary muscle (producing accommodation = shaping the lens so that objects nearby come into focus) and smooth muscle in the constrictor pupillae muscle (producing constriction of pupils, so less light enters the eye).
This reflex adjusts the diameter of the pupil in response to the intensity of light striking the retina.
Afferent portion = Optic nerve
Efferent portion = Oculomotor nerve, specifically the two neuron parasympathetic pathway that synapses in the ciliary ganglion.
The postganglionic PS neurons in the short ciliary nerves stimulate the constrictor pupillae muscle in the iris to contract, thus reducing the size of the pupil in response to bright light. In an intact reflex, the pupil in the illuminated eye constricts (called the direct response) as does the pupil in the non-illuminated eye (consensual response). Loss of the pupillary reflex may indicate a lesion of the optic nerve, oculomotor nerve, or brainstem.
A template for drawing the pathway of the oculomotor nerve is also provided in the Cranial Nerves Appendix.
Cranial nerve IV: Trochlear nerve
Functions
Innervates the superior oblique—an extrinsic muscle of the eye (skeletal motor).
Skull opening
Superior orbital fissure.
Attachment to brainstem
Dorsal surface of midbrain.
Nucleus of origin/destination in CNS
Motor fibers originate from the trochlear nucleus in the midbrain.
Details
Figure 29.8 Relationship of CN III and IV to circle of Willis and Cavernous Sinus (in blue). GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 7.29.
This nerve is named because of the trochlea—a pulley-like structure made of cartilage in the upper medial part of the orbit—through which the tendon of the superior oblique muscle passes. Passage of the tendon through the trochlea changes the direction of the muscle pull on the eyeball.
The trochlear nucleus is in the midbrain. The motor fibers of IV decussate (cross) within the midbrain then exit from the posterior surface of the midbrain below the inferior colliculus. The nerve is very long and slender—it winds around the midbrain and like CN III it passes between the posterior cerebral and superior cerebellar arteries, then traverses the cavernous sinus. It enters the orbit through the superior orbital fissure and penetrates the superior surface of the superior oblique muscle.
Cool fact: The trochlear nerve is the ONLY motor nerve—cranial or spinal—that arises from the dorsal aspect of the central nervous system.
A template for drawing the pathway of the trochlear nerve is also provided in the Cranial Nerves Appendix.
Cranial nerve VI: Abducens nerve
Functions
Innervates the lateral rectus—an extrinsic muscle of the eye (skeletal motor).
Skull opening
Superior orbital fissure.
Attachment to brainstem
Pons/medulla junction, almost at the midline.
Nucleus of origin/destination in CNS
Skeletal motor fibers originate from the abducens nucleus in the pons.
Details
Figure 29.9 Summary of innervation of extra-ocular muscles. ESSENTIAL NEUROSCIENCE, 2ND ED., FIGURE 14-10.
Cranial nerve VI supplies one extrinsic eye muscle = the lateral rectus. When this muscle contracts, the eye swivels laterally and the gaze is directed away from the midline. CN VI is named from its role in abduction of the eye.
The abducens nucleus is in the caudal pons. CN VI leaves the brainstem at the junction of the pons and medulla. It passes superiorly in the subarachnoid space within the posterior cranial fossa, penetrates the dura, and then bends sharply over the apex of the petrous part ofthe temporal bone to enter the cavernous sinus. It runs in the center of the sinus alongside the internal carotid artery. It enters the orbit through the superior orbital fissure. CN VI enters the medial surface of the lateral rectus muscle.
A template for drawing the pathway of the abducens nerve is also provided in the Cranial Nerves Appendix.
Cranial nerve XI: Spinal accessory nerve
Functions
Innervates sternocleidomastoid and trapezius muscles (skeletal motor).
Skull opening
Jugular foramen.
Attachments to CNS
Does not attach to the brainstem. Instead, it attaches to the lateral side of the cervical spinal cord.
Nucleus of origin/destination in CNS
Skeletal motor fibers originate from the accessory nuclei, located in the ventral gray column of the upper 5 or 6 cervical spinal cord segments.
Details
Figure 29.10 Topography of CN XI. GILROY, ATLAS OF ANATOMY.
The spinal accessory nerve is formed from rootlets that emerge from the lateral spinal cord—from the upper six cervical segments. The cell bodies of these neurons are in the accessory nuclei, a column of gray matter in the lateral part of the ventral horn of gray in the spinal cord, extending from the medulla/spinal cord junction to C-6 spinal cord segment. Once outside the spinal cord the rootlets unite to form a trunk that ascends in the vertebralcanal and passes into the cranial cavity via foramen magnum.
CN XI exits the skull through the jugular foramen. Even though XI does NOT attach to the brain, it is considered a proper cranial nerve since it exits the skull through a bony opening.
Once outside the skull, CN XI passes deep to the styloid process and posterior belly of the digastric muscle. CN XI enters the deep surface of the sternocleidomastoid (SCM), which it innervates, then leaves the posterior border of SCM near its midpoint to enter the posterior triangle of the neck. XI crosses the posterior triangle obliquely (where it is vulnerable to trauma injury) then enters the anterior border of trapezius, which it also innervates. CN XI is tested by having the patient turn their head from side-to-side and shrug their shoulders against resistance.
A template for drawing the pathway of the spinal accessory nerve is also provided in the Cranial Nerves Appendix.
Cranial nerve XII: Hypoglossal nerve
Functions
Innervates intrinsic and extrinsic muscles of the tongue (skeletal motor).
Skull opening
Hypoglossal canal.
Attachments to CNS
Arises bilaterally as rootlets from the anterior surface of the medulla, in the sulcus between the pyramid and olive.
Nucleus of origin/destination in CNS
Skeletal motor fibers originate from the hypoglossal nucleus in the medulla.
Details
Figure 29.11 Topography of CN XII and “hitch-a-ride” cervical spinal nerve fibers. CLINICALLY ORIENTED ANATOMY, FIGURE 9.18.
CN XII is the motor nerve of the tongue. It arises from the hypoglossal nucleus in the medulla. Rootlets of the nerve leave the medulla between the pyramid and olive, penetrate the dura, and then fuse to form CN XII as it exits the skull through the hypoglossal canal. CN XII descends between the internal carotid artery and internal jugular vein and appears in the neck as a loop, just inferior to the digastric and stylohyoid muscles. CN XII then ascends deep to the digastric and mylohyoid muscles to enter the tongue from below (thus the name: hypoglossal).
CN XII innervates all muscles of the tongue, both intrinsic (= produce fine movements, such as wiggling the tip of the tongue) and extrinsic (= produce gross movements—like protruding the tongue out of the mouth). All muscles with the suffix “glossus” are innervated by CN XII (hyoglossus, styloglossus, and genioglossus), EXCEPT one = palatoglossus, which is a muscle of the palate supplied by CN X.
Several branches of the cervical plexus hitch-a-ride with CN XII to reach muscles in the neck. NOTE: These are NOT hypoglossal nerve fibers—but fibers derived from ventral rami of cervical spinal nerves.
C-1 fibers ride along the hypoglossal nerve, then drop off in the neck to form the superior root of the ansa cervicalis. The superior root joins the inferior root (fibers from C-2 and C-3 that do not follow CN XII) to complete the loop-shaped ansa cervicalis. The ansa is a motor nerve that innervates infrahyoid (“strap”) muscles in the neck.
Other C-1 fibers that join the hypoglossal nerve ride further forward in the neck to innervate the geniohyoid muscle in the floor of the mouth and the thyrohyoid muscle, one of the infrahyoid muscles.
