Motor cranial nerves

Cranial nerves list and chart

The cranial nerves (CN) are 12 pairs of nerves that primarily supply the head and neck region. With the exception of the spinal accessory nerve (CN XI), the cranial nerves originate in the brain.

The cranial nerves include:

1. CN I: olfactory nerve
2. CN II: optic nerve
3. CN III: oculomotor nerve
4. CN IV: trochlear nerve
5. CN V: trigeminal nerve
6. CN VI: abducens nerve
7. CN VII: facial nerve
8. CN VIII: vestibulocochlear nerve
9. CN IX: glossopharyngeal nerve
10. CN X: vagus nerve
11. CN XI: spinal accessory nerve
12. CN XII: hypoglossal nerve

Motor cranial nerves

Cranial nerves can have sensory or motor functions, and mixed cranial nerves contribute to multiple functions. All motor cranial nerves (oculomotor, trochlear, abducens, spinal accessory and hypoglossal nerves) have general somatic efferent fibers. The oculomotor nerve (CN III) also has general visceral efferent fibers, responsible for parasympathetic responses like pupil constriction and ocular accommodation.

In terms of functions of the motor cranial nerves, three are mainly responsible for moving the eyeball within its socket: the trochlear (CN IV) and the abducens (CN VI) nerves supply one extra-ocular muscle each, whereas the others are innervated by the oculomotor nerve (CN III). The spinal accessory nerve (CN XI) supplies some of the muscles of the neck and back responsible for head and scapular movements. The hypoglossal nerve (CN XII) innervates most tongue muscles, so it is primarily responsible for tongue movements. Other motor functions like facial expressions and phonation are mainly regulated by efferent branches of mixed cranial nerves.

Interested in knowing more about the cranial nerves in general or the nuclei they originate from? Explore these study units!

Oculomotor nerve (CN III)

The oculomotor nerve originates in the midbrain at the level of the superior colliculus, extends through the cavernous sinus, and enters the orbit via the superior orbital fissure.

The somatic fibers of the oculomotor nerve originate from the oculomotor nuclear complex and innervate most of the extraocular muscles. The superior division innervates the superior rectus and levator palpebrae superioris, whereas the inferior division innervates the medial rectus, the inferior rectus and the inferior oblique. These muscles are responsible for movements of the eyeball within its socket and for elevating the upper eyelid.

The visceral fibers of the oculomotor nerve originate from the accessory (Edinger-Westphal) nucleus, synapse in the ciliary ganglion, then supply the intraocular muscles. In response to stimulation of the optic nerve, these fibers contribute to two main parasympathetic functions: the pupillary light reflex (reflexive constriction of the pupil in response to light) and the accommodation reflex (change in refractive power of the eye for near vision).

To dive deeper into the motor cranial nerves responsible for the motor functions of the eye, check out the study unit below.

Trochlear nerve (CN IV)

The nucleus of the trochlear nerve is located in the midbrain tegmentum, at the level of the inferior colliculus.

The nerve emerges from the dorsal midbrain, travels through the cavernous sinus, and enters the orbit via the superior orbital fissure to reach the superior oblique extraocular muscle.

This cranial nerve only has somatic efferent fibers, and innervates the superior oblique muscle, responsible for internally rotating, abducting and depressing the eyeball.

Are you a visual learner? Memorize the cranial nerves more quickly with our labeling exercises, available to download for free.

Abducens Nerve (CN VI)

Like the trochlear nerve (CN IV), the abducens nerve only has somatic efferent fibers and it innervates a single extraocular muscle.

The nucleus of the abducens nerve is located in the tegmentum of the pons. The nerve fibers travel through the cavernous sinus and enter the orbit via the superior orbital fissure to reach the lateral rectus muscle.

The main function of this muscle is abduction of the eyeball.

How much do you know about the cranial nerves responsible for the motor functions of the eye? Assess your learning in this quiz.

Spinal accessory nerve (CN XI)

The spinal accessory nerve, or accessory nerve, is composed of spinal roots that arise from the first through fifth segments of the cervical spinal cord. These fibers ascend through the foramen magnum and into the cranial cavity, then travel through the posterior cranial fossa and leave the skull via the jugular foramen.

The spinal accessory nerve descends to innervate the sternocleidomastoid and trapezius muscles, and is therefore responsible for carrying motor commands of neck lateral flexion and rotation, and scapular elevation, adduction and depression.

Hypoglossal nerve (CN XII)

The hypoglossal nerve emerges from the anterior surface of the medulla, travels laterally across the posterior cranial fossa, and exits the skull via the hypoglossal canal.

The hypoglossal nerve innervates all the intrinsic muscles of the tongue (superior longitudinal, vertical, transverse and inferior longitudinal muscles), and the majority of extrinsic muscles of the tongue (genioglossus, hyoglossus, styloglossus). The palatoglossus is the only tongue muscle innervated by a different cranial nerve, the vagus nerve (CN X).

Explore the anatomy and function of the accessory and hypoglossal nerves in these study units.