Motor cranial nerves

List

The cranial nerves (CN) are twelve pairs of nerves that, with the exception of the spinal accessory nerve (CN XI), originate in the brain and contribute to the peripheral nervous system (PNS), supplying the head and neck.

These 12 paired nerves, and their main branches, include:

• The olfactory nerve (CN I)
• The optic nerve (CN II)
• The oculomotor nerve (CN III)
• The trochlear nerve (CN IV)
• The trigeminal nerve (CN V)
  • Ophthalmic branch (V1)
  • Maxillary branch (V2)
  • Mandibular branch (V3)
• The abducens nerve (CN VI)
• The facial nerve (CN VII)
  • Temporal branch
  • Zygomatic branch
  • Buccal branch
  • Mandibular branch
  • Cervical branch
• The vestibulocochlear nerve (CN VIII)
• The glossopharyngeal nerve (CN IX)
• The vagus nerve (CN X)
• The spinal accessory nerve (CN XI)
• The hypoglossal nerve (CN XII)

Motor or sensory

Of these nerves, some have special sensory functions, some have somatic sensory functions, some have autonomic functions, some have somatic motor functions, and some have a combination of the aforementioned functions. The olfactory nerve, the optic nerve, the facial nerve, the vestibulocochlear nerve, the glossopharyngeal nerve, and the vagus nerve each play roles in special sensory functions (i.e. olfaction, vision, gustation, audition, and balance).

The trigeminal nerve (all three branches: the ophthalmic, maxillary, and mandibular) and the glossopharyngeal nerve play roles in somatic sensory functions.

The oculomotor nerve, the facial nerve, the glossopharyngeal nerve, and the vagus nerve have important autonomic functions.

Finally, the oculomotor nerve, the trochlear nerve, the mandibular branch of the trigeminal nerve (V3), the abducens nerve, the facial nerve, the glossopharyngeal nerve, the vagus nerve, the spinal accessory nerve, and the hypoglossal nerve are responsible for motor functions.

This is a lot of information to take in; but one way to try to simplify efforts to understand each nerve and each of its separate functions is to separate the 12 cranial nerves into smaller groups. This can be done by first dividing the overarching group of 12 nerves into:

• nerves which are considered to have primarily motor functions
• nerves which are considered to have primarily sensory functions
• nerves which have a combination of both motor and sensory components.

To memorize all the names of the cranial nerves and whether they are primarily motor, sensory or both, check this cranial nerves mnemonics video:

This article will provide an introduction to the cranial nerves which are considered primarily motor nerves, which includes the oculomotor nerve, the trochlear nerve, the abducens nerve, the spinal accessory nerve, and the hypoglossal nerve.

Somatic efferent component

The somatic efferent axons of the oculomotor nerve originate in a region of the midbrain called the oculomotor nuclear complex. From here, they travel through the red nucleus and emerge from the ventral midbrain medial to the crus cerebri. After these nerve fibers travel through the the cavernous sinus (along with the trochlear nerve, ophthalmic, and maxillary divisions of the trigeminal nerve, and abducens nerve), they separate into small superior and large inferior divisions. Accompanied by the trochlear nerve, the ophthalmic division of the trigeminal nerve (V1), and the abducens nerve, the somatic efferent oculomotor nerve fibers enter the orbit by way of the superior orbital fissure.

The majority of extraocular muscles are innervated by this component of the oculomotor nerve. 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.

Activation of the nerves innervating the extraocular muscles allows for movement of the eye within its socket. The superior rectus muscle is the main muscle responsible for upward gaze: contraction of this muscle elevates, adducts, and medially rotates the eye. The inferior rectus muscle is the main muscle responsible for downward gaze: its contraction depresses, adducts, and laterally rotates the eye. The medial rectus muscle is responsible for medial gaze: its contraction adducts the eye. Simultaneous contraction of both medial recti results in convergence (i.e. crossing) of the eyes. The inferior oblique muscle is responsible for upward, outward gaze: its contraction elevates, abducts, and laterally rotates the eye.

Finally, the levator palpebrae superioris functions to open the eye: contraction of this muscle elevates the upper eyelid.

Visceral efferent (parasympathetic) component

The parasympathetic fibers coursing within the oculomotor nerve originate from the Edinger-Westphal nucleus, a region of cell bodies in the midbrain. These cell bodies give off preganglionic visceral motor axons, which travel through the midbrain accompanied by the somatic efferents. Together, these constitute the oculomotor nerve.

These parasympathetic fibers eventually branch off from the main nerve, specifically from the nerve innervating the inferior oblique muscle, and terminate in the ciliary ganglion in the bony orbit. After synapsing at this ganglion, postganglionic axons extend from the ciliary ganglion in the form of 6-10 short ciliary nerves. These short nerves perforate the sclera and supply the cornea, choroid, iris, sphincter pupillae muscle, ciliary body and sclera, and are responsible for parasympathetic responses in the eye including pupil constriction and ocular accommodation.

Role in the pupillary light reflex

The pupillary light reflex is the reflexive constriction of the pupils in response to light. There are two main components of this reflex: an afferent (sensory) component involving the optic nerve and an efferent component involving the oculomotor nerve. The reflex is triggered when cells in the retina are activated by light, sending signals down the optic nerve (the afferent component of the reflex).

Optic nerve fibers project and synapse in the pretectal region of the midbrain, which contributes to the circuit for the pupillary light reflex. Neurons from the pretectal region then project and synapse in the oculomotor nucleus. From here, preganglionic parasympathetic neurons from the Edinger-Westphal nucleus send axons via the oculomotor nerve to synapse with postganglionic neurons in the ciliary ganglion, which sends signals to the sphincter pupillae muscle to contract and leads to constriction of the pupil (the efferent component of the reflex).

Role in occular accommodation

The term accommodation refers to the adaptation of the eyes for near vision. This is made possible by increasing the curvature of the lens, constricting the pupil, and converging the eyes. When the ciliary muscle is innervated by oculomotor nerve parasympathetic fibers originating from the Edinger-Westphal nucleus in the midbrain, the ciliary muscle contracts. When this occurs, the suspensory ligament of the lens is released, allowing the lens to relax. When the lens relaxes, its degree of curvature increases, making it rounder.

Simultaneously, the sphincter pupillae muscle is instructed by oculomotor parasympathetic fibers muscles to constrict, thereby reducing the size of the pupil and sharpening the image on retina. Convergence of the eyes contributes to accommodation by allowing both eyes to direct their gaze to an object closer to the midline. As previously noted, this is made possible by simultaneous contraction of both medial recti muscles, innervated by the somatic efferent component of the  oculomotor nerve.