Motor Cranial Nerves
Anatomy is a tough subject for many students, especially when it comes to learning about the nervous system. Trying to learn about innervations of the head and neck takes this challenge to a whole new level! With so many structures packed in this small region each having numerous functions, it can be intimidating to figure out where to begin. This article is one of a series of articles which aims to help students break down this difficult topic by providing an introduction to the nerves giving sensory and motor innervations to the head and neck.
The Twelve Cranial Nerves
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)
- 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)
Types & Functions
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.
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.
The Oculomotor Nerve (CN III)
Originating in the midbrain at the level of the superior colliculus, the oculomotor nerve extends through the cavernous sinus and enters the orbit via the superior orbital fissure. It contains both somatic efferent (motor) fibers and special visceral efferent fibers. These special visceral efferent fibers are autonomic parasympathetic fibers that originate from a group of neuron cell bodies called the Edinger-Westphal nucleus located in the midbrain.
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 Ocular 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.
The Trochlear Nerve (CN IV)
The trochlear nerve originates from the trochlear nucleus, a group of cell bodies at the level of the inferior colliculus in the midbrain tegmentum. The fibers in this nerve are somatic efferent only.
Axons from the trochlear nucleus are unique in that, after they travel dorsally around the periaqueductal grey matter and cerebral aqueduct, they cross the midline and emerge from the dorsal surface of the caudal midbrain below the inferior colliculi on the contralateral side. The nerve fibers travel through the cavernous sinus (along with the oculomotor nerve, the ophthalmic and maxillary divisions of the trigeminal nerve, and the abducens nerve), just lateral to the internal carotid artery.
Once through the sinus, it enters the orbit via the superior orbital fissure, accompanied by the oculomotor nerve, the ophthalmic division of the trigeminal nerve, and the abducens nerve. The nerve courses medially and along the roof of the orbit until it reaches the superior oblique extraocular muscle.
Innervated by the trochlear nerve, the superior oblique muscle passes through a fibrocartilaginous structure called the trochlea and inserts on the outer superior surface of the eye. When signals are sent via the trochlear nerve for this muscle to contract, the eye becomes depressed, abducted, and medially rotated. This allows for downward and outward gaze.
The Abducens Nerve (CN VI)
Like the trochlear nerve, the abducens nerve contains somatic efferent fibers only, and innervates only one extraocular muscle: the lateral rectus. The abducens nerve originates from the cell bodies comprising the abducens nucleus, which is located in the pontine tegmentum, a region in the dorsomedial part of the posterior pons just ventral to fourth ventricle. Axons of the cells within this nucleus project ventrally and emerge from the brainstem at the pontine-medullary border (the junction of the pons and medullary pyramids).
It then travels ventro-laterally in the subarachnoid space of the posterior cranial fossa, penetrating the dura lateral to the dorsum sellae of sphenoid bone. The nerve fibers then travel between dura and apex of petrous temporal bone in order to enter the cavernous sinus, and enters the orbit via the superior orbital fissure along with the oculomotor nerve, the trochlear nerve, and the ophthalmic division of the trigeminal nerve.
Finally, the abducens nerve arrives and synapses on the lateral rectus muscle. The lateral rectus muscle is responsible for lateral gaze: contraction of the lateral rectus abducts the eye.
The Spinal Accessory Nerve (CN XI)
The spinal accessory nerve carries somatic efferent fibers to innervate the sternocleidomastoid and trapezius muscles. The spinal accessory nerve is composed of spinal roots which arise from motor neurons in the first through fifth cervical segments of the spinal cord. The rootlets branch from the lateral surface of the spinal cord and join together to ascend through the foramen magnum and into the cranial cavity as a single nerve.
The spinal accessory nerve then travels through the posterior fossa and leaves the skull via the jugular foramen, posterior to the glossopharyngeal and vagus nerves, and additionally accompanied by the internal jugular vein. From the jugular foramen, the spinal accessory nerve descends to innervate the trapezius and sternocleidomastoid muscles, synapsing on their deep surfaces.
It should be noted that the accessory nerve also has cranial roots, which arise from the anterolateral surface of the caudal part of the medulla, just inferior to where the vagus nerve roots emerge from the medulla. After leaving the medulla, they follow the same path as the spinal roots into the jugular foramen; but after traveling through this foramen, rather than continuing on with the spinal roots, they branch off and accompany the vagus nerve. From this point on, they are considered as part of the vagus nerve, and contribute to the innervation of the pharyngeal musculature.
The Hypoglossal Nerve (CN XII)
The hypoglossal nerve carries somatic sensory efferent fibers which innervate all of the intrinsic muscles and the majority of extrinsic muscles of the tongue, including the hyoglossus, styloglossus, and genioglossus muscles. The only tongue muscle not innervated by the hypoglossal nerve is palatoglossus, which is innervated by the vagus nerve.
The nerve is composed of rootlets which emerge from the anterior surface of the medulla, travel laterally across the posterior cranial fossa, and exit the skull via the hypoglossal canal.