Human Anatomy Lesson 15
Lesson 15 - Cranial Nerves
Now that we have covered the skull and cranial base, we can turn to the cranial nerves, which provide innervation to the head and structures in the neck, thorax, and abdominopelvic cavity. There are 11 nerves that come off the brain and innervate structures in the head, neck, upper limbs, thorax, and abdomen, and one nerve that comes off the spinal cord and innervates structures in the neck and shoulder. These 12 nerves are called cranial nerves.
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Learning Objectives - By the end of this lesson, you should be able to ...
- List the cranial nerves
- Understand the differences between cranial nerves and spinal nerves
- Understand on a very basic level how embryological development is related to the course of cranial nerves in the skull
- Describe where the cranial nerves originate on the brain or spine, the course they take through the cranial cavity, and where they exit the cranial cavity
- List the types of fibers carried by each cranial nerve
- Describe what muscles and other structures are innervated by each cranial nerve
A Few Notes about Cranial Nerves
One "cranial nerve," the accessory nerve, was mistakenly classified as a cranial nerve in the mid-19th century, since "spinal" fibers innervate structures in the neck and shoulder and "cranial" fibers run with the vagus nerve. However, the "cranial part" of the accessory nerve is now considered to be part of the vagus nerve, so the accessory nerve is more properly considered to be a spinal nerve, and should be called the spinal accessory nerve. There are really 11 cranial nerves, then, although the concept of 12 cranial nerves is so deeply ingrained in the anatomico-medical literature that the "accessory nerve" is still listed with the cranial nerves.
To date, we have talked about four different types of nerve fibers: (1) somatic motor fibers that innervate muscles derived from embryonic somites; (2) somatic sensory fibers that receive sensation from muscles derived from embryonic somites; (3) visceral motor fibers, including sympathetic and parasympathetic fibers; and (4) visceral sensory fibers, that receive sensation from structures innervated by the visceral motor system. We will have to tweak this system when discussing the innervation of structures in the head and neck. Muscles and other structures in the head are not from somites, so it is not technically correct to call them somatic structures, or to call the nerves that innervate them somatic motor or somatic sensory nerves. During embryonic development, paraxial mesoderm superior to the cervical somites develops into four occipital somites, and paraxial mesoderm superior to this develop into seven crude somite-like structures called somitomeres. The occipital somites develop into the four growth centers of the occipital bone, but the somitomeres do not develop in quite as organized a fashion as the somites that make up the body wall. The somitomeres develop into most of the cranial vault, and each of the 11 paraxial blocks (4 occipital, 7 above this) is associated with striated voluntary muscle and a cranial nerve, just like for somites. Starting in the foregut, neural crest cells evaginate toward the gut tube in a segmental fashion, creating four pharyngeal pouches where endoderm and ectoderm contact each other, and endoderm and ectoderm between each pouch make up a series of 5-6 branchial arches. Therefore, instead of calling the muscles and nerves associated with these branchial arches somatic muscles and somatic nerves, we will call them branchial muscles and branchial nerves. Cranial nerves V, VII, IX, X, and XI are associated with branchial arches during embryological development, and so they carry branchial motor fibers to associated muscles, whereas cranial nerves III, IV, VI, and XII carry "normal" motor fibers. So, Instead of using the term "somatic nerve," we will use the term "branchial nerve" to refer to nerves associated with muscles that derive from branchial arches. (Note: if you are a student in my spring 2017 Human Anatomy course, you can ignore this distinction, since we treated the non-visceral motor and sensory fibers as if they were synonymous with somatic motor and somatic sensory fibers).
Balanced against this complication are a few things that make our job easier: first, cranial nerves do not carry sympathetic fibers. Remember from our discussion of the autonomic division of the peripheral nervous system that sympathetic fibers run in paravertebral ganglia in the sympathetic chain, hitchhike on arteries running to the neck and head, and synapse in superior, middle, and inferior cervical ganglia in the neck before sending postganglionic fibers to structures in the head and neck. Cranial nerves therefore do not carry sympathetic fibers. Cranial nerves III, VII, IX, and X carry preganglionic parasympathetic fibers to ganglia in the head which contain cell bodies and send off postganglionic fibers to structures in the head and neck. Second, cranial nerves carry special sensory fibers for vision, smell, and taste.
Students may find a mnemonic helpful for remembering the order of cranial nerves, although I never have. You can find a full list at the link below (but note, they range from G-rated to quite raunchy):
One G-rated mnemonic used at Duke University Medical School is:
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It may also be helpful to use a mnemonic to remember the types of fibers that each cranial nerve carries (Motor, Sensory, Both):
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- Cranial Nerve Mnemonics
This is a more complete list of mnemonics for the cranial nerves
Cranial Nerve I
CN I carries special sensory fibers for the sense of smell. Special sensory receptors are in the roof and upper parts of the nasal cavity, and these receptors send nerve rootlets down into the nasal cavity. These rootlets and receptors join together into small bundles and pass through the cribriform plate of the ethmoid bone, synapsing with secondary neurons in the olfactory bulbs. The olfactory bulbs sit on top of the cribriform plate and run posteriorly to enter the telencephalic portion of the brain, so that CN I is one of two cranial nerves that does not enter the brainstem.
Cranial Nerve II
The optic nerve carries special sensory fibers for vision, which return information from photoreceptors in the retina to the dienchephalic part of the brain. Since it is derived directly from an outpouching of the diencephalon, is covered by myelin produced by oligodendrocytes, and is ensheathed in the three meningeal layers that surround the brain and spinal cord, it is more properly considered to be a part of the CNS rather than a cranial nerve. It is, along with CN I, one of two cranial nerves that does not enter or leave from the brainstem. CN II runs from the eyeball through the optic canal at the back of the orbit, and meets its mate at the optic chiasm, where fibers from the nasal visual fields of the two eyes partially cross. Note that the optic nerve is important for vision, but does not carry motor fibers, so it is not one of the cranial nerves that moves the eyeball.
Cranial Nerve III
The oculomotor nerve carries two types of fibers: somatic motor fibers that innervate 4/6 of the muscles that move the eyeball and one that moves the eyelid, and visceral motor (in this case, parasympathetic) fibers. CN III leaves the anterior surface of the brainstem between the midbrain and pons, enters the anterior edge of the tentorium cerebelli, runs in the lateral wall of the cavernous sinus, and enters the orbit through the superior orbital fissure. Somatic motor fibers innervate levator palpebrae superioris, the superior rectus, inferior rectus, and medial rectus muscles, and the inferior oblique muscle. Preganglionic parasympathetic fibers synapse in the ciliary ganglion and innervate the sphincter pupillae muscles (constrict the pupils) and the ciliary muscles (change the shape of the lens for near vision).
Cranial Nerve IV
This is one of the nerves that doesn't quite pull its weight - the only thing it does is provide somatic motor innervation to the superior oblique muscle. CN IV arises in the midbrain, enters the free edge of the tentorium cerebelli, and, like the oculomotor nerve, runs in the lateral wall of the cavernous sinus to enter the orbit through the superior orbital fissure.
Cranial Nerve V - Trigeminal Nerve
If CN IV doesn't quite do its share, CN V picks up the slack! The trigeminal nerve carries two types of fibers: somatic sensory fibers out to the face, anterior 1/2 of the scalp, mucous membranes of the oral and nasal cavities and paranasal sinuses, part of the tympanic membrane, the eye and conjunctiva, and dura mater in the anterior and middle cranial fossa, and branchial motor fibers to the muscles of mastication, tensor tympani, tensor veli palatini, mylohyoid, and the anterior belly of the digastric. The trigeminal nerve exits from the anterolateral surface of the pons in a large sensory root and small motor root that pass through the posterior cranial fossa, over the medial tip of the petrous part of the temporal bone, and into the middle cranial fossa. The sensory root expands into the trigeminal ganglion in the middle cranial fossa, where it sits in a depression on the anterior surface of the petrous part of the temporal bone called the trigeminal cave. The three terminal divisions of the trigeminal nerve arise from the trigeminal ganglion:
- Ophthalmic nerve (V1): runs in the lateral wall of the cavernous sinus then enters the orbit through the superior orbital fissure. The ophthalmic nerve carries sensory branches from the eyes, conjunctiva, orbital contents including the lacrimal gland, nasal cavity, frontal and ethmoid sinuses, upper eyelid, dorsum of the nose, and anterior part of the scalp.
- Maxillary nerve (V2): runs in the lateral wall of the cavernous sinus inferior to V1, then leaves the cranial cavity through the foramen rotundum to enter the pterygopalatine fossa. The maxillary nerve receives sensory branches from dura in the anterior and middle cranial fossae, nasopharynx, palate, nasal cavity, teeth of the upper jaw, maxillary sinus, skin covering the side of the nose, lower eyelid, cheek, and upper lip.
- Mandibular nerve (V3): leaves the skull through the foramen ovale along with the motor root of the trigeminal nerve. Branchial motor fibers of V3 innervate the muscles of mastication (temporalis, masseter, medial and lateral pterygoid muscles), tensor tympani, tensor veli palatini, anterior belly of the digastric, and mylohyoid muscles. Sensory branches innervate the skin of the lower face, cheek, lower lip, ear, external acoustic meatus, temporal region, anterior 2/3 of the tongue, teeth of the lower jaw, mastoid air cells, mucous membranes of the cheek, mandible, and dura in the middle cranial fossa.
Cranial Nerve VI
The abducent nerve carries somatic motor fibers to the lateral rectus muscle that moves the eyeball. Like CN IV, CN VI does not do its fair share of work. CN VI arises from the brainstem between the pons and medulla, pierces the dura covering the clivus, crosses the superior edge of the petrous temporal bone, and enters the cavernous sinus inferolateral to the internal carotid artery, finally entering the orbit through the superior orbital fissure.
Cranial Nerve VII
The facial nerve carries branchial motor, somatic sensory, and visceral motor fibers, in addition to special sensory fibers for taste to the anterior 2/3 of the tongue. The somatic sensory fibers innervate the external acoustic meatus and a small amount of skin posterior to the ear. Visceral motor fibers in CN VII are preganglionic parasympathetic fibers that stimulate secretomotor activity in the lacrimal glands and mucous membranes of the nasal cavity, hard palate, and soft palate. Branchial motor fibers provide innervation to the muscles of facial expression, parts of the scalp, stapedius, posterior belly of the digastric, and the stylohyoid muscles. As noted above, special sensory fibers provide innervation to the anterior 2/3 of the tongue.
The facial nerve runs in two separate roots from the lateral surface of the brainstem between the pons and medulla, across the posterior cranial fossa, and out of the skull through the internal acoustic meatus. The larger of the two roots is the motor root, and carries branchial motor fibers to the various muscles listed above. The smaller of the two roots is the "sensory root," and contains somatic sensory fibers, special sensory fibers for taste, and visceral motor fibers. In the petrous part of the temporal bone, the two roots run in the facial canal for a short distance before fusing together to form the facial nerve proper, at which point the nerve enlarges to form the geniculate ganglion that contains cell bodies for sensory neurons. The geniculate ganglion immediately gives off the greater petrosal nerve then the nerve to stapedius and chorda tympani before exiting the skull through the stylomastoid foramen. The greater petrosal nerve carries preganglionic parasympathetic fibers to the pterygopalatine ganglion, which sends postganglionic fibers to the lacrimal gland and mucous glands of the nasal cavity, maxillary sinus, and palate. The chorda tympani carries special sensory fibers of taste from the anterior 2/3 of the tongue and preganglionic parasympathetic fibers destined for the submandibular ganglion, which sends postganglionic fibers to the submandibular and sublingual glands.
Cranial Nerve VIII
The vestibulocochlear nerve carries special sensory fibers for hearing and balance. It runs from the lateral surface of the brainstem between the pons and medulla, across the posterior cranial fossa, into the internal acoustic meatus. CN VIII is made up of two components, a vestibular component for balance and a cochlear component for hearing, which combine into a single nerve in the petrous temporal.
Cranial Nerve IX
The glossopharyngeal nerve carries branchial motor, visceral motor, and visceral sensory fibers, in addition to special sensory fibers for taste from the posterior 1/3 of the tongue. Branchial motor fibers provide innervation to the stylopharyngeus muscle. Visceral sensory fibers provide sensory input from the carotid body and sinus, posterior 1/3 of the tongue, palatine tonsils, upper pharynx, and mucosa of the middle ear and pharyngotympanic tube. Visceral motor fibers are preganglionic parasympathetic fibers that stimulate secretomotor activity in the parotid salivary gland. As noted above, special sensory taste fibers are from the posterior 1/3 of the tongue.
Fibers of the glossopharyngeal nerve run from the anterolateral surface of the upper medulla oblongata, cross the posterior cranial fossa, and enter the jugular foramen, where they merge to form the glossopharyngeal nerve proper. Two ganglia (the superior and inferior ganglia) within the jugular foramen contain the cell bodies of sensory neurons. The tympanic nerve branches from CN IX within the jugular foramen. The tympanic nerve re-enters the temporal bone and participates in the formation of the tympanic plexus in the middle ear cavity. This nerve provides sensory innervation to the mucosa in the middle ear, pharyngotympanic tube, and mastoid air cells. The tympanic nerve also contributes visceral motor (preganglionic parasympathetic) fibers via the lesser petrosal nerve, which exits the temporal bone into the middle cranial fossa, descends through the foramen ovale, and runs to the otic ganglion, where the fibers synapse and send postganglionic fibers to the parotid gland.
Cranial Nerve X
Along with cranial nerves V and VII, CN X is one of the true powerhouse cranial nerves, carrying somatic motor, somatic sensory, visceral motor, visceral sensory, and special sense fibers to the head, neck, pharynx, and structures in the thorax and abdomen. Somatic motor fibers innervate the palatoglossus muscle and muscles of the soft palate (minus tensor veli palatini), pharynx (minus stylopharyngeus), and the larynx. Somatic sensory fibers innervate the skin posterior to the ear and external acoustic meatus and dura in the posterior cranial fossa. Visceral motor (preganglionic parasympathatetic) fibers stimulate smooth muscle and glands in the pharynx, larynx, thoracic viscera, and abdominal viscera that derive from the embryonic foregut and midgut. Visceral sensory fibers receive input from aortic chemoreceptors and baroreceptors and the mucous membranes of the pharynx, larynx, esophagus, bronchi, lungs, heart, and abdominal viscera that derive from the embryonic foregut and midgut. Special sensory taste fibers are taste fibers from the epiglottis.
The vagus nerve runs from the anterolateral surface of the medulla oblongata just inferior to the origin of CN IX, crosses the posterior cranial fossa, and enters the jugular foramen. Fibers of CN X merge inside the jugular foramen to form the vagus nerve proper. The superior and inferior ganglia inside the the jugular foramen contain the cell bodies of sensory neurons in the vagus nerve.
Cranial Nerve XI
Spinal Accessory Nerve
The spinal accessory nerve is made up of two roots, a spinal root and cranial root. As noted in the introduction, the spinal accessory nerve is unique as a cranial nerve since its spinal fibers do not originate from the brain, but instead arise from motor neurons in the cervical part of the spinal cord. These fibers join together , enter the cranial cavity through the foramen magnum into the posterior cranial fossa before exiting through the jugular foramen. CN XI then descends to the deep surface of the sternocleidomastoid and trapezius muscles and provides motor innervation to these two muscles.
The "cranial root" of the accessory nerve refers to fibers from the caudal part of the medulla oblongata on the anterolateral surface. These fibers course with the spinal fibers into the jugular foramen, where they join together and are distributed to the pharyngeal musculature innervated by the vagus nerve.
Cranial Nerve XII
The hypoglossal nerve carries somatic motor fibers to the three extrinsic tongue muscles (the hyoglossus, styloglossus, and genioglossus muscles) and all the intrinsic muscles of the tongue. CN XII arises from the anterior surface of the medulla, passes laterally across the posterior cranial fossa, and exits the cranial cavity through the hypoglossal canal.
© 2014 Robert McCarthy