The cranial nerves are 12 paired nerves that arise from the brain and travel through foramina and fissures in the cranium to innervate various structures, primarily in the head and neck. Just like spinal nerves, which originate from the spinal cord, cranial nerves originate from the brain. But unlike spinal nerves, which are pretty similar to each other, with all of them being mixed nerves, the cranial nerves are a mix and match of motor, sensory, and autonomic functions. Simply put, some cranial nerves are motor nerves, some are sensory nerves, some are mixed, and some carry parasympathetic fibers as well. Funnily enough, some of them aren't even nerves at all.
The first and second cranial nerves, which are the olfactory and optic nerves, are actually brain projections that belong to the olfactory and optic tracts. But everyone likes to call them cranial nerves, so we will too. Let's start by naming the 12 pairs of cranial nerves in order from rostral, or front of the brain, to the caudal, or back of the brain, on an image of the ventral surface of the brain.
First olfactory, second optic, third oculomotor, fourth trochlear, fifth trigeminal, six abducens, seventh facial, eighth vestibulocochlear, ninth glossopharyngeal, tenth vagus. 11th accessory, and 12th hypoglossal. There are many mnemonics out there to help you remember these, so choose one that best fits you, but here at Osmosis we like to remember these nerves by saying O-O-O to touch and feel very green vegetables.
Ah. Now, these nerves don't all originate from the same place, and they enter or exit at different parts of the brain. Cranial nerves 1 and 2 enter the cerebrum, but most of the cranial nerves enter or exit at various levels of the brainstem.
Cranial nerves 3 and 4 emerge from the midbrain, and it's worth mentioning that cranial nerve 4 is the only nerve to arise from the dorsal aspect of the brainstem. Cranial nerve 5, or the trigeminal nerve, arises from the pons, while cranial nerves 6, 7, and 8 enter or exit at the pontomedullary junction. Lastly, cranial nerves 9, 10, and 12 all enter or exit from the medulla. Finally, cranial nerve 11 arises mostly from the spinal cord.
Okay, now let's talk about the embryological development of the cranial nerves. See, way back when we all looked a bit like intrauterine shrimp, there were six sets of tissue bands called pharyngeal arches. Originally, there were six of them, numbered logically from 1 to 6. However, the fifth doesn't develop, so we are left with five pharyngeal arches numbered 1, 2, 3, 4, and 6. During development, each pharyngeal arch develops along with a cranial nerve or one of its branches, so whatever structure develops from a particular arch is innervated by the associated nerve.
So, the first arch is associated with the trigeminal nerve, or cranial nerve 5. This arch gives rise to the myelohyoid, tensor tympani, tensor veli palatini, the anterior belly of digastric, and the muscles of mastication. The second arch is associated with and innervated by the facial nerve, or cranial nerve 7, and gives rise to the stapedius, stylohyoid, posterior belly of digastric, and the muscles of facial expression. The third arch is associated with the glossopharyngeal nerve, or cranial nerve 9, and gives rise to the stylopharyngeus muscle. The fourth arch is associated with the superior laryngeal nerve, which is a branch of the vagus nerve, or cranial nerve 10. This arch gives rise to the cricothyroid muscle, cricopharyngeus, muscles of the pharynx except for the stylopharyngeus, and the muscles of the soft palate except for the tensor veli palatini.
Finally, the sixth arch is associated with the recurrent laryngeal nerve. another branch of the vagus nerve, and gives rise to intrinsic muscles of the larynx, except for the cricothyroid muscle, and to the upper muscles of the esophagus. Now, to better understand the anatomy and function of the cranial nerves, let's think of the cranial nerves like tiny highways. Just like cars traveling along highways, information travels through the cranial nerves. This information can travel in one direction, or both.
The two lanes that make up our cranial nerve highway are motor and sensory pathways. Motor fibers originate in the brain and carry motor signals from the brain to the structure they innervate, so they are referred to as efferent fibers. And to remember this, the E in efferent stands for exit.
Sensory fibers carry signals in the opposite direction, from their receptors in the periphery to the brain, which is why they can be referred to as afferent fibers, with the A standing for arriving. Now, there are three types of motor fibers. Somatic motor, which innervate striated muscles under voluntary control.
somatic branchial motor that can only be found in cranial nerves and innervate voluntary muscles that develop embryologically from the pharyngeal or branchial arches, and visceral motor, specifically parasympathetic fibers, which constitute the cranial outflow of the parasympathetic division of the autonomic nervous system, and innervate smooth muscle, glands, and the heart. As for the sensory fibers, there are also three types. Somatic sensory, which carries sensory information such as pain, temperature, touch, and proprioception from the skin, mucous membranes, and muscles.
Visceral sensory, which carry information from the carotid body and sinus, pharynx, larynx, as well as thoracic and abdominal viscera. And lastly, special sensory, which is associated with smell, sight, taste, hearing, and balance. Now let's see what type of information each cranial nerve carries. Generally, cranial nerves will carry either motor information, sensory information, or both, making it a mixed nerve. Now, we have a good mnemonic to remember what type of information each cranial nerve carries.
Some say marry money, but my brother says big brains matter more. The 12 words correspond with cranial nerves 1 through 12. If it starts with an M, it carries only motor information. If it starts with an S, it carries only sensory information.
And if it starts with a B, it carries both. It's worth noting that in addition to carrying somatic motor fibers as well as sensory fibers, there are four cranial nerves that also carry parasympathetic information, specifically the oculomotor, facial, glossopharyngeal, and vagus nerves. Now, let's talk about the main functions of each cranial nerve.
So, the olfactory nerve has a special sensory function and carries information about smell. The optic nerve is also special sensory and carries information responsible for vision. The oculomotor nerve has a somatic motor function as it innervates most of the extraocular muscles, which are responsible for movements of the eyeball. It also carries parasympathetic information to the sphincter pupillae muscle, which constricts the pupil, and the ciliary muscle, which alters the curvature of the lens.
The trochlear nerve provides somatic motor innervation to one of the extraocular muscles called the superior oblique. The trigeminal nerve, as the name suggests, has three divisions, the ophthalmic, maxillary, and mandibular nerves. All three nerves carry sensory information from different parts of the face, scalp, and mouth.
The trigeminal nerve also has a somatic motor function, as its mandibular division innervates the muscles of mastication that are responsible for chewing movements. The abducens nerve provides somatic motor innervation to one extraocular muscle called the lateral rectus. The facial nerve provides somatic motor innervation to several muscles, including the muscles of facial expression, and it also carries parasympathetic fibers for innervation of the submandibular, sublingual, lacrimal, and nasal glands. The facial nerve also has a sensory function as it innervates a small area of skin on the external ear. Finally, it also carries special sensory fibers that sense taste from the anterior two-thirds of the tongue.
The vestibulocochlear is a special sensory nerve in charge of hearing and balance. The glossopharyngeal nerve provides somatic motor innervation to the stylopharyngeus muscle needed for swallowing, while also carrying parasympathetic fibers to the parotid gland. It also carries sensory information from the oropharynx as well as the carotid sinus and carotid body. and finally it has special sensory fibers that sense taste from the posterior one-third of the tongue. The vagus nerve provides somatic motor innervation to the muscles of the larynx and pharynx needed for phonation and swallowing.
The nerve also carries parasympathetic fibers to the heart and lungs, as well as muscles and glands of the gastrointestinal tract and abdominal viscera. It carries sensory information from the larynx, pharynx, heart, lungs, and abdominal viscera. Finally, it has special sensory fibers that sense taste from the epiglottic region and laryngopharynx.
The spinal accessory nerve provides somatic motor innervation to the sternocleidomastoid and trapezius muscles. Finally, the hypoglossal nerve also has exclusively a somatic motor function as it innervates the muscles of the tongue responsible for tongue movements. As a quick break, let's try to label the 12 cranial nerves on this image of the undersurface of the brain.
Alright, as a quick recap, here are the 12 cranial nerve pairs. Cranial nerve 1, or the olfactory nerve. Cranial nerve 2, or the optic nerve. Cranial nerve 3, or the oculomotor nerve.
Cranial nerve 4, or the trochlear nerve. Cranial nerve 5, or the trigeminal nerve. cranial nerve 6 or the abducens nerve, cranial nerve 7 or the facial nerve, cranial nerve 8 or the vestibulocochlear nerve, cranial nerve 9 or the glossopharyngeal nerve, cranial nerve 10 or the vagus nerve, cranial nerve 11 or the accessory nerve, and cranial nerve 12 or the hypoglossal nerve. They can carry different information. somatic motor which innervates striated muscles under voluntary control, parasympathetic which are visceral motor fibers that innervate involuntary structures like glands, smooth muscle, such as in the gastrointestinal tract, and the heart, and somatic branchial motor that innervate voluntary muscles that developed embryologically from the branchial arches, also known as pharyngeal arches.
Then there are somatic sensory which carry pain, temperature, tactile, and proprioceptive impulses from the skin, mucous membranes, and muscle. Visceral sensory, which convey sensory information from organs such as blood vessels, the gastrointestinal tract, and other thoracic and abdominal viscera. And lastly, special sensory, which is associated with smell, sight, taste, hearing, and balance. Finally, the cranial nerves all carry varying types of information.
which we can remember by using our mnemonic. Some say marry money, but my brother says big brains matter more. Furthermore, cranial nerves 3, 7, 9, and 10 all serve as pathways for parasympathetic innervation, while cranial nerves 1, 2, 7, 8, 9, and 10 all carry special sensory information.
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