AssessmentsIntroduction to the cranial nerves
Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
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: 1st Olfactory, 2nd Optic, 3rd Oculomotor, 4th Trochlear, 5th Trigeminal, 6th Abducens, 7th Facial, 8th Vestibulocochlear, 9th Glossopharyngeal, 10th 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:
Oh Oh Oh To Touch And Feel Very Green Vegetables A H!
Now, these nerves don’t all originate from the same place and they enter or exit at different parts of the brain.
Lastly, cranial nerves nine, ten and twelve, all enter or exit from the medulla.
Finally, cranial nerve eleven arises mostly from the spinal cord.
Ok, now let’s talk about the embryological development of the cranial nerves.
Originally there were six of them, numbered logically from 1 to 6.
However the 5th doesn’t develop, so we are left with 5 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 five.
The second arch is associated with and innervated by the facial nerve, or cranial nerve seven, and gives rise to the stapedius, stylohyoid, posterior belly of digastric and the muscles of facial expression.
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 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 from 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 carry 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.