CRANIAL NERVES 101
In THE GOLDEN SPIDER, Lady Amanda Ravensburg invents a clockwork spider that can re-weave nerves. Well, almost. Because, as we learn, there are complications. Many of them.
As to the nerves themselves, there is an important difference between cranial and peripheral nerves—and how our hero and heroine must go about approaching them.
I thought it would be fun (yes, former biology professor here) to take a few minutes to introduce (or for some of you, review) each of the twelve cranial nerves. Yes, yes, I know you don’t want a lecture. I’ve done my best to make it brief AND interesting. Below is a list of their names, a quick overview of how you’re using them on a day-to-day basis, and at least one interesting fact to go with each.
(For those of you hyperventilating at the idea of SO MANY NERVES, you need only give a quick perusal of those cranial nerves that control the eye muscles (III, IV, and VI) and those of the ear (VIII) to understand those cranial nerves I write about in THE GOLDEN SPIDER. Actually, you really don’t need to read this article at all. Anything you need to know is inside the story and written for those with no prior knowledge. Feel free to skip this article entirely and just enjoy the adventure.)
Let’s start with a few basics.
The nervous system can be divided into two parts:
1) The central nervous system (CNS) which is made up of the brain, brainstem, and spinal cord.
2) The peripheral nervous system (PNS) which is all the nervous tissue outside of the brain and spinal cord.
Nerves have two basic functions. They either make something happen (via motor efferent nerve fibers) or they bring in information from our environment (via sensory afferent nerve fibers). To read more about the difference between effect/affect, you can read A Grammar Gripe: Affect vs. Effect.
There are two kinds of nerve fibers, but three kinds of nerves:
1) A motor nerve is a nerve that makes something happen, like make a muscle contract. It’s made up of only motor fibers.
2) A sensory nerve is a nerve that carries sensory information to the brain. It’s made up of only sensory fibers.
3) A mixed nerve is a nerve that has both motor and sensory fibers. Depending upon the nerve’s duties, the ratios of the two different kinds of fibers will vary.
Now that we have the basics of nerves down, we can move on to the coolest of all nerves: The Cranial Nerves!
Cranial nerves come in pairs (one on each side of our body) and originate directly from the brain or brainstem. Not only do they have names, they each have a designated a number that roughly corresponds to where the nerve emerges, moving from the front of the brain to the back.
(For those of you who want MORE, look for the blue color that indicates a link.)
Allow me to first introduce the Terminal Nerve, cranial nerve 0. Often overlooked and rarely mentioned, this nerve is nonetheless fascinating. While his* exact function is still under debate, he might well be the nerve that senses pheromones and is involved in regulating sexual behavior in mammals. But for now, this nerve is holding tightly to many of his secrets.
- This nerve may be responsible for mate selection (yes, even among humans) and the use of birth control pills might alter a woman’s choice. You can read more in my blog post Birth Control Pills Can Do What?
Meet the Olfactory Nerve, otherwise known as Cranial nerve I. She is devoted purely to providing you with your sense of smell (100% sensory).
- Humans only have 350 olfactory receptor genes… Rats have 1400 genes and can be trained to sniff out things like land mines and tuberculosis.
- Accidentally inhaling zinc, a component of many “cold remedies” along with other things (chronic congestion, smoking, etc.) can damage your sense of smell and thereby ruin your enjoyment of food. Though this often leads to weight loss, please don’t approach that as an applied technique.
Next in line is the Optic Nerve, Cranial Nerve II. He’s the nerve connecting your brain directly to the retina at the back of the eye. Purely sensory, all information travels through this nerve to the lower back part of your brain.
- Blindness can result if you fall backwards and smash the back of your head badly enough. Or if someone (murder mysteries, anyone?) smashes it for you. Either scenario damages the part of the brain (occipital cortex) that receives and processes all visual input. This part of the brain can also be damaged by medical conditions such as lesions and strokes. The condition is called cortical blindness.
- As visual information travels from the retinas to the back of your brain, the two optic nerves “share” information by partially swapping some of their nerve fibers (partial decussation) to provide you with binocular vision. This neat feature is involved in a fun poke-your-eye experiment that you can do (gently!) right now in the comfort of your own home.
With purely motor concerns, the Oculomotor Nerve, Cranial Nerve III, has been entrusted with the sole duty of moving four of the six muscles that move your eyeball and innervating a muscle that lifts your upper eyelid. She’s also in charge of controlling the muscles inside your eyeball that 1) alter the diameter of your pupil and 2) change the shape of your lens.
- Treat her nicely. When damaged, left and right eyes can stop coordinating… sometimes leading to double vision.
- You know how doctors flick a light in one eye and look to see if the pupil contracts? They’re testing for damage to this nerve via your pupillary reflex. Not only will she respond to a bright light by constricting that exact pupil, but via partial decussation (see CN II) the pupil of your other eye will constrict. Two nerves tested with nothing but a mini flashlight—and safe enough to try at home!
This guy has one job. ONE. The Trochlear Nerve, Cranial Nerve IV, innervates one eye muscle. ONE. The muscle that lets you look down and to the side.
- Damage can lead to double vision.
- Movements that might indicate when you are thinking or preparing to lie.
Like her friend, CN IV, the Abducens Nerve, Cranial Nerve VI, also innervates only one eye muscle. The muscle that lets you look directly to the side.
- In all mammals besides humans, it innervates a muscle (that we don’t have) that can retract (pull back) the eyeball in order to protect it.
Amblyopia (Lazy Eye) is a condition in which one eye will drift. Perhaps you’ve seen young children wearing eye patches? While the condition is obvious to onlookers, the affected individual is unaware and does not see double. How can this be? When weakness or scaring of these eye muscles (or bad vision or damage to the nerves) prevents both eyes from tracking together, the brain takes note and “turns off” the information from the “bad” eye. This is a problem for little kids because if the brain keeps that eye “turned off” long enough, the nerves will actually lose their connections with the visual cortex (back of the brain, see CN II) and leave them permanently blind in that eye. The treatment can be as simple as patching the “good eye” and forcing the “bad eye” to work for part of the day. Hence the eye patches.
Why, yes, we did skip over Cranial Nerve V. Not because I hold anything against him (he’s actually one of my favorite nerves), but merely so those CNs innervating the eye muscles could hang out together and give CN V his proper due.
Meet the Trigeminal Nerve. A mixed nerve, this guy has three branches—all of which innervate muscles that you use to chew your food (motor fibers). He also innervates three very distinct sections (dermatomes) of the skin covering your face (see image below) with sensory fibers.
- Opthalmic (V1): eyes, nose, forehead (the skin in that general area)
- Maxillary (V2): lower eyelid & upper lip (and skin in that general area), gums, and part of the mouth and throat
- Mandibular (V3): lower gums, teeth, lips (and skin in that general area), palate and tongue
- By targeting only specific branches of V2 or V3, your dentist can refine the use of Novocaine to numb very specific portions of this nerve prior to applying whatever torture event you’re in for.
- An ice cream headache (brain freeze!) happens when you eat cold food quickly. You’ll feel pain in your forehead and at the temples. Why? Referred pain. The cold sensors in your mouth inform your brain (via CN V) they’re freezing. The brain “reads” this cold/pain as coming from your face rather than your mouth. It’s called referred pain (something also seen in heart attacks when pain runs down the patient’s left arm).
Meet your Facial Nerve, Cranial Nerve VII. She is a mixed nerve. Her motor fibers work to control the muscles of facial expression (wrinkling your brow, bearing your teeth, frowning, squeezing your eyes shut, puffing your cheeks, and pursing your lips to make a duck face). Her sensory fibers provide you with your sense of taste from the most forward (anterior) two-thirds of your tongue. There’s also a tiny patch of skin on the back of your ear that she innervates for sensation.
- When doctors ask you to make various facial expressions, they’re testing this nerve.
- This cranial nerve is involved in corneal reflexive blinking. Stimulate one eye, and a signal is sent to the brain via the trigeminal V2 branch… after which you’ll blink because the motor fibers of the facial nerve (VII) will make you.
Ah, at last we reach the Vestibulocochlear Nerve, Cranial Nerve VIII, a purely sensory nerve. He makes an appearance in the first chapter of THE GOLDEN SPIDER when we meet two agents trying to deal with a malfunctioning acousticocept (an invented steampunk device).
Like the trigeminal, this nerve can also be divided into distinct branches.
The vestibular branch is involved in sending your brain information about your balance and equilibrium. It picks up information from tiny hairs inside your three semicircular canals.
- Damage to this nerve (ototoxic antibiotics, Ménière’s disease, encephalitis, rare autoimmune diseases) can bring on attacks of dizziness, nausea, and the inability to maintain correct balance.
The cochlear branch provides you with your ability to hear. It relays and information (sound waves) that touch the ear drum to the brain. Damage to this nerve can cause hearing loss.
- A cochlear implant can be used to bypass a damaged cochlear nerve to allow a person to hear electronically. There are two parts to such an implant. 1) The part worn outside (usually behind the ear) that will detect sound waves and sends an electronic signal to 2) an implant that has electronics inserting directly into the cochlea and cochlear nerve to stimulate the sending of signals to the brain.
The Glossopharyngeal Nerve, Cranial Nerve IX, is a mixed nerve. Anything you taste (sensory) on the back one-third of your tongue is mediated by her. She also has sensory fibers coming from a special section of your carotid artery allowing your brain to monitor the composition of blood flowing through it (oxygen, carbon dioxide, pH and temperature.) On the motor end of things, she innervates a single muscle involved in swallowing and innervates your parotid gland (saliva for eating).
- She is responsible for the gag reflex (make of that what you will…) The sensory component of this reflex is mediated by CN IX, but the motor reaction is caused by CN X.
Meet your longest cranial nerve, the Vagus Nerve, Cranial Nerve X, whose name comes from the Latin word for “wandering”. Though a mixed nerve, he is 90% sensory.
Motor fibers (not shown here) are involved in a few movements of the mouth and throat, including those involved in speech and swallowing.
A good portion of its job is to mediate a number of parasympathetic responses (automatic and calming effects), a topic worthy of its own set of lectures. So, in a nutshell:
- It initiates relaxation.
- It helps you breath (diaphragm innervation).
- It is responsible for controlling your heart rate (slows it down and lowers blood pressure).
- It regulates sweating.
- It helps your gut move things through by causing waves of contraction (peristalsis).
- It innervates glands (e.g. gallbladder).
- Though I’m oversimplifying a little, this nerve is involved in the primitive response to stress that can cause you to faint. (E.g. the sight of blood. Watch Lady Olivia closely in THE SILVER SKULL.) Blood pressure and heart rate drop, and you will lose consciousness – faint, aka fall to the ground. If you feel this happening, sit or lie down quickly.
- A few sensory branches innervate the ear canal and explains why some people will cough when using a cotton-tipped swab.
- This nerve provides you with “gut” feelings. The brain-stomach connection is very real.
- Vagal stimulation (depending on location) plays a role in the gag reflux.
- Stimulating this nerve can reduce inflammation.
- Speech and swallowing can be impacted during a thyroidectomy if the recurrent laryngeal nerve—a branch of CNX—is damaged.
- Stomach inflammation can stimulate vomiting.
Your Accessory Nerve, Cranial Nerve XI, is not all about the bling. She is, however, essential for speech. Via motor fibers, she controls the muscles of your mouth, throat and voice box. She’s also involved in voluntary swallowing (save the jokes, people). On top of that, she innervates two muscles of your neck and back (sternocleidomastoid and trapezius)
- She’s the only nerve that originates outside the skull.
- This nerve lets you shrug your shoulders.
This guy, your Hypoglossal Nerve, Cranial Nerve XII, is all motor and in charge of your voluntary tongue movements. He’s really important for speech and articulation.
- He’s very important to all small children as he’s responsible for allowing them to stick out their tongue at adults. (Which is what the doctors are checking when they ask you to do this. Any damage to this nerve, and your tongue will move toward the affected side.)
*Nerves have been anthropomorphized purely for our entertainment. Hence the unusual use of pronouns.
THE GOLDEN SPIDER
Lady Amanda is tired of having both her intelligence and her work dismissed.
After blackmailing her way into medical school, she catches the eye of her anatomy professor from the moment she walks into his lecture hall. Is he interested in her? Or only her invention–a clockwork spider that can spin artificial nerves?
Lord Thornton, a prominent neurobiologist, has been betrayed.
Secret government technology has been stolen from his laboratory, and a foreign spy is attempting to perfect it via a grisly procedure… using gypsies as test subjects. The last thing he needs is the distraction of a beautiful–and brilliant–new student, even if her spider could heal a deteriorating personal injury.
Until her device is stolen and used in the latest murder.
Lord Thornton has no option but to bring her into his laboratory as well as the investigation where they must fight their growing, yet forbidden, attraction. Bodies accumulate and fragile bonds are tested as they race across London, trying to catch the spy before it’s too late.
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Though USA TODAY bestselling author Anne Renwick holds a Ph.D. in biology and greatly enjoyed tormenting the overburdened undergraduates who were her students, fiction has always been her first love. Today, she writes steampunk romance, placing a new kind of biotech in the hands of mad scientists, proper young ladies and determined villains.
Anne brings an unusual perspective to steampunk. A number of years spent locked inside the bowels of a biological research facility left her permanently altered. In her steampunk world, the Victorian fascination with all things anatomical led to a number of alarming biotechnological advances. Ones that the enemies of Britain would dearly love to possess.
To chat with Anne, stop by on Facebook or join the Department of Cryptobiology Facebook group. You can also join her newsletter list to have cover reveals, sneak peaks, sales and giveaways delivered straight to your inbox.
Good article on the function of the 12 cranial nerves. I didn’t know that the auditory nerve (VIII) is 100% sensory or that the Vagus nerve was 90% sensory either.
I actually created a song to help students memorize the nerves by which is set to the 12 Days of Christmas – https://m.youtube.com/watch?v=kxStDYFK2Cc&t=2s
I lost my sense of smell 40 years ago (except for a short period in my early 20s). Things taste *different* to me than they do to my husband, but it hasn’t discouraged my appetite – unfortunately. Contact me if you want to talk about it, Ms Renwick. 🙂
I really enjoyed the cranial nerve piece! I love it that you know that we are not all brain dead. Really appreciate these extra gifts from you!
Thank you, Carolyn O
I like the Facial Nerve, Cranial Nerve VII, It is just neat.