CRANIAL NERVES

Tavishi

coolest brother who DID consent to this <3

This is my brother, Yash. More importantly, he's one of Kradigan's biggest fans.

Jokes aside, he can do something funny with his eye because he has Marcus-Gunn syndrome, otherwise known as jaw-winking syndrome.

Whenever my brother opens his mouth, his left eye opens, too. He can control it to some extent, but anyone who knows my brother has watched his eye twitch every time he takes a bite of his food.

case in point

Why does this happen? Because my brother has silly cranial nerves.

The cranial nerves are part of the peripheral nervous system, as are pretty much all nerves. There are twelve cranial nerves, labeled I-XII, or in Roman numerals.

Cranial nerve I is the olfactory nerve, which is used for smell. The olfactory nerve, unlike most cranial nerves, does not originate from the brain stem. Rather, the olfactory nerve originates from the olfactory mucosa, an upper part of the nasal cavity. The olfactory nerve's synapses terminate in the olfactory bulb of the forebrain.

The olfactory nerve consists of sensory cells called olfactory receptor neurons. The axons, or the extended spaghetti-looking part of the neuron, forms the nerve itself. However, odorants (things that cause smells) interact with the cell body itself.

On the ciliated surface of olfactory receptor neurons are olfactory receptors. When odorants bind to olfactory receptors, a subunit of a G protein dissociates, activates adenylyl cyclase II, and causes the production of cAMP from ATP, and causes depolarization of the cell which then produces an action potential, or a signal.

yummy

Next, cranial nerves II, III, IV, and VI all are related to the eye.

Cranial nerve II is the optic nerve. The optic nerve is an iffy cranial nerve, as it's technically part of the central nervous system. Arguably, it's one of the more important cranial nerves.

The optic nerve receives signals in both eyes, and both paths of the nerve cross over each other. The nerve then terminates in the lateral geniculate nucleus of the thalamus, the pretectum (midbrain), and the superior colliculus (midbrain). From these areas, additional neurons from the LGN carry information to the occipital lobe. Information about vision carries through and is "processed" in the thalamus, whereas the superior colliculus and pretectum control eye movement. The pretectum allows for the pupil of the eye to expand in response to changes in light. The right eye's visual information is processed on the left side of the brain, and vice versa. The place where the nerves cross is called the optic chiasm.

hate this image

Whereas cranial nerves III, IV, and VI control eye movement, the optic nerve actually receives signals about vision.

Cranial nerve III is the oculomotor nerve, and it originates from the superior colliculus, which is part of the midbrain. The superior colliculus also plays a small role in optic sensation, as well.

The oculomotor nerve controls the opening of the eyelids, narrowing of the pupil, and focusing of the eye. While most of the oculomotor's function is focused (see what I did there?) on the extraorbital muscles, it also has some intraorbital functions, too, like focusing of vision. However, the most important function for this post is that the oculomotor controls your eyelids opening and closing.

Cranial nerve IV is the trochlear nerve and is irrelevant. Jokes aside, the trochlear nerve's exclusive function is the superior oblique muscle's function. The superior oblique muscle controls the rotation of the eye. Look left with your left eye, and right with your right eye. Look at the bottom of the nose, and the top of the nose. Congrats, you just used your trochlear nerve! Next, cranial nerve VI is the abducens nerve, and it controls the lateral rectus muscle, which controls the outward gaze of the eye. For example- there's a seal on your wall, go look at it! Congrats, you just used cranial nerve VI!

That concludes the cranial nerves that exclusively control the eye. Next, cranial nerve V- the trigeminal nerve. The trigeminal nerve has three branches and functions- ophthalmic, and the maxillary and mandibular nerves. The ophthalmic nerve carries sensation from a variety of sources, including the forehead, tongue, and the cornea.

The maxillary and mandibular nerves, however, control jaw movement and sensation. The maxillary nerve is purely sensation, and the mandibular nerve controls the movement of the jaws.

For Yash, a defect with his oculomotor and trigeminal nerves produces jaw-winking syndrome. Yash's oculomotor nerve and the maxillary nerve of his trigeminal nerve produces this:

the brother of all time

Next, cranial nerve VII is the good ol' facial nerve. The facial nerve controls facial expressions (duh) and facial sensations, including gustation (taste sensation). Overall, it has a variety of functions: far too many to list in one generalized post about the cranial nerves. Y'all don't have a long enough attention span for that, smh.

Cranial nerve VIII is the vestibulocochlear nerve, another nerve my brother is familiar with. The vestibulocochlear nerve's functions are balance and sound. I don't like the ear or hearing, so I will not be continuing further. Ew earwax.

Cranial nerve IX, however, is the glossopharyngeal nerve. This nerve has a variety of functions, but among my favorite, is its role in taste. The glossopharyngeal nerve is one of many nerves that plays a role in tongue sensation. Additionally, the glossopharyngeal nerve innervates the tonsils, sinuses, and finally, the middle ear via the tympanic nerve.

Cranial nerve X is the vagus nerve, and has both motor and sensory function. But while a lot of the past nerves like cranial nerve IV had much more narrow functions, the vagus nerve's function is significantly more generalized, and rather, plays a role in parasympathetic control, and is one of the largest nerves in the body. In relation to the last post, stimulation of the vagus nerve has proven to be an effective treatment for epilepsy.

Cranial nerve XI is a mostly motor nerve, and is the accessory nerve. The accessory nerve controls the function of the sternocleidomastoid and trapezius muscles, both of which are located around the shoulders, but not quite near the head. Shrug your shoulders, and shake your head- congrats, you've used your accessory nerve.

Finally, the hypoglossal nerve is cranial nerve XII, and controls tongue movement. It's pretty straightforward- the hypoglossal nerve is how you're able to move your tongue up, down, and all around. Speech of any sort utilizes the hypoglossal nerve.