(Neuro)Optometry in Focus: Cranial Nerve Basics I-III

Howdy, y’all!

Since it’s been a while since I wrote any neuro-related posts, I figure it’s as good of time as any to begin a discussion on cranial nerves.  Let’s get started!


What are cranial nerves?

Cranial nerves are simply the 12 pairs of nerves that originate in the brain, and then travel to various parts of the body.  Typically, these 12 nerves are referred to by roman numerals.

Before we dive further into each of the 12 cranial nerves, it’s nice to have some understanding of, well, nerves.  So here it goes:

Nerves can be classified in several different ways, but the most common are:

  • Size
  • Conduction speed
  • Function
  • Origin
For right now, I primarily want to focus on function.

One major functional distinction is sensory versus motor.

  • Sensory nerves carry information regarding your senses to the brain.
  • Since they are going to the brain, they are considered afferent
  • Motor nerves carry information from the brain to the body to create movement.
  • Since they are coming from the brain, they are considered efferent.

Nerves may also be classified as special or general.

  • Special nerves are those related to special senses – sight, smell, hearing, taste, and balance.
  • General nerves are for everything else.
Finally, nerves can be responsible for either somatic (general body) or visceral (internal organ) functions.
Okay, I think that covers the basics. Now onto cranial nerves!

CN I: Olfactory

The Olfactory nerve is responsible for smell.  There’s some debate as to its classification – special somatic afferent or special visceral afferent.  Regardless, it’s a sensory nerve for a special sense!
Interestingly, this nerve doesn’t have a technical nucleus, but rather originates in the nasal cavity, with its branches passing through the cribriform plate of the ethmoid bone.  This creates a perfect location for picking up smells!
The fibers then travel to the olfactory bulb to become the olfactory tract, and then pass to other parts of the brain to play roles in memory!

CN II: Optic

The optic nerve is most optometrists’ favorite.  After all, it’s the one we work with all the time!
Due to its role in sight, the optic nerve is again a special nerve that carries sensory information to the brain – special somatic afferent!  Like CN I, CN II does not have a nucleus.
You can read more about the pathway of the optic nerve in my post: How Do You See?

CN III: Oculomotor Nerve

With the oculomotor nerve, we’re starting to get into more interesting territory!
CN III is a motor nerve that actually has two branches – one for somatic muscle and one for visceral muscle! For this reason, the oculomotor nerve is both general somatic efferent AND general visceral efferent.
Unlike CN I and II, CN III has a true nucleus – the oculomotor nucleus.  This structure is located in the midbrain (uppermost) part of the brainstem. Fibers then pass forward (towards the face) through the cavernous sinus and into the superior orbital fissure where it divides into two branches: superior and inferior.
The superior branch of the oculomotor nerve is responsible for somatic innervation of levator and superior rectus.  As an optometrist, these two muscles are extremely important, as the levator is responsible for movement of the upper eyelid, while the superior rectus helps to lift the eye up!
The inferior branch of the oculomotor nerve provides somatic innervation to the medial and inferior rectus muscles, as well as the inferior oblique.  Once again, these muscles are important in optometry, as the medial rectus turns the eye in, the inferior pulls the eye down, and the inferior oblique helps to rotate the eye!
The inferior branch of the oculomotor nerve additionally carries visceral (parasympathetic) innervation to ciliary muscle and pupillary constrictor.  These are once again important to optometrists, as the ciliary muscles drives accommodation (or the ability to focus), while the pupillary constrictor muscle is responsible for contracting the pupil!
Well, that’s enough typing for now.  Check back soon for Part II of this series!
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