AnatomyLove

Introduction to cranial nerves

Cranial nerves

There are 12 cranial nerves, numbered in order from front to back on the ventral aspect, or base of the brain. These 12 are termed cranial nerves because their nuclei lie within the brain stem. From their origin in the brain stem these nerves travel through the cranial cavity, through fissures and foramina in bone, to get to their targets, most of which are in the head and neck, though there are a few below the neck. In order they are:

  1. Olfactory
  2. Optic
  3. Oculomotor
  4. Trochlear
  5. Trigeminal
  6. Abducens
  7. Facial
  8. Vestibulocochlear
  9. Glossopharyngeal
  10. Vagus
  11. Accessory
  12. Hypoglossal

Think of how many things go on in your head – it is home to special senses like:

  • hearing
  • balance
  • vision
  • taste
  • smell

But, we also:

  • make expressive faces
  • blink
  • speak and vocalize
  • chew
  • salivate
  • swallow
  • cry
  • snot
  • look around (different than vision)
  • feel things on our skin and inside our mouth, nose, and eyes
  • move our head, neck, and shoulders

Types of fibers in cranial nerves

To accomplish this wide variety of tasks, cranial nerves incorporate different types of fibers. Anatomists use a series of three letters to quickly describe the types of fibers each cranial nerve has. 

The first letter is either G or S, which stands for general or special types of fibers.

  • General fibers innervate structures for things like touch, pain, temperature, and proprioception.
  • Special fibers innervate special sense structures for vision, balance, hearing, smell, and taste.

The next letter is either V or S. V stands for visceral, S stands for somatic.

  • Visceral structures are those that derive from a branchial arch in a developing embryo, and are involved in special senses.
  • Somatic structures are everything else, having come from somites of a developing embryo. 

The last letter is either A or E, which stands for afferent (sensory) and efferent (motor) types of innervation. These two operate through feedback loops whereby a sensory stimulus causes a motor response. 

This yields the following combinations:

  • GSE general somatic efferent
  • SVE special visceral efferent*
  • GVE general visceral efferent
  • GSA general somatic afferent
  • SSA special somatic afferent
  • GVA general visceral afferent
  • SVA special visceral afferent

Efferent fibers

GSE fibers are general somatic efferent nerves to skeletal muscle derived from somites in an embryo.

  • These are the muscles that: make up the tongue, and move the eyes around.
  • Nerves with GSE fibers in them are: CN 3, 4, and 6 (the eye muscles), and CN 12 (to the tongue).
  • Spinal nerves to the rest of the muscles in the body are also GSE motor fibers.

SVE, or sometimes shown as SSE* are special visceral efferent fibers to skeletal muscle derived from branchial arch mesoderm, not somites. A developing embryo forms 5 arches and pouches in between, out of which most structures of the head and neck develop (everything that isn’t from somites). Each arch has a dedicated cranial nerve to it, meaning anything coming from that arch is innervated by that cranial nerve. Their targets tend to fit a “visceral” conception of the head and neck (skeletal muscles for facial expression, mastication, speech, and swallowing). 

  • SVE is specifically to skeletal muscle from the arches, innervated by CN 5 (only the 3rd portion), 7, 9, 10, or 11

GVE – maybe my favorite type of fiber – stands for general visceral efferent fibers with a secretomotor effect on smooth muscle, glands, and cardiac muscle.

  • The only cranial nerves that contain GVE fibers are 3, 7, 9, and 10.
  • These are the parasympathetic cranial nerves, responsible for rest and digest functions. The antagonist to the parasympathetic rest and digest functions are sympathetic fight or flight innervations, which come from the sympathetic chain. They too, are GVE nerves, though they are not cranial nerves. 

Afferent fibers

Let’s look at the sensory side of things. Here we’ll see GSA, SSA, GVA, and SVA types of fibers. 

GSA stands for general somatic afferent, meaning the sense of touch, pain, temperature, and proprioception from somatic targets like the skin, joints, bone, and skeletal muscle.

  • Several cranial nerves do this, including all of 5, with help from 7, 9, and 10.

SSA is special somatic afferent fibers for vision, balance, and hearing.

  • The retina and vestibulocochlear apparatus develop from somites so CN 2 and 8 contain SSA fibers. 

GVA, or general visceral afferents, are sensations from glands and viscera in the head and cavities below the neck.

  • The two nerves that allow us feeling from organs in those locations are CN 9 and 10

SVA fibers are special visceral afferents responsible for smell and taste.

  • The special receptor cells in the nose and mouth are innervated by CN 1 for smell, and 7, 9, and 10 for taste.

Motor, sensory, or both types of fibers

Now, with all those letters out of the way, I’d like to wrap up this introduction to cranial nerves by reviewing which cranial nerves contain either sensory, motor, or both types of fibers. This is the A or E letter in the three-letter fiber types, and I find this particularly useful for organizing my thoughts about cranial nerves. 

Review the list of nerves in order at the top of this page. Since there are 12 cranial nerves it helps to have a mnemonic to keep their motor/sensory/or both types of fibers straight.

  • There are many mnemonics, but a clean one is: some say marry money, but my brother says big brains matter most.
  • From 1 to 12, the nerves contain sensory, sensory, motor, motor, both, motor, both, sensory, both, both, motor, motor.
    • 1 sensory
    • 2 sensory
    • 3 motor
    • 4 motor
    • 5 both
    • 6 motor
    • 7 both
    • 8 sensory
    • 9 both
    • 10 both
    • 11 motor
    • 12 motor

I hope you found this overview useful! Working through the fiber types within cranial nerves can be daunting! But take it one step at a time, start with the motor/sensory divide, and work out from there!

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