Cranial Nerves Notes

Exercise 14-2: The Cranial Nerves

Materials

  • Brain models
  • Preserved brain specimens
  • Penlight
  • Snellen vision chart
  • Tuning fork
  • Unknown samples to smell
  • PTC, thiourea, and sodium benzoate tasting papers

Overview of Cranial Nerves

The 12 pairs of cranial nerves originate from or bring information to the brain (Fig. 14.4). Each nerve has two names:

  1. A sequential Roman numeral indicating the order of its attachment to the brain.
  2. A descriptive name related to the nerve's location or function.
  • Example: Cranial nerve III is the third cranial nerve attached to the brain and is called the oculomotor nerve because it provides motor axons to some eye muscles.
  • Most cranial nerves innervate structures in the head and neck.
  • Three cranial nerves are purely sensory.
  • Four cranial nerves are mixed (sensory and motor axons).
  • Five cranial nerves are primarily motor.

Cranial Nerves and Their Functions

1. CN I: Olfactory Nerve

  • Purely sensory nerve.
  • Innervates the olfactory mucosa in the superior nasal cavity.
  • Provides the sense of smell.

2. CN II: Optic Nerve

  • Purely sensory nerve.
  • Provides the sense of vision.
  • Axons emerge from the retina of the eye.
  • Axons meet at the optic chiasma, where partial exchange occurs before forming the optic tracts.

3. CN III: Oculomotor Nerve

  • Motor cranial nerve.
  • Innervates four of the six extrinsic eye muscles that move the eyeball.
  • Innervates the muscle that opens the eyelid.
  • Innervates the muscle that constricts the pupil.
  • Innervates the muscle that changes the shape of the lens for near vision (accommodation).

4. CN IV: Trochlear Nerve

  • Small motor nerve.
  • Innervates one of the six extrinsic eye muscles that move the eyeball (the superior oblique muscle).

5. CN V: Trigeminal Nerve

  • Large mixed nerve.
  • Three branches provide sensory innervation from the face.
  • Provides motor innervation to the muscles of mastication (chewing).