Cranial Nerves Notes

Learning Objectives

• List the modalities carried by each of the cranial nerves.
• Identify each of the cranial nerves on the brainstem.
• Describe the pathway and innervation of each of the cranial nerves.
• Understand actions of the extraocular muscles on the eye.

Overview of Cranial Nerves

• There are twelve pairs of cranial nerves.
• They are numbered using Roman numerals (I - XII) from cranial to caudal.
• Cranial nerves originate from the brainstem and exit via skull foramina.

Types of Cranial Nerves

• Motor Nerves (Efferent): Convey motor information and exit the brain.
• Sensory Nerves (Afferent): Convey sensory information and enter the brain/brainstem.
• Mixed Nerves: Convey both motor and sensory information.

Mnemonic for Cranial Nerves

• Mnemonic to remember cranial nerves: "Some say marry money, but my brother says: big brains matter more."

Cranial Nerve Pathways and Functions

1. CN I - Olfactory Nerve: Special sensory (olfaction)
   • Origin: Olfactory bulb, exits via the cribriform plate.
2. CN II - Optic Nerve: Special sensory (vision)
   • Origin: Retinal ganglia cell axons form optic nerve, exits via optic canal.
   • Visual pathway: Optic nerve -> Optic chiasm -> Optic tract -> Lateral Geniculate Nucleus (LGN) -> Visual cortex.
3. CN III - Oculomotor Nerve: Innervates most extraocular muscles, parasympathetic fibers control pupil constriction.
4. CN IV - Trochlear Nerve: Innervates superior oblique muscle.
5. CN VI - Abducens Nerve: Innervates lateral rectus muscle.
   • (CN III, IV, and VI exit via superior orbital fissure).
6. CN V - Trigeminal Nerve: Sensory from the face; divided into three branches - V1 (ophthalmic), V2 (maxillary), V3 (mandibular, also motor for mastication).
7. CN VII - Facial Nerve: Motor for facial expression, special sensory (taste) from anterior 2/3 of the tongue.
8. CN VIII - Vestibulocochlear Nerve: Special sensory (hearing and balance).
9. CN IX - Glossopharyngeal Nerve: Motor for stylopharyngeus, sensory from posterior 1/3 of the tongue.
10. CN X - Vagus Nerve: Motor and sensory for visceral organs and autonomic functions.
11. CN XI - Accessory Nerve: Innervates sternocleidomastoid and trapezius muscles.
12. CN XII - Hypoglossal Nerve: Motor for tongue muscles.

Cranial Nerve Exit Points

• CN I: Cribriform plate.
• CN II: Optic canal.
• CN III, IV, VI: Superior orbital fissure.
• CN V1: Superior orbital fissure.
• CN V2: Foramen rotundum.
• CN V3: Foramen ovale.
• CN IX: Jugular foramen.
• CN X: Jugular foramen.
• CN XI: Jugular foramen.
• CN XII: Hypoglossal canal.

Extraocular Muscles

• Muscles controlling eye movement include:
  • Superior rectus
  • Inferior rectus
  • Lateral rectus
  • Medial rectus
  • Superior oblique
  • Inferior oblique
• Common tendinous ring serves as the origin point for all extraocular muscles.

Eye Movement Actions

• Muscles often work in pairs for movements such as:
  • Abduction and adduction.
  • Elevation and depression.

Clinical Correlate Exercises

• Assessment exercises can involve observing eye movements to determine potential nerve paralysis.

Trigeminal Nerve Pathway

• Trigeminal ganglion divides into three main branches:
  • V1 (Ophthalmic): Sensory from upper face.
  • V2 (Maxillary): Sensory from middle face.
  • V3 (Mandibular): Sensory from lower face, also carries motor fibers for mastication.

Thalamic Nuclei Functions

• VPL: General sensory information from the body.
• LGN: Visual information.
• VPM: General sensory information from the face.

References

• Grant’s Atlas of Anatomy, 8th edition.
• Grant’s Dissector, 16th edition.
• Gray’s Anatomy: anatomical basis for clinical practice.
• Moore’s Clinically Oriented Anatomy, 7th edition.
• Netter’s Atlas of Human Anatomy, 5th edition.