Cranial Nerves I–III: Detailed Study Notes

Introduction

• Second podcast of a three-part series on cranial nerves (Dr. Stuart Ingalls)
  • Part 1: general overview, exit points, foramina
  • Part 2 (current): deep dive into first six cranial nerves (CN I–VI) and post-cranial course, with clinical correlations
  • Part 3: will address remaining six nerves (CN VII–XII)
• All material arranged around “sensory-motor-mixed” functional categories

Sensory–Motor–Mixed Classification Mnemonic

• Phrase: “Some Say Merry Money, But My Brother Says Big Brains Mean More.”
  • Each word’s initial letter = S (Sensory) / M (Motor) / B (Both = Mixed)
  • “Brains” sometimes swapped for an anatomical term that is not PG-rated

Cranial Nerve I – Olfactory Nerve (CN I)

• Purely sensory; conveys the special sense of smell
• Anatomical course
  • Olfactory receptor neurons in the superior nasal mucosa
  • Filaments pass superiorly through the cribriform plate of ethmoid bone
  • Synapse in the olfactory bulbs → olfactory tracts → CNS
• Physiology
  • Inhaled air swirls via nasal conchae → odorant aerosols dissolve in mucus → bind chemoreceptors
  • High discriminatory power (e.g., distinguishing dark roast vs. vanilla-bean latte)
• Clinical correlations
  • Shearing injury at cribriform plate (e.g., severe head trauma) → anosmia (partial or complete loss of smell ± taste)

Cranial Nerve II – Optic Nerve (CN II)

• Purely sensory; conveys vision via photoreceptors in retina
• Developmental nuance
  • Retina derived from optic cup (outgrowth of diencephalon) → optic “nerve” is technically a CNS tract
• Course
  • Axons exit globe through optic canal → optic nerve → optic chiasm → optic tracts → lateral geniculate nucleus → visual cortex
• Partial Decussation at Optic Chiasm
  • Medial (nasal) retinal fibers cross
  • Lateral (temporal) retinal fibers remain ipsilateral
  • Functional result: each cerebral hemisphere receives the contralateral visual field
  • Right visual field → left hemisphere
  • Left visual field → right hemisphere
• Lesion patterns & visual field defects
  1. Pre-chiasmatic lesion (optic nerve / eye)
  • Monocular blindness; loss of stereopsis; example: closing one eye blocks lateral field due to nose
  1. Chiasmatic lesion (e.g., pituitary tumor, anterior communicating artery aneurysm)
  • Loss of decussating fibers → bitemporal hemianopsia (tunnel vision); mimicked by outer “blinders”
  1. Post-chiasmatic lesion (optic tract, stroke, trauma)
  • Homonymous hemianopsia (loss of same visual field side in both eyes)
• Demonstrative examples
  • Stereoscopic photos of Paris: loss of Eiffel Tower (left eye injury); loss of both Eiffel Tower & Army Museum domes (chiasmal compression)

Cranial Nerve III – Oculomotor Nerve (CN III)

• Mixed: somatic motor + parasympathetic (GVE)
• Skull exit: Superior orbital fissure; divides into superior & inferior branches
• Somatic motor targets (5 extra-ocular muscles)
  • Levator palpebrae superioris (eyelid elevation)
  • Superior rectus (up)
  • Medial rectus (in)
  • Inferior rectus (down)
  • Inferior oblique (up + out)
• Parasympathetic pathway
  • Preganglionic fibers from Edinger-Westphal nucleus (midbrain)
  • Travel within CN III → superior orbital fissure → ciliary ganglion (synapse)
  • Short ciliary nerves → eye
  • Sphincter pupillae (pupil constriction/miosis)
  • Ciliary muscle (lens accommodation)
• Sympathetic contrast (NOT via CN III)
  • Preganglionic in T1–T2 → superior cervical ganglion → internal carotid plexus → orbit via long & short ciliary nerves
  • Innervates dilator pupillae (pupil dilation/mydriasis); fibers traverse ciliary ganglion without synapse

Oculomotor Nerve Palsy ("Third-nerve palsy")

• Etiologies: ischemia (e.g., diabetes), aneurysm (PComm), trauma, herniation
• Unaffected muscles: Superior oblique (CN IV) + Lateral rectus (CN VI)
• Classic clinical triad
  1. “Down & out” gaze
  • Unopposed pull of lateral rectus (abduction) + superior oblique (depression)
  1. Ptosis
  • Loss of levator palpebrae superioris
  1. Mydriasis & loss of light reflex
  • Parasympathetic failure to sphincter pupillae
• Diplopia scenario
  • Double vision in most gaze directions; lateral gaze often least symptomatic because lateral rectus is intact
• Neurological exam pearl
  • Pen-light test: “equal & reactive” = intact CN III; “fixed & dilated” (blown pupil) = emergency sign of CN III compression (e.g., uncal herniation)

Quick Look-Ahead (teaser)

• Next nerves (CN IV–VI) introduced in upcoming segment but not yet covered in detail here

Integrative & Clinical Connections

• Olfaction loss may impact taste → nutritional & safety implications (e.g., detecting smoke)
• Visual field testing essential in endocrinology (pituitary tumors) & neurology (strokes)
• Oculomotor pupillary reflex used for rapid brain-stem assessment in comatose patients; autonomic reflex independent of patient cooperation

Study Checklist / Self-Test Prompts

• Can you reproduce the “Some Say Merry Money…” mnemonic and label each nerve’s modality?
• Trace the pathway of light from right visual field to cerebral cortex and predict effects of lesions at eye, chiasm, and tract.
• Draw extra-ocular muscles, label CN III targets vs. CN IV & VI, and explain the “down & out” sign.
• Explain why parasympathetic fibers of CN III are more sensitive to compression than motor fibers.
• Simulate bitemporal hemianopsia using your own hands and relate it to pituitary macroadenoma growth.