Recording-2025-07-02T11:38:37.734Z

Cranial Nerve IV – Trochlear

• Basic facts

  • CN\ IV is the smallest cranial nerve and has the longest intracranial course ⟶ high vulnerability to trauma.
  • Exits the midbrain dorsally, decussates, then enters the orbit through the superior orbital fissure.
  • Pierces the belly of the superior oblique (SO) muscle and supplies it exclusively.

• Primary action of the SO

  • Intorsion: rotation of the superior pole of the eyeball toward the nose.
  • Secondary actions: slight depression and abduction when the eye is adducted (~<!20^\circ from the mid-line).

• "Why rotate your eye?" – physiologic head-tilt compensation

  • When the head tilts left:
  • The left globe rotates clockwise (superior pole from 12{\small :}00 to 2{\small :}00).
    (12{:}00 \rightarrow 02{:}00)
  • Without correction, the visual scene would appear rotated.
  • SO contracts ⟶ intorsion restores the vertical axis of the left eye.
  • The right eye simultaneously performs extorsion (inferior pole swings medially) via inferior oblique (IO).
  • Practical illustration: you can lie on your side and still perceive the TV upright.

• Trochlear (IV) palsy

  • Pathophysiology: denervation of SO → unopposed IO & inferior rectus → eye rests in extorted position.
  • Symptoms: vertical diplopia, worse when looking down/reading/walking downstairs.
  • Typical compensation: patient tilts head toward the unaffected side ⟶ generates a physiologic extorsion in the healthy eye and diminishes diplopia.

Cranial Nerve V – Trigeminal

• General
  • Largest cranial nerve; mixed (sensory + motor).
  • Root emerges lateral to the pons; trigeminal (semilunar) ganglion houses the primary sensory neurons.
  • Three divisions: V1 ophthalmic, V2 maxillary, V_3 mandibular.

V₁ – Ophthalmic Division

• Route: cavernous sinus → superior orbital fissure → orbit.
• Branches (mnemonic "Fingers Like Nuts"):
  • Frontal n. (largest, runs above levator palpebrae):
  • Splits into supratrochlear n. (medial, passes above trochlea) and supraorbital n. (lateral, through supra-orbital foramen).
  • Both supply skin of anterior scalp/forehead.
  • Lacrimal n.
  • Reaches lacrimal gland carrying sensory fibres + autonomic hitch-hikers (sympa & parasympa).
  • Also sensation to lateral conjunctiva & eyelids.
  • Nasociliary n. (deepest; courses in orbital fat toward medial wall)
  • Long ciliary nn. → cornea & eyeball (pain, touch).
  • Infratrochlear n. → bridge of nose, medial lids.
  • Anterior & posterior ethmoidal nn. → ethmoid air cells; anterior branch continues as internal nasal n. to anterior nasal septum & lateral wall.

V₂ – Maxillary Division

• Route: trigeminal ganglion → foramen rotundum → superior pterygopalatine fossa (PPF).

• Key landmark: pterygopalatine (sphenopalatine) ganglion in the PPF

  • Analogies in lecture: “Grand Central Station” or “O’Hare Airport” – many fibres synapse or simply pass through.
  • Spatial analogy: triangular bathtub standing on end; PPF = tub, pterygomaxillary fissure = rim, sphenopalatine foramen = drain into nasal cavity.

• Branches

  1. Zygomatic n. (through pterygomaxillary fissure → zygomaticotemporal & zygomaticofacial) → skin over temple & cheek.
  2. Nasal branches (via sphenopalatine foramen) → mucosa of lateral nasal wall.
  3. Nasopalatine n. → across roof of nasal cavity to nasal septum; descends through incisive canal to anterior hard palate.
  4. Greater & lesser palatine nn. (through palatine canal → palatine foramina) → mucosa of hard & soft palate.
  5. **Posterior, middle