Cranial Nerve III - The Oculomotor Nerve

In-Situ Anatomy (Real Image)

Fig. 13.2 - Oculomotor Nerves (CN III) In Situ. View from above with the forebrain removed, showing the oculomotor nuclei in the rostral midbrain and CN III in relation to adjacent structures. - Neuroanatomy through Clinical Cases, 3rd Ed.

Orbital Course (Real Image)

Fig. 8.106 - CN III branches in the orbit. - Gray's Anatomy for Students

Key Facts: CN III (Oculomotor Nerve)

Fiber Types

• General Somatic Efferent (GSE) - innervates most extraocular muscles
• General Visceral Efferent (GVE) - preganglionic parasympathetic fibers

Course

1. Origin: Nucleus in the rostral midbrain at the level of the superior colliculus
2. Exits: Anterior surface of the brainstem (between midbrain and pons), as exiting fascicles
3. Subarachnoid space: Passes between the posterior cerebral artery (above) and superior cerebellar artery (below); close relationship to the posterior communicating artery - the parasympathetic fibers run superficially here, making them vulnerable to aneurysm compression
4. Cavernous sinus: Travels in the lateral wall
5. Entry to orbit: Through the superior orbital fissure, within the common tendinous ring, dividing into superior and inferior branches

Muscles Innervated

Edinger-Westphal Nucleus

CN III Palsy - Classic Signs

• Ptosis (levator palpebrae superioris failure)
• "Down and out" eye position (unopposed CN IV + CN VI)
• Mydriasis (dilated, unreactive pupil) - if parasympathetics involved
• Surgical CN III palsy (e.g., PComm aneurysm) = pupil involved
• Medical CN III palsy (e.g., diabetes) = pupil spared (ischemia spares superficial parasympathetic fibers)

Course of the Facial Nerve (CN VII)

Schematic Diagram

Figure 10-1: Schematic diagram of CN VII (facial nerve) - Localization in Clinical Neurology, 8e

Overview

• Motor fibers - muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric
• Special sensory (taste) - anterior 2/3 of tongue via chorda tympani
• Parasympathetic - secretomotor to lacrimal, sublingual, submandibular glands
• Cutaneous sensory - skin of posterior concha, external auditory canal

Four Segments

1. Nucleus and Fascicles (Pontine Segment)

• The facial motor nucleus lies in the ventrolateral pons
• Fascicles loop around the abducens nucleus (CN VI), forming the internal genu - this is why pontine lesions often affect both CN VI and VII together (e.g., Möbius syndrome)
• The nerve exits from the ventrolateral pons at the pontomedullary junction, together with the nervus intermedius (CN VII sensory/parasympathetic root)

2. Meatal (Cisternal + Canal) Segment

• CN VII and the nervus intermedius cross the cerebellopontine angle (CPA) laterally, alongside CN VIII (cochleovestibular nerve)
• They enter the internal acoustic meatus together with the internal auditory artery and vein
• In the meatus: the motor division lies on the superoanterior surface of CN VIII; nervus intermedius lies between them
• No branches arise here

3. Intratemporal Segment (Facial/Fallopian Canal)

4. Peripheral Segment (After Stylomastoid Foramen)

• CN VII exits the skull at the stylomastoid foramen
• Immediately gives off:
  • Posterior auricular nerve (occipitalis, auricular muscles)
  • Digastric branch (posterior belly)
  • Stylohyoid branch
• Enters the parotid gland and divides at the pes anserinus ("goose's foot") into 5 terminal branches:

Key Clinical Points

The Geniculate Ganglion

Real Cadaveric Images

Figure 112.4 - Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 2

What is it?

Key Characteristics

What it Contains

1. General Somatic Afferent (GSA) neurons
   • Carry pain and touch from the skin of the external auditory meatus (EAM) and part of the pinna
   • Central processes travel in the nervus intermedius and terminate in the spinal trigeminal nucleus (nucleus of V)
2. Special Visceral Afferent (SVA) neurons - taste
   • Carry taste from the anterior 2/3 of the tongue via the chorda tympani
   • Central processes terminate somatotopically in the gustatory nucleus (rostral nucleus tractus solitarius)

• Those destined for the pterygopalatine (sphenopalatine) ganglion leave as the greater petrosal nerve (GSPN) - the very first branch of CN VII
• Those destined for the submandibular ganglion continue onward in CN VII, eventually leaving as the chorda tympani

The Greater Petrosal Nerve (GSPN) - First Branch

• Runs anteriorly and medially, through a hiatus on the anterior surface of the petrous bone into the middle cranial fossa
• Passes beneath the trigeminal ganglion
• At the foramen lacerum, joins the deep petrosal nerve (postganglionic sympathetics from internal carotid plexus) to form the Vidian nerve (nerve of the pterygoid canal)
• Reaches the pterygopalatine ganglion - postganglionic fibres supply lacrimal, palatal, and nasal glands
• Is a key surgical landmark for middle cranial fossa approaches to the IAM and petrous apex

Relations

• Posteriorly/inferiorly: Superior semicircular canal and cochlea cradle the ganglion
• Superiorly: Floor of the middle cranial fossa (thin bone, often dehiscent)
• Anteriorly: GSPN runs forward toward foramen lacerum
• Medially: Subarachnoid space may extend as far as the ganglion
• The greater and lesser petrosal nerves diverge in the area medial to the ganglion (contrary to most textbook depictions)

Clinical Significance

Why Does the Geniculate Ganglion Form, and What Forms It?

Who Forms It - The Two Sources

1. The Epibranchial Placode (Primary source)

• A thickened patch of surface ectoderm located just caudal to the first branchial groove (the groove between branchial arches 1 and 2)
• Specifically, it is the epibranchial placode of the second branchial arch
• Epibranchial placodes are specialised ectodermal thickenings that give rise to the sensory ganglia of cranial nerves VII, IX, and X
• The cells of this placode differentiate into neuroblasts which become the sensory pseudounipolar cell bodies of the geniculate ganglion

2. The Rhombencephalon (Contributing source)

• Primordial facial nerve tissue arises from the rhombencephalon (the embryonic hindbrain)
• This tissue grows outward and makes contact with the epibranchial placode
• The interaction between these two tissues at ~4 weeks triggers neuroblast differentiation and ganglion formation

Also independently - Neural Crest Cells

• The nervus intermedius (the sensory root that connects the geniculate ganglion back to the brainstem) develops independently from the geniculate ganglion itself and extends to the brainstem, lying between the motor root of CN VII and CN VIII

The Timeline - Step by Step

Why Does It Form - The Functional Reason

• The spot where the labyrinthine segment transitions to the tympanic segment (requiring a sharp turn)
• Close enough to the inner ear to serve as the relay for taste (chorda tympani), ear skin sensation (EAM), and the origin point for the GSPN (lacrimal/nasal parasympathetics)
• A position where the nerve can give off its main branches before it enters the narrower, more constrained mastoid segment

Key Concept: Epibranchial Placodes