Week 5 HUBS - Cranial Nerves

Cranial Nerve $I$ — Olfactory

• Type: SSA (special sensory)
• Foramen: Cribriform plate (ethmoid bone)
• Origin: Olfactory receptors in nasal epithelium
• CNS nucleus: Olfactory bulb
• Pathway: Olfactory nerves → olfactory bulb → olfactory tract → cortex
• Function: Sense of smell
• Key clinical note: Lesion causes anosmia (loss of smell); only cranial nerve with direct connection to cerebrum

Cranial Nerve $II$ — Optic

• Type: SSA
• Foramen: Optic canal
• Origin: Retina
• CNS nucleus: Lateral geniculate nucleus (LGN) of the thalamus; visual cortex in occipital lobe
• Pathway: Retina → optic nerve → optic chiasm (nasal fibers decussate) → optic tract → LGN → optic radiations → visual cortex
• Function: Vision
• Lesion: Partial or complete visual field loss depending on location

Cranial Nerve $III$ — Oculomotor

• Type: GSE (motor) and GVE (parasympathetic)
• Foramen: Superior orbital fissure
• Skeletal muscle innervation (GSE): levator palpebrae superioris; superior rectus; medial rectus; inferior rectus; inferior oblique
• Parasympathetic (GVE): sphincter pupillae; ciliary muscle via ciliary ganglion
• CNS nuclei: Oculomotor nucleus; Edinger–Westphal nucleus (parasympathetic)
• Pathway: GSE fibers to extraocular muscles; GVE fibers to pupil constriction and accommodation
• Lesion: Ptosis, eye deviated down-and-out, dilated pupil, loss of pupillary reflex

Cranial Nerve $IV$ — Trochlear

• Type: GSE
• Foramen: Superior orbital fissure
• Innervation: Superior oblique muscle
• CNS nucleus: Trochlear nucleus (midbrain)
• Pathway: Emerges dorsally from brainstem, travels in cavernous sinus, through superior orbital fissure
• Lesion: Inability to look down when the eye is adducted

Cranial Nerve $V$ — Trigeminal

• Type: GSA (sensory) and SVE (motor)
• Foramina: $V<em>1$: superior orbital fissure; $V</em>2$: foramen rotundum; $V_3$: foramen ovale
• Three divisions (sensory for all; motor only for V3):
  • $V_1$ (Ophthalmic): sensory from forehead, scalp, cornea, nasal cavity, lacrimal gland
  • $V_2$ (Maxillary): sensory from midface, maxilla, nasopharynx, teeth of maxilla, palate
  • $V_3$ (Mandibular): sensory from lower face; motor to muscles of mastication, tensor tympani, tensor veli palatini, anterior belly of digastric, mylohyoid
• Nuclei: Trigeminal sensory nuclei (GSA, spinal and principal), Trigeminal motor nucleus (SVE for V3)
• Notable branches carrying parasympathetic information (via hitchhiking fibers):
  • Lacrimal gland: via V1 pathways (pterygopalatine ganglion via greater petrosal from VII)
  • Submandibular/sublingual glands: via V3 (chorda tympani from VII joins lingual nerve to submandibular ganglion)
  • Parotid gland: via IX (lesser petrosal to otic ganglion)
• Pathways: Sensory information to brainstem; motor to muscles of mastication; proprioception from jaw muscles
• Lesion: Loss of sensation in all three divisions; V3 motor deficit (weakness of mastication)

Cranial Nerve $VI$ — Abducens

• Type: GSE
• Foramen: Superior orbital fissure
• Innervation: Lateral rectus muscle
• CNS nucleus: Abducens nucleus (pons)
• Pathway: Travels through cavernous sinus to eye
• Lesion: Inability to abduct the eye; medial deviation of the affected eye

Cranial Nerve $VII$ — Facial

• Type: GSE (facial expression); GVE (salivary/lacrimal); GSA (skin around ear); SSA (taste anterior 2/3 of tongue)
• Foramen: Internal acoustic meatus → stylomastoid foramen
• Nuclei: Facial nucleus; Superior salivatory nucleus; Spinal trigeminal nucleus; Solitary nucleus (taste)
• Branches (terminal divisions): Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical; Nervus intermedius carries parasympathetic and taste fibers
• Parasympathetic pathways:
  • Lacrimal gland: greater petrosal nerve → pterygopalatine ganglion → lacrimal gland
  • Submandibular and sublingual glands: chorda tympani joining lingual nerve → submandibular ganglion
  • Mucosal glands of nasal/oral cavities via facial nerve contributions
• Special functions: Taste from anterior 2/3 tongue (via chorda tympani)
• Lesion: Bell’s palsy – unilateral facial weakness; possible decreased lacrimation/salivation and loss of taste anterior 2/3 tongue

Cranial Nerve $VIII$ — Vestibulocochlear

• Type: SSA
• Foramen: Internal acoustic meatus
• Sensory components: Hearing (cochlear) and balance (vestibular)
• CNS nuclei: Cochlear nucleus; Vestibular nucleus
• Pathway: Cochlea/vestibular apparatus → vestibulocochlear nerve → brainstem
• Lesion: Hearing loss, tinnitus, dizziness, vertigo

Cranial Nerve $IX$ — Glossopharyngeal

• Type: GVA, SSA, GVE, GSE
• Foramen: Jugular foramen
• Sensory (GVA): carotid body/sinus, posterior 1/3 tongue (general + taste), tonsils, upper pharynx, middle ear, auditory tube
• Taste and somatic sensation: posterior 1/3 tongue; pharynx
• Parasympathetic (GVE): parotid gland via lesser petrosal nerve → otic ganglion
• Motor (GSE): stylopharyngeus
• Nuclei: Spinal trigeminal nucleus (GVA); solitary nucleus (taste); inferior salivatory nucleus (GVE); nucleus ambiguus (GSE)
• Lesion: Loss of taste from posterior 1/3 tongue; reduced or absent pharyngeal sensation; impaired parotid secretion

Cranial Nerve $X$ — Vagus

• Type: GSA, GVA, SSA, GVE, GSE
• Foramen: Jugular foramen
• Motor: pharyngeal, soft palate, laryngeal muscles; thoracic and abdominal viscera (GVE)
• Nuclei: Ambiguus (branchial motor); Dorsal motor nucleus of X (parasympathetic); Solitary nucleus (GVA/GSA/SSA)
• Branches (key): Pharyngeal branches; Superior laryngeal nerve; Recurrent laryngeal nerve
• Pathway: Vagus fibers extend from brainstem to thorax and abdomen via esophageal/gastric plexuses
• Lesion: Soft palate deviation; vocal cord paralysis; broad autonomic disturbances

Cranial Nerve $XI$ — Accessory

• Type: GSE
• Foramen: Jugular foramen
• Origin: Spinal root from C1–C5; ascends via foramen magnum; exits via jugular foramen
• Innervation: Sternocleidomastoid; Trapezius
• Nuclei: Ambiguus and accessorius
• Lesion: Paralysis of SCM and trapezius

Cranial Nerve $XII$ — Hypoglossal

• Type: GSE
• Foramen: Hypoglossal canal
• Innervation: Intrinsic tongue muscles; extrinsic tongue muscles (genioglossus, hyoglossus, styloglossus); some tongue muscles via ansa cervicalis to infrahyoid muscles
• CNS Nucleus: Hypoglossal nucleus
• Lesion: Tongue atrophy, deviation of tongue toward the side of the lesion on protrusion

Quick reference: General concepts to recall

• Cranial nerves: $I$–$XII$; Roman numerals denote order; many have multiple modalities
• Afferent vs efferent: Afferent = sensory; Efferent = motor
• Major functional categories:
  • GSA: general somatic afferent
  • GVA: general visceral afferent
  • SSA: special sensory
  • GSE: general somatic efferent
  • GVE: general visceral efferent
  • SVE: special visceral efferent
• Autonomic divisions: Sympathetic (thoracolumbar) vs Parasympathetic (craniosacral)
• Foramina and exits (high-yield):
  • I: cribriform plate
  • II: optic canal
  • III, IV, VI: superior orbital fissure
  • V1: superior orbital fissure; V2: foramen rotundum; V3: foramen ovale
  • VII, IX, X, XI: jugular foramen
  • XII: hypoglossal canal
• Nuclei are mostly in the brainstem (except I and II)
• Key clinical links: lesions produce characteristic motor, sensory, or autonomic deficits per nerve name