Cranial Nerves – Comprehensive Bullet-Point Study Notes

General Organization of Peripheral Nerves

• Two broad categories reach the periphery
  • Spinal nerves
  • (31) pairs, segmentally arranged
  • Convey general somatic & visceral efferent/afferent fibres
  • Motor (efferent) somata: anterior & lateral grey horns of spinal cord ➜ exit via inter-vertebral foramina to effectors
  • Sensory (afferent) somata: dorsal-root ganglia ➜ central processes synapse in dorsal grey horn, then ascend to sensory cortex
  • Cranial nerves
  • (12) pairs, numbered I–XII from anterior ➜ posterior brain/brain-stem attachments
  • Share the four general functional components above plus
    • Special visceral efferent (branchial motor) – e.g. muscles of mastication, facial expression, phonation
    • Special somatic/visceral afferent – the five “special senses” (smell, vision, taste, hearing, balance)
  • Nuclei reside in specific brain / brain-stem columns, NOT spinal cord
  • Exit skull via matching foramina; many mnemonic sentences exist to memorise order (omitted here for propriety)

Cranial Nerve I – Olfactory

• Pure special visceral afferent (smell)
• Primary olfactory neurons
  • Bipolar cells in olfactory epithelium (roof of nasal cavity, nasal septum, medial superior concha)
  • Dendrites 3"sample" chemical aerosols; axons bundle into ≈ (20) fila ➜ pierce cribriform plate, dura & arachnoid ➜ olfactory bulb
• Olfactory bulb
  • Contains mitral cell somata; synaptic glomeruli serve as first relay
• Olfactory tract
  • Axons of mitral cells form tract ➜ bifurcate into
  • Lateral olfactory stria ➜ anterior temporal lobe (primary olfactory cortex) – discrimination of quality & intensity
  • Medial olfactory stria ➜ cross midline via anterior commissure to contralateral bulb/cortex; efferent fibres modulate baseline firing (phasic down-regulation)

Cranial Nerve II – Optic

• Pure special somatic afferent (vision)
• Begins where ganglion-cell axons pierce sclera posterior to optic disc (blind spot)
• Retina
  • Rods – light intensity (scotopic vision)
  • Cones – colour discrimination (photopic vision)
  • Photoreceptor → bipolar → ganglion hierarchy; ganglion axons converge at optic disc
• Course
  • Each nerve runs through optic canal ➜ unite at optic chiasm (middle cranial fossa)
  • Nasal half of each retina crosses ➜ results: left visual field ➜ right tract and vice-versa; foundation of stereopsis/depth perception
  • Optic tracts ➜ lateral geniculate bodies (thalamus) ➜ optic radiations ➜ occipital (striate) cortex

Cranial Nerve III – Oculomotor

• Mixed somatic motor + visceral (parasympathetic) motor
• Nuclei – mid-brain
  • Somatic motor nucleus
  • Axons ➜ superior orbital fissure ➜ split
    • Superior division: superior rectus, levator palpebrae superioris (elevates eyelid)
    • Inferior division: medial rectus, inferior rectus, inferior oblique
  • Edinger–Westphal (accessory) nucleus – pre-ganglionic PS
  • Ride within inferior division ➜ ciliary ganglion
  • Post-ganglionic short ciliary nn. ➜ sphincter pupillae (miosis) & ciliary muscle (accommodation)
• Provides minor proprioceptive feedback to co-ordinate smooth pursuit

Cranial Nerve IV – Trochlear

• Pure somatic efferent to superior oblique muscle
• Unique features
  • ONLY CN to exit dorsally from brain-stem
  • Long intracranial course around mid-brain ➜ superior orbital fissure ➜ superior oblique (tendon through trochlea d"pulley")
• Small proprioceptive component

Cranial Nerve V – Trigeminal

• Largest CN; principal general sensory nerve of face; plus branchial motor to muscles of mastication & several suprahyoid/palatal muscles
• Trigeminal (semilunar) ganglion – analogue of dorsal-root ganglion; houses sensory somata
• Three divisions ("tri-gemini")
  1. Ophthalmic (V1) – sensory only; enters via superior orbital fissure
  • Supraorbital, supratrochlear, infratrochlear – scalp/forehead
  • Nasal brs. of anterior ethmoidal – bridge of nose
  • Anterior & posterior ethmoidal – eye/orbit mucosa
  1. Maxillary (V2) – sensory only; via foramen rotundum
  • Zygomatic & infra-orbital – maxillary skin
  • Dental branches – upper teeth, palate
  1. Mandibular (V3) – mixed; via foramen ovale
  • Sensory:
    • Inferior alveolar ➜ lower teeth; terminal mental nerve ➜ chin skin
    • Lingual ➜ anterior \tfrac{2}{3} of tongue (touch, pain)
    • Auriculotemporal ➜ temporal scalp
  • Motor:
    • Branches from motor nucleus (pons) piggy-back on V3 trunks
    • Muscles: masseter, temporalis, pterygoids, tensor tympani, tensor veli palatini, mylohyoid, anterior digastric belly
• Central sensory processing: mesencephalic (proprioception), principal sensory (touch), spinal tract nucleus (pain/temperature) ➜ ascend to cortex

Cranial Nerve VI – Abducens

• Pure somatic motor to lateral rectus ("cheater muscle" – abducts eye)
• Nucleus in caudal pons; fibres exit at pontomedullary junction ➜ superior orbital fissure

Cranial Nerve VII – Facial

• Four functional components
  1. Branchial motor – muscles of facial expression + stapedius, stylohyoid, posterior digastric
  2. Special sensory – taste anterior \tfrac{2}{3} tongue
  3. General somatic sensory – small ear patch (external acoustic meatus)
  4. Parasympathetic – lacrimal, submandibular, sublingual glands
• Emerges as motor root + nervus intermedius from pontomedullary angle ➜ internal acoustic meatus ➜ traverses facial canal ➜ exits stylomastoid foramen
• Motor branches (within parotid) – "To Zanzibar By Motor Car"
  • Temporal, Zygomatic, Buccal, Mandibular, Cervical (+ posterior auricular/occipital)
• Parasympathetic pathways
  • Greater petrosal ➜ pterygopalatine ganglion ➜ lacrimal & nasal glands
  • Chorda tympani (joins lingual n.) ➜ submandibular ganglion ➜ submandibular & sublingual salivary glands
  • Post-synaptic sympathetic fibres accompany arteries, usually oppose parasympathetic action (decrease secretions)

Cranial Nerve VIII – Vestibulocochlear

• Pure special sensory – hearing & equilibrium
• Two distinct parts share internal acoustic meatus
  • Cochlear nerve
  • Spiral ganglion in cochlea
  • Frequency coding: base (high \,f) ➜ apex (low \,f); amplitude coding via vibration magnitude
  • Vestibular nerve
  • Vestibular (Scarpa) ganglion in facial canal
  • Otolith organs – utricle (horizontal) & saccule (vertical) acceleration; "cherry-cheesecake" analogy (hair cells + otolith gel)
  • Semicircular canal ampullae – rotation in 3 planes; "bowl of cheerios" (endolymph inertia vs cupula)
• Central nuclei: four vestibular + two cochlear nuclei lateral/medial to inferior cerebellar peduncle ➜ cerebellum, thalamus, eye movement pathways

Cranial Nerve IX – Glossopharyngeal

• Mixed; key areas: posterior \tfrac{1}{3} tongue, pharynx, carotid body/sinus, parotid gland
• Nuclei (medulla)
  • Solitary tract nucleus – taste & visceral afferent (posterior tongue, carotid body)
  • Spinal tract nucleus – general somatic afferent (pharynx, middle ear)
  • Nucleus ambiguus – branchial motor ➜ stylopharyngeus (elevates pharynx during swallowing)
  • Inferior salivatory nucleus – pre-gang PS ➜ tympanic n. ➜ lesser petrosal ➜ otic ganglion ➜ parotid gland (salivation)
• Sensory ganglia: superior & inferior glossopharyngeal just outside jugular foramen

Cranial Nerve X – Vagus

• Name = "wanderer"; widest distribution (pharynx ➜ splenic flexure)
• Functional components
  • Branchial motor – pharyngeal constrictors, intrinsic laryngeal mm. (phonation)
  • General somatic sensory – external ear, meninges
  • Special sensory – taste from epiglottis/root of tongue
  • Visceral sensory – thoraco-abdominal organs (state monitoring)
  • Parasympathetic – heart (HR↓), bronchoconstriction, GI motility & secretion up to mid-transverse colon
• Nuclei (medulla)
  • Nucleus ambiguus – motor to pharyngeal ✓ superior laryngeal & recurrent laryngeal nn.
  • Solitary tract & spinal tract nuclei – sensory input; ganglia: superior (jugular) for somatic, inferior (nodose) for visceral
  • Dorsal vagal nucleus – pre-gang PS + visceral afferents
• Key branches
  • Cardiac branches (sup. cervical, inf. cervical, thoracic) – SA-node regulation & feedback
  • Left recurrent laryngeal loops under aortic arch; right loops under subclavian ➜ larynx; supply trachea & upper oesophagus en route
  • Anterior & posterior vagal trunks along oesophagus ➜ abdominal plexuses (foregut & midgut organs)

Cranial Nerve XI – Accessory

• Pure somatic motor (with proprioception) – sternocleidomastoid & trapezius
• Dual origin
  • Spinal nucleus (C1–C5/6 ventral horn) – fibres ascend through foramen magnum
  • Nucleus ambiguus (medulla) – merges briefly
• Exit skull via jugular foramen; spinal root then descends along internal carotid ➜ divides to SCM & through posterior triangle to trapezius
• C2–C4 cervical plexus provide additional motor + sensory feedback to target muscles

Cranial Nerve XII – Hypoglossal

• Pure somatic motor to intrinsic (longitudinal, transverse, vertical) & extrinsic (styloglossus, hyoglossus, genioglossus) tongue muscles
• Nucleus: medulla; fibres exit via multiple rootlets between pyramid & olive ➜ hypoglossal canal ➜ descend then anterior to tongue
• Communicates with C1 fibres (hitch-hiking)
  • Ansa cervicalis (C1–C3) formed; supplies infrahyoid muscles
  • C1 meningeal sensory branch re-enters cranial cavity soon after canal

Tabular Quick Reference (condensed)

• I – Olfactory – S – smell
• II – Optic – S – vision
• III – Oculomotor – M – most eye mm.; pupil & lens PS
• IV – Trochlear – M – superior oblique
• V – Trigeminal – B – facial sensation, mastication mm.
• VI – Abducens – M – lateral rectus
• VII – Facial – B – facial expression, taste ant \tfrac{2}{3}, PS glands
• VIII – Vestibulocochlear – S – hearing & balance
• IX – Glossopharyngeal – B – post \tfrac{1}{3} tongue sensation & taste, stylopharyngeus, PS parotid
• X – Vagus – B – phonation, visceral sensation, wide PS supply
• XI – Accessory – M – SCM & trapezius
• XII – Hypoglossal – M – tongue movements

Integrative & Clinical Connections

• Decussation patterns (optic chiasm) underpin binocular depth perception; lesions produce characteristic field defects (e.g. bitemporal hemianopia).
• Gag reflex spans CN IX (afferent) & X (efferent); testing assesses medullary integrity.
• Corneal blink reflex: afferent V1 ➜ efferent VII; demonstrates trigemino-facial circuitry.
• Vestibulo-ocular reflex (VIII ➜ III, IV, VI) stabilises gaze during head turns; clinical caloric testing exploits endolymph physics.
• Accessory nerve superficial course in posterior triangle makes it vulnerable in lymph-node biopsies – results in shoulder droop.
• Hypoglossal palsy: tongue deviates toward lesion (unopposed contralateral genioglossus).
• Parasympathetic lesions (III, VII, IX, X) manifest as fixed dilated pupil, dry eye/mouth, absent carotid sinus reflex, tachycardia, respectively.