Cranial Nerves

Classification: Nervous
Figure 1 Origination of the 12 cranial nerves off the brain’s ventral surface (Source: Wikimedia Commons)

There are 12 cranial nerves, enumerated by Roman numerals. They are specialized PNS nerve bundles originating from the brain (or cranium), and are visible on the brain’s ventral surface

Functional classification

  • Sensory: I (Olfactory), II (Optic), VIII (Vestibulocochlear)
  • Motor: III (Oculomotor), IV (Trochlear), VI (Abducens), XI (Accessory), XII (Hypoglossal)
  • Mixed (both sensory/motor functions): V (Trigeminal), VII (Facial), IX (Glossopharyngeal), X (Vagus)
  • Special sensory: I (Smell), II (Vision), VIII (Equilibrium/Hearing), VII/IX/X (Taste)

Embryological classification

  • Branchial (pharyngeal arch) nerves: V, VII, IX, X
  • Somite-derived (eye/tongue muscles): III, IV, VI, XII

Skull foramina classification: anatomical exit sites

  • Specifics in Table 1

Cranial nerve comparison

Special examples

  • Taste: VII, IX, X
  • Blinking: V1 (afferent), VII (efferent)
  • Reflexes
    • Vestibulo-ocular (VOR): VIII (afferent), III, IV, VI (efferent)
    • Acoustic (stapedius muscle): VIII (afferent), VII (efferent)
    • Pupillary light: II (afferent), III (efferent)
    • Corneal: V1 (afferent), VII (efferent)
    • Gag: IX (afferent), X (efferent)
CNOriginForaminaDestinationFunctional classification
I (Olfactory)1Olfactory epitheliumCribriform plate of ethmoidOlfactory bulbsSensory (special sense)
II (Optic)RetinaOptic canals of sphenoidOptic chiasm (converging point) Sensory (special sense)
III (Oculomotor)Midbrain (oculomotor nucleus)Superior orbital fissures of sphenoidExtrinsic eye muscles (4/6), levator palpebrae superioris, pupillary constrictorMotor
IV (Trochlear)Midbrain (trochlear nucleus)Superior orbital fissures of sphenoidSuperior obliquesMotor
V (Trigeminal)Cornea, forehead, upper eyelid (V1), midface, teeth, nasal cavity (V2), lower face/teeth (V3)Superior orbital fissures (V1/ophthalmic branch), foramen rotundum (V2/maxillary), foramen ovale (V3/mandibular)Pons sensory nuclei (V1-3), mastication muscles (V3)Mixed (face)
VI (Abducens)PonsSuperior orbital fissures of sphenoidLateral rectus muscleMotor
VII (Facial) Anterior 2/3 of tongue (sensory), motor nuclei of pons (motor)Internal acoustic meatusNucleus solitarius (sensory), facial expression muscles, salivary/lacrimal glands (motor)Mixed (face)
VIII (Vestibulocochlear)Vestibular apparatus (vestibular branch), cochlear/Organ of Corti (cochlear branch)Internal acoustic meatus of temporal Vestibular/cochlear nuclei of pons and medulla oblongataSensory (special sense)
IX (Glossopharyngeal)Posterior 1/3 tongue, pharynx/palate, carotids(sensory), nucleus ambiguus of medulla oblongata (motor)Jugular foramina between occipital/temporal Nucleus solitarius (sensory), stylopharyngeus muscle, parotid gland (motor)Mixed (head, neck): sensory (superior/jugular and inferior/petrosal ganglia)
X (Vagus)Pharynx, epiglottis taste, external ear apparatus, diaphragm, visceral thoracic-abdominopelvic organs (sensory), motor nuclei in MO (motor)Jugular foramina between occipital/temporal Sensory nuclei, autonomic centers of MO (sensory), larynx/pharynx muscles (motor)Mixed (thorax, abdomen): sensory (superior/jugular and inferior/nodose ganglia)
XI (Accessory)Motor nuclei of SC and MOJugular foramina between occipital/temporal Voluntary muscles of palate/pharynx/larynx (internal branch, joining vagus), sternocleidomastoid/trapezius (external branch)Motor
XII (Hypoglossal)Motor nuclei of MOHypoglossal canals of occipitalExtrinsic/intrinsic tongue musclesMotor
Table 1 1The olfactory nerve (I) is the cranial nerve that bypasses synapses with the thalamus, directly terminating in the cerebrum.

Functional tests (physical exam)

I: smell

II: visual acuity (Snellen’s), pupillary reflex

III: EOMs/H Test (finger in H pattern to test for normal movement of extra-ocular muscles), accommodation reflex, eyelid droop

IV: eyes look down/in

V: test light touch/pain, jaw clench, corneal reflex (afferent; doesn’t feel stimulus, no consensual blinking)

VI: eyes look side/laterally, Parks-Bielschowsky-Helveston head tilt Test

VII: facial symmetry, taste test, corneal reflex (efferent; feels stimulus, no consensual blinking)

VIII: finger rub, Rinne/Weber, Romberg Test (stand feet together eyes closed and open, observe for significant loss of balance)

IX: gag reflex (afferent), swallowing reflex

X: swallowing reflex

XI: turning head, shrug shoulder

XII: sticking out tongue

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