Cranial nerves

The table below lists the major characteristics of the 12 cranial nerves:

Nerve _ Functions _Clinical _ Pathway/foramen| I (Olfactory) _ Smell_ _ Cribriform plate| II (Optic) _ Sight_ _ Optic canal| III (Oculomotor) _ Eye movement (MR, IO, SR, IR) Pupil constriction Accomodation Eyelid opening_ Palsy results in

• ptosis
• 'down and out' eye
• dilated, fixed pupil_Superior orbital fissure (SOF)|

IV (Trochlear) _Eye movement (SO) _ Palsy results in defective downward gaze --> vertical diplopia _ SOF | V (Trigeminal) _ Facial sensation Mastication _ Lesions may cause:

• trigeminal neuralgia
• loss of corneal reflex (afferent)
• loss of facial sensation
• paralysis of mastication muscles
• deviation of jaw to weak side_ V1: SOF,

V2: Foramen rotundum, V3: Foramen ovale | VI (Abducens) _Eye movement (LR) _ Palsy results in defective abduction --> horizontal diplopia_ SOF | VII (Facial) _ Facial movement Taste (anterior 2/3rds of tongue) Lacrimation Salivation_ Lesions may result in:

• flaccid paralysis of upper + lower face
• loss of corneal reflex (efferent)
• loss of taste
• hyperacusis_ Internal auditory meatus |

VIII (Vestibulocochlear) _ Hearing, balance _ Hearing loss Vertigo, nystagmus Acoustic neuromas are Schwann cell tumours of the cochlear nerve_ Internal auditory meatus| IX (Glossopharyngeal) _Taste (posterior 1/3rd of tongue) Salivation Swallowing Mediates input from carotid body & sinus_Lesions may result in;

• hypersensitive carotid sinus reflex
• loss of gag reflex (afferent)_ Jugular foramen |

X (Vagus) _ Phonation Swallowing Innervates viscera _ Lesions may result in;

• uvula deviates away from site of lesion
• loss of gag reflex (efferent)_ Jugular foramen|

XI (Accessory) _ Head and shoulder movement _ Lesions may result in;

• weakness turning head to contralateral side_ Jugular foramen |

XII (Hypoglossal) _ Tongue movement_ Tongue deviates towards side of lesion_ Hypoglossal canal |

Some cranial nerves are motor, some sensory and some are both. The most useful mnemonic is given below.

CN I ------------------------------------------------------------------------>XII

Some Say Marry Money But My Brother Says Big Brains Matter Most

S = Sensory, M = Motor, B = Both

swb018.png View from the inferior surface of the brain showing the emergence of the cranial nerves swb019.png Diagram showing the nuclei of the cranial nerves in the brainstem

Cranial nerve reflexes

Reflex _ Afferent limb_ Efferent limb| Corneal _ Ophthalmic nerve (V1) _ Facial nerve (VII)| Jaw jerk _ Mandibular nerve (V3) _ Mandibular nerve (V3)| Gag _ Glossopharyngeal nerve (IX) _ Vagal nerve (X) | Carotid sinus_ Glossopharyngeal nerve (IX) _ Vagal nerve (X) | Pupillary light _ Optic nerve (II) _ Oculomotor nerve (III)| Lacrimation_ Ophthalmic nerve (V1) _ Facial nerve (VII)|