Laboratory 7.3 Cranial Nerves

Cranial nerves — overview

12 paired nerves originating in the brain; part of the PNS; primarily innervate the head and neck; mixed nerves with both sensory (afferent) and motor (efferent) fibers; numbered I–XII from front to back of brain

Where CN I and CN II originate

Cerebrum (all other cranial nerves originate from the brainstem)

Mnemonic for CN I–XII order

"Only One Of The Two Athletes Felt Very Good, Victorious, And Healthy" (O=Olfactory, O=Optic, O=Oculomotor, T=Trochlear, T=Trigeminal, A=Abducens, F=Facial, V=Vestibulocochlear, G=Glossopharyngeal, V=Vagus, A=Accessory, H=Hypoglossal)

Vagus nerve (CN X) — notable feature

Extends beyond the head and neck into the thoracic and abdominal cavities; provides parasympathetic innervation to the heart, lungs, and digestive tract; controls heart rate, GI peristalsis, and other visceral functions

CN I — Olfactory nerve

Function: smell; Test: identify different odors with each nostril separately; Disorder: anosmia (loss of smell)

CN II — Optic nerve

Function: vision; Test: Snellen chart for visual acuity; confrontation method for visual fields; Disorder: loss of vision or partial vision loss

CN III — Oculomotor nerve

Function: eye movement, pupil constriction, eyelid opening; Test: pupil light reflex (shine light in each eye); follow moving object to test eye movements; Disorder: weakness of eye muscles, nystagmus

CN IV — Trochlear nerve

Function: eye movement (superior oblique muscle); Test: ask patient to follow object downward and inward; Disorder: see CN III disorders

CN V — Trigeminal nerve

Function: facial sensation and mastication; Test: light touch, pain, and temperature on face (sensory); ask patient to clench teeth and assess jaw strength (motor); Disorder: loss of facial sensation

CN VI — Abducens nerve

Function: eye movement (lateral rectus muscle — abduction); Test: ask patient to look laterally; Disorder: inability to abduct the eye; see CN III/IV disorders

CN VII — Facial nerve

Function: facial expression, taste (anterior 2/3 tongue), salivation, tear secretion; Test: ask patient to frown, raise eyebrows, show teeth (observe for asymmetry); apply sweet/salty substances to anterior 2/3 tongue; Disorder: Bell's palsy, facial nerve paralysis

CN VIII — Vestibulocochlear nerve

Function: hearing and balance; Test: standard hearing tests (whisper, tuning fork); Romberg test or gait assessment for balance; Disorder: Meniere's disease, hearing loss

CN IX — Glossopharyngeal nerve

Function: taste (posterior 1/3 tongue), salivation, swallowing, carotid body monitoring; Test: gag reflex (touch back of throat), taste on posterior 1/3 tongue, observe swallowing; Disorder: glossopharyngeal neuralgia

CN X — Vagus nerve

Function: taste, swallowing, speech, organ regulation; Test: gag reflex, assess voice quality, observe swallowing for difficulty or asymmetry; Disorder: dysphagia, vocal cord paralysis

CN XI — Accessory nerve

Function: shoulder and neck muscle movement; Test: shrug shoulders against resistance; turn head against resistance; Disorder: accessory nerve palsy

CN XII — Hypoglossal nerve

Function: tongue movement; Test: ask patient to stick out tongue and move it side to side; observe for deviation or atrophy; Disorder: hypoglossal nerve paralysis

Trigeminal nerve (CN V) — 3 branches

Ophthalmic, Maxillary, and Mandibular branches

CN V ophthalmic branch — innervation

Eye structures (cornea, ciliary body, lacrimal gland, conjunctiva, eyelid), forehead, and part of the nasal mucosa

CN V maxillary branch — innervation

Lower eyelid, cheek, nostril, upper lip, and upper teeth

CN V mandibular branch — innervation

Lower teeth, lower lip, chin, part of the external ear, and the muscles of mastication

Facial nerve (CN VII) — 5 branches

Temporal, Zygomatic, Buccal, Marginal mandibular, and Cervical branches

CN VII temporal branch — innervation

Frontalis muscle, orbicularis oculi, and part of the orbicularis oris

CN VII zygomatic branch — innervation

Muscles in the zygomatic area, mainly the orbicularis oculi

CN VII buccal branch — innervation

Buccinator muscle and parts of the orbicularis oris

CN VII marginal mandibular branch — innervation

Muscles around the lower lip and chin (e.g., depressor labii inferioris)

CN VII cervical branch — innervation

Platysma muscle

Vestibulocochlear nerve (CN VIII) — 2 components

Cochlear nerve (hearing — innervates cochlea) and Vestibular nerve (balance — innervates semicircular canals, utricle, and saccule)

Bell's palsy

Disorder of CN VII (facial nerve); results in unilateral facial paralysis/weakness; revealed clinically by asymmetry when asking patient to frown, raise eyebrows, or show teeth

Clinical importance of cranial nerve assessment

Cranial nerve testing helps assess CNS integrity; disorders can indicate localized infections/injuries or systemic diseases such as diabetes or multiple sclerosis