Cranial Nerves (I-XII)

Cheatsheet Content

### Cranial Nerve I: Olfactory Nerve - **Type:** Sensory - **Function:** Sense of smell - **Origin:** Olfactory receptor neurons in the olfactory mucosa of the nasal cavity - **Pathway:** Axons form fascicles that pass through the cribriform plate of the ethmoid bone to synapse in the olfactory bulb. Second-order neurons then project to the primary olfactory cortex. - **Clinical Relevance:** - **Anosmia:** Loss of smell, often due to head trauma (shearing of olfactory filaments), viral infections (e.g., common cold, COVID-19), or neurological disorders. - **Hyposmia:** Reduced sense of smell. - **Phantosmia:** Smelling odors that aren't there. - **Parosmia:** Altered sense of smell. - **Testing:** Presenting familiar, non-irritating odors (e.g., coffee, vanilla) to each nostril while the other is occluded and eyes are closed. Avoid irritants like ammonia. ### Cranial Nerve II: Optic Nerve - **Type:** Sensory - **Function:** Vision - **Origin:** Ganglion cells of the retina - **Pathway:** Axons from retinal ganglion cells converge to form the optic nerve. Each optic nerve exits the orbit via the optic canal, and the two optic nerves meet at the optic chiasm. Here, fibers from the nasal (medial) half of each retina cross to the contralateral side, while temporal (lateral) fibers remain ipsilateral. Beyond the chiasm, the fibers form the optic tracts, which mostly terminate in the lateral geniculate nucleus of the thalamus. From there, optic radiations project to the primary visual cortex in the occipital lobe. - **Clinical Relevance:** - **Anopia:** Blindness in one eye (e.g., optic nerve lesion). - **Hemianopia:** Blindness in half of the visual field (e.g., optic tract or optic radiation lesion). - **Scotoma:** Blind spot in the visual field. - **Papilledema:** Swelling of the optic disc due to increased intracranial pressure. - **Optic Neuritis:** Inflammation of the optic nerve, often associated with multiple sclerosis. - **Testing:** - **Visual Acuity:** Snellen chart. - **Visual Fields:** Confrontation testing or formal perimetry. - **Fundoscopy:** Examination of the optic disc and retina. - **Pupillary Light Reflex:** Both direct and consensual responses (afferent limb of reflex). ### Cranial Nerve III: Oculomotor Nerve - **Type:** Motor (Somatic and Parasympathetic) - **Function:** - **Somatic:** Innervates four of the six extraocular muscles: superior rectus, inferior rectus, medial rectus, inferior oblique. Also innervates the levator palpebrae superioris (lifts eyelid). - **Parasympathetic:** Innervates the constrictor pupillae (pupil constriction) and ciliary muscle (lens accommodation for near vision). - **Origin:** Oculomotor nucleus and Edinger-Westphal nucleus (parasympathetic) in the midbrain. - **Pathway:** Exits the brainstem anteriorly, passes through the superior orbital fissure into the orbit. - **Clinical Relevance:** - **Oculomotor Nerve Palsy:** - **"Down and Out" eye position:** Due to unopposed action of lateral rectus (CN VI) and superior oblique (CN IV). - **Ptosis:** Drooping eyelid (paralysis of levator palpebrae superioris). - **Mydriasis:** Dilated pupil (paralysis of constrictor pupillae, unopposed sympathetic). - **Loss of accommodation.** - **Causes:** Aneurysms (especially posterior communicating artery), diabetes (often spares pupil), tumors, trauma. - **Testing:** - **Extraocular Movements:** H-test to assess eye movements. - **Pupillary Light Reflex:** Direct and consensual (efferent limb of reflex). - **Accommodation Reflex:** Near response (pupil constriction, lens thickening, convergence). - **Eyelid position.** ### Cranial Nerve IV: Trochlear Nerve - **Type:** Motor - **Function:** Innervates the superior oblique muscle, which depresses and intorts the eyeball. - **Origin:** Trochlear nucleus in the midbrain. It is unique as it is the only cranial nerve to emerge from the posterior aspect of the brainstem and decussate (cross) before exiting. - **Pathway:** Exits posteriorly, wraps around the midbrain, passes through the superior orbital fissure into the orbit. - **Clinical Relevance:** - **Trochlear Nerve Palsy:** - **Vertical diplopia:** Double vision, especially when looking down and in. - **Head tilt:** Patients often tilt their head to the opposite shoulder to compensate for the intorsion deficit (Bielschowsky's head tilt test). - **Causes:** Head trauma (due to its long, slender course), congenital defects, microvascular ischemia. - **Testing:** - **Extraocular Movements:** Assess for downward and inward rotation of the eye. - **Bielschowsky's Head Tilt Test:** Ask the patient to tilt their head to each shoulder; the diplopia worsens when tilting towards the affected side. ### Cranial Nerve V: Trigeminal Nerve - **Type:** Mixed (Sensory and Motor) - **Function:** - **Sensory:** Principal sensory nerve of the face, scalp, teeth, and mucous membranes of the oral and nasal cavities. It has three main divisions: - **Ophthalmic (V1):** Forehead, upper eyelid, cornea, conjunctiva, nasal dorsum. - **Maxillary (V2):** Lower eyelid, upper lip, cheek, upper teeth, nasal mucosa, palate. - **Mandibular (V3):** Lower lip, chin, lower teeth, anterior 2/3 of tongue (general sensation, not taste), temporal region, external auditory meatus. - **Motor:** Innervates muscles of mastication (masseter, temporalis, medial and lateral pterygoids), as well as tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of digastric. - **Origin:** Sensory nuclei throughout the brainstem; Motor nucleus in the pons. - **Pathway:** Emerges from the lateral pons, forms the trigeminal ganglion (Gasserian ganglion), then divides into V1, V2, V3. - **Clinical Relevance:** - **Trigeminal Neuralgia:** Severe, sudden, shock-like pain in the distribution of one or more trigeminal divisions, often triggered by light touch, chewing, or talking. - **Trigeminal Nerve Palsy:** - **Sensory loss:** Numbness in the face. - **Motor weakness:** Difficulty chewing, deviation of the jaw towards the affected side when opening the mouth (due to unopposed pterygoid muscle). - **Corneal Reflex:** Afferent limb of the reflex (sensory from cornea). - **Testing:** - **Sensory:** Test light touch, pain, and temperature sensation over the three divisions of the face. - **Motor:** Palpate masseter and temporalis during clenching; ask patient to open mouth and observe for jaw deviation. - **Corneal Reflex:** Gently touch cornea with cotton wisp (should elicit bilateral blink). ### Cranial Nerve VI: Abducens Nerve - **Type:** Motor - **Function:** Innervates the lateral rectus muscle, which abducts (moves outward) the eyeball. - **Origin:** Abducens nucleus in the pons. - **Pathway:** Exits the brainstem between the pons and medulla, passes through the superior orbital fissure into the orbit. It has a long intracranial course, making it vulnerable to injury. - **Clinical Relevance:** - **Abducens Nerve Palsy:** - **Medial Strabismus:** Eye turns inward at rest (esotropia) due to unopposed action of the medial rectus (CN III). - **Diplopia:** Double vision, especially on lateral gaze towards the affected side. - **Inability to abduct the eye past the midline.** - **Causes:** Increased intracranial pressure (false localizing sign), trauma, tumors, microvascular ischemia (diabetes, hypertension). - **Testing:** - **Extraocular Movements:** Ask the patient to look laterally. Observe for inability to move the eye outward. ### Cranial Nerve VII: Facial Nerve - **Type:** Mixed (Motor, Sensory, Parasympathetic) - **Function:** - **Motor:** Innervates muscles of facial expression (e.g., frontalis, orbicularis oculi, orbicularis oris, buccinator), stapedius muscle, posterior belly of digastric, stylohyoid. - **Sensory:** Taste sensation from the anterior 2/3 of the tongue, and general sensation from a small area around the external auditory meatus. - **Parasympathetic:** Innervates lacrimal glands (tear production), submandibular and sublingual salivary glands. - **Origin:** Motor nucleus in the pons; sensory and parasympathetic nuclei in the pons and medulla. - **Pathway:** Emerges from the pontomedullary junction, enters the internal acoustic meatus, travels through the facial canal in the temporal bone, and exits via the stylomastoid foramen to supply facial muscles. - **Clinical Relevance:** - **Bell's Palsy:** Idiopathic unilateral facial paralysis, often viral in origin, affecting all branches of the facial nerve. Patients cannot wrinkle forehead, close eye, or smile on the affected side. - **Upper Motor Neuron Lesion:** (e.g., stroke) affects contralateral lower face, but spares the forehead (due to bilateral cortical innervation of the upper facial muscles). - **Crocodile Tears Syndrome (Gustolacrimal Reflex):** Aberrant regeneration of facial nerve fibers, causing tears instead of salivation when eating. - **Hyperacusis:** Increased sensitivity to sound (paralysis of stapedius muscle). - **Testing:** - **Motor:** Ask patient to wrinkle forehead, close eyes tightly, smile, puff out cheeks, show teeth. Observe for asymmetry. - **Sensory:** Test taste on anterior 2/3 of tongue (sweet, salty, sour, bitter). - **Schirmer Test:** To assess lacrimal gland function. ### Cranial Nerve VIII: Vestibulocochlear Nerve - **Type:** Sensory - **Function:** - **Cochlear Division:** Hearing. - **Vestibular Division:** Balance, equilibrium, and spatial orientation. - **Origin:** Spiral ganglion (cochlear) and vestibular ganglion (vestibular) in the inner ear. - **Pathway:** Emerges from the pontomedullary junction, enters the internal acoustic meatus, and terminates in the cochlear nuclei and vestibular nuclei in the brainstem. - **Clinical Relevance:** - **Hearing Loss:** - **Conductive:** Problem with sound conduction to inner ear (e.g., earwax, otitis media, otosclerosis). - **Sensorineural:** Damage to cochlea, auditory nerve, or central auditory pathways (e.g., presbycusis, noise exposure, Meniere's disease, acoustic neuroma). - **Vertigo:** Sensation of spinning or whirling. - **Nystagmus:** Involuntary rhythmic eye movements. - **Tinnitus:** Ringing or buzzing in the ears. - **Acoustic Neuroma (Vestibular Schwannoma):** Benign tumor on the vestibular part of the nerve, causing progressive unilateral hearing loss, tinnitus, and balance issues. - **Testing:** - **Hearing:** - **Whisper Test:** Simple screening. - **Weber Test:** Tuning fork on top of head. Sound lateralizes to affected ear in conductive loss, to unaffected ear in sensorineural loss. - **Rinne Test:** Tuning fork on mastoid bone then by ear canal. Air conduction > bone conduction (positive Rinne) is normal. Bone conduction > air conduction (negative Rinne) indicates conductive loss. - **Balance:** - **Romberg Test:** Stand with feet together, eyes open then closed. - **Gait assessment.** - **Dix-Hallpike Maneuver:** For benign paroxysmal positional vertigo (BPPV). ### Cranial Nerve IX: Glossopharyngeal Nerve - **Type:** Mixed (Sensory, Motor, Parasympathetic) - **Function:** - **Sensory:** Taste from posterior 1/3 of tongue, general sensation from posterior 1/3 of tongue, tonsils, pharynx, middle ear, Eustachian tube. Carotid sinus (baroreceptors) and carotid body (chemoreceptors) afferents. - **Motor:** Innervates the stylopharyngeus muscle (elevates pharynx during swallowing and speaking). - **Parasympathetic:** Innervates the parotid salivary gland (via otic ganglion). - **Origin:** Nuclei in the medulla (nucleus ambiguus for motor, solitary nucleus for taste, inferior salivatory nucleus for parasympathetic). - **Pathway:** Exits the brainstem from the medulla, passes through the jugular foramen. - **Clinical Relevance:** - **Glossopharyngeal Neuralgia:** Severe, episodic pain in the throat, tonsil, posterior tongue, or ear, often triggered by swallowing, coughing, or talking. - **Dysphagia:** Difficulty swallowing. - **Loss of gag reflex:** Afferent limb of the gag reflex. - **Loss of taste:** On posterior 1/3 of tongue. - **Syncope:** Carotid sinus hypersensitivity can lead to fainting. - **Testing:** - **Gag Reflex:** Touch posterior pharynx (should elicit gag). - **Swallowing:** Ask patient to swallow water. - **Taste:** Test taste on posterior 1/3 of tongue. - **Speech:** Listen for hoarseness or nasal quality. ### Cranial Nerve X: Vagus Nerve - **Type:** Mixed (Motor, Sensory, Parasympathetic) - **Function:** The "wandering" nerve, with widespread distribution. - **Motor:** Innervates most muscles of the pharynx, larynx, and soft palate (except stylopharyngeus, tensor veli palatini). Involved in swallowing and voice production. - **Sensory:** General sensation from pharynx, larynx, part of external ear, and dura mater. Taste from epiglottis. Visceral sensation from thoracic and abdominal organs (e.g., heart, lungs, GI tract). - **Parasympathetic:** Major parasympathetic supply to thoracic and abdominal viscera, regulating heart rate, respiration, and digestion. - **Origin:** Nuclei in the medulla (nucleus ambiguus for motor, solitary nucleus for taste and visceral sensation, dorsal motor nucleus of vagus for parasympathetic). - **Pathway:** Exits the brainstem from the medulla, passes through the jugular foramen, and descends into the neck, thorax, and abdomen. - **Clinical Relevance:** - **Vagal Nerve Palsy:** - **Dysphonia:** Hoarseness (recurrent laryngeal nerve lesion, a branch of vagus). - **Dysphagia:** Difficulty swallowing. - **Palatal deviation:** Uvula deviates to the unaffected side when patient says "Ahhh" (due to weakness of palatal muscles on affected side). - **Loss of gag reflex:** Efferent limb. - **Cardiac arrhythmias, digestive issues.** - **Vasovagal Syncope:** Common faint due to overactivity of the vagus nerve, leading to bradycardia and hypotension. - **Testing:** - **Voice quality:** Listen for hoarseness. - **Palatal movement:** Ask patient to say "Ahhh" and observe uvula. - **Gag Reflex:** Efferent limb. - **Swallowing assessment.** ### Cranial Nerve XI: Accessory Nerve (Spinal Accessory Nerve) - **Type:** Motor - **Function:** Innervates the sternocleidomastoid muscle (turns head to opposite side, flexes neck) and the trapezius muscle (shrugs shoulders, elevates scapula). - **Origin:** - **Cranial Root:** Arises from nucleus ambiguus in the medulla (often considered part of vagus nerve). - **Spinal Root:** Arises from motor neurons in the ventral horn of the upper cervical spinal cord (C1-C5/C6). - **Pathway:** Spinal root ascends through the foramen magnum to join the cranial root briefly, then exits the skull via the jugular foramen. The spinal part then separates and descends to innervate the muscles. - **Clinical Relevance:** - **Accessory Nerve Palsy:** - **Weakness in shoulder shrugging:** Drooping of the shoulder on the affected side. - **Difficulty turning head:** Weakness in rotating head to the opposite side against resistance. - **Muscle atrophy:** Of sternocleidomastoid and trapezius. - **Causes:** Neck dissection surgery, trauma, tumors in the jugular foramen. - **Testing:** - **Shoulder Shrug:** Ask patient to shrug shoulders against resistance. - **Head Rotation:** Ask patient to turn head against resistance to each side. ### Cranial Nerve XII: Hypoglossal Nerve - **Type:** Motor - **Function:** Innervates all intrinsic and most extrinsic muscles of the tongue (genioglossus, hyoglossus, styloglossus). Essential for speech (articulation) and swallowing. - **Origin:** Hypoglossal nucleus in the medulla. - **Pathway:** Exits the brainstem from the medulla, passes through the hypoglossal canal. - **Clinical Relevance:** - **Hypoglossal Nerve Palsy:** - **Tongue Deviation:** When protruded, the tongue deviates towards the side of the lesion (due to unopposed action of the genioglossus muscle on the healthy side pushing the tongue). - **Dysarthria:** Difficulty with articulation of speech. - **Dysphagia:** Difficulty swallowing. - **Atrophy and fasciculations:** On the affected side of the tongue (in LMN lesions). - **Causes:** Tumors, trauma, stroke affecting the medulla, neurodegenerative diseases (e.g., ALS). - **Testing:** - **Tongue Protrusion:** Ask patient to stick out their tongue. Observe for deviation. - **Tongue Strength:** Ask patient to push tongue against cheek while you palpate externally. - **Observe for atrophy or fasciculations.**