Premed · Premed · Anatomy Physiology 1
Lecture 18: Cranial Nerves
Anatomy and Physiology I
Learning Objectives
By the end of this lecture, students will be able to:
- List the twelve cranial nerves by name, number, and type (sensory, motor, or mixed)
- Identify the origin and exit point of each cranial nerve from the brain and skull
- Describe the major functions and target structures of each cranial nerve
- Explain the clinical tests used to assess each cranial nerve
- Predict the deficits that result from damage to specific cranial nerves
Lecture Content
I. Overview of the Cranial Nerves
The twelve pairs of cranial nerves emerge directly from the brain rather than the spinal cord. They are numbered I through XII from anterior to posterior based on their points of attachment to the brain and fall into three functional types. Sensory (afferent) nerves carry information TO the brain: I, II, and VIII. Motor (efferent) nerves carry commands FROM the brain: III, IV, VI, XI, and XII. Mixed (both sensory and motor) nerves include V, VII, IX, and X. A useful mnemonic for the nerve names is "Oh Oh Oh To Touch And Feel Very Green Vegetables Such Happiness" (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Spinal Accessory, Hypoglossal). A mnemonic for the types is "Some Say Marry Money But My Brother Says Big Brains Matter More" (Sensory, Sensory, Motor, Motor, Both, Motor, Both, Sensory, Both, Both, Motor, Motor).
II. Individual Cranial Nerves
CN I — Olfactory Nerve
The olfactory nerve is sensory, attached to the olfactory bulb on the inferior frontal lobe, and its filaments pass through the cribriform plate of the ethmoid bone. It provides the sense of smell (olfaction). Olfactory receptor neurons in the nasal epithelium send axons through the cribriform plate to synapse in the olfactory bulb; the olfactory tract then carries signals to the primary olfactory cortex (piriform cortex), amygdala, and entorhinal cortex. This pathway is unique in bypassing the thalamus. Clinical testing involves asking the patient to identify familiar odors (coffee, vanilla) with eyes closed, one nostril at a time. Damage produces anosmia (loss of smell), commonly caused by head trauma shearing the olfactory filaments at the cribriform plate, upper respiratory infection, or neurodegenerative disease.
CN II — Optic Nerve
The optic nerve is sensory, attached to the diencephalon at the optic chiasm, and exits through the optic canal. It provides vision. Retinal ganglion cell axons form the optic nerve, partially crossing at the optic chiasm (nasal fibers cross, temporal fibers do not), then continuing as the optic tract to the lateral geniculate nucleus (LGN) of the thalamus and finally to the primary visual cortex (V1) via the optic radiations. Clinical tests include visual acuity (Snellen chart), visual field testing (confrontation test), and fundoscopic examination (ophthalmoscope). Damage produces visual field deficits depending on lesion location: an optic nerve lesion causes ipsilateral blindness, while an optic chiasm lesion (for example, from a pituitary tumor) causes bitemporal hemianopia.
CN III — Oculomotor Nerve
The oculomotor nerve is motor (somatic motor plus parasympathetic), attached to the midbrain at the interpeduncular fossa, and exits through the superior orbital fissure. Its somatic motor component innervates four of the six extrinsic eye muscles — the superior rectus, inferior rectus, medial rectus, and inferior oblique — as well as the levator palpebrae superioris, which elevates the upper eyelid. Its parasympathetic component constricts the pupil (sphincter pupillae) and controls accommodation (ciliary muscle) via the ciliary ganglion. Clinical tests include testing eye movements in an H-pattern, checking the pupillary light reflex, and checking for ptosis. Damage causes oculomotor palsy, which presents with ptosis (drooping eyelid), the eye turning laterally and downward ("down and out" due to the unopposed action of the lateral rectus and superior oblique), a dilated pupil (mydriasis), and loss of accommodation.
CN IV — Trochlear Nerve
The trochlear nerve is motor, attached to the posterior midbrain (it is the only cranial nerve to exit from the posterior brainstem) and is the thinnest cranial nerve. It exits through the superior orbital fissure. It innervates the superior oblique muscle, which depresses and intorts the eye, turning it inferiorly when the eye is adducted (as when looking down at a book while reading). Clinical testing asks the patient to look down and medially. Damage causes difficulty looking down and inward, and the patient may tilt the head to compensate; diplopia (double vision) occurs especially when descending stairs or reading.
CN V — Trigeminal Nerve
The trigeminal nerve is mixed (the major sensory nerve of the face plus motor to the muscles of mastication), attached to the lateral pons. It has three divisions. The V1 (Ophthalmic) division is sensory only, exits through the superior orbital fissure, and provides sensation from the forehead, upper eyelid, cornea, nasal bridge, and superior nasal mucosa. The V2 (Maxillary) division is sensory only, exits through the foramen rotundum, and provides sensation from the cheek, upper lip, upper teeth, nasal cavity, and palate. The V3 (Mandibular) division is mixed, exits through the foramen ovale, and provides sensory innervation to the lower face, chin, lower teeth, the anterior two-thirds of the tongue (general sensation only, NOT taste), and the temporal region; its motor fibers supply the muscles of mastication (temporalis, masseter, medial and lateral pterygoids) plus the mylohyoid, anterior belly of digastric, tensor tympani, and tensor veli palatini.
Clinical testing of the sensory component involves testing light touch and pain across all three divisions and eliciting the corneal (blink) reflex (touching the cornea, with V1 as the afferent limb and VII as the efferent limb). Motor testing involves asking the patient to clench the teeth (palpating the masseter and temporalis) and open the jaw against resistance. Damage can produce trigeminal neuralgia (tic douloureux), characterized by episodes of severe, stabbing facial pain; loss of facial sensation; and jaw deviation toward the affected side upon opening (due to the unopposed lateral pterygoid on the normal side).
CN VI — Abducens Nerve
The abducens nerve is motor, attached at the junction of the pons and medulla (anterior), and exits through the superior orbital fissure. It innervates the lateral rectus muscle, which abducts the eye (turns it laterally). Clinical testing asks the patient to look laterally (abduct the eye). Damage causes medial strabismus (the eye turns medially at rest due to the unopposed medial rectus), inability to abduct the affected eye, and diplopia on lateral gaze.
CN VII — Facial Nerve
The facial nerve is mixed (the major motor nerve of the face plus taste plus parasympathetic), attached at the junction of the pons and medulla (lateral). It enters the internal acoustic meatus, travels through the facial canal, and exits at the stylomastoid foramen. Its motor component innervates the muscles of facial expression (frontalis, orbicularis oculi, orbicularis oris, buccinator, platysma, and others) and the stapedius muscle in the middle ear. Its sensory component provides taste from the anterior two-thirds of the tongue via the chorda tympani. Its parasympathetic component stimulates the submandibular and sublingual salivary glands (via the chorda tympani and submandibular ganglion) and the lacrimal gland (via the greater petrosal nerve and pterygopalatine ganglion).
Clinical testing involves testing facial expressions (raise eyebrows, close eyes tightly, smile, puff cheeks) and testing taste on the anterior tongue. Bell palsy is a peripheral CN VII lesion in which the entire ipsilateral face is paralyzed (upper AND lower face), so the patient cannot close the eye, raise the eyebrow, or smile on the affected side. Hyperacusis may occur due to stapedius paralysis, and taste is lost on the anterior two-thirds of the tongue. In contrast, an upper motor neuron lesion (such as from a stroke) paralyzes only the contralateral LOWER face, sparing the forehead because the upper face receives bilateral cortical input.
<image>An inferior view of the brain showing the attachment points of all twelve cranial nerves. CN I (olfactory) at the olfactory bulb on the inferior frontal lobe, CN II (optic) at the optic chiasm, CN III (oculomotor) from the midbrain interpeduncular fossa, CN IV (trochlear) from the posterior midbrain wrapping around anteriorly, CN V (trigeminal) from the lateral pons, CN VI (abducens) from the pontomedullary junction, CN VII (facial) and CN VIII (vestibulocochlear) from the cerebellopontine angle, CN IX (glossopharyngeal), CN X (vagus), and CN XI (spinal accessory) emerging from the lateral medulla, and CN XII (hypoglossal) from the anterior medulla between the pyramid and olive. Each nerve is color-coded by type: blue for sensory (I, II, VIII), red for motor (III, IV, VI, XI, XII), and purple for mixed (V, VII, IX, X). An adjacent table lists each nerve's skull foramen exit point.</image>
CN VIII — Vestibulocochlear Nerve
The vestibulocochlear nerve is sensory, attached at the junction of the pons and medulla (cerebellopontine angle), and enters the internal acoustic meatus (it does not actually exit the skull, terminating in the inner ear). It has two divisions. The cochlear nerve provides hearing, carrying auditory impulses from the cochlea (organ of Corti) to the cochlear nuclei in the brainstem. The vestibular nerve provides balance and equilibrium, carrying signals from the vestibular apparatus (semicircular canals, utricle, and saccule) to the vestibular nuclei in the brainstem. Clinical tests for hearing include the whisper test and Weber and Rinne tuning fork tests; balance is tested with the Romberg test. Damage produces sensorineural hearing loss, tinnitus (ringing in the ears), vertigo, and nystagmus. An acoustic neuroma (vestibular schwannoma) is a common tumor at the cerebellopontine angle.
CN IX — Glossopharyngeal Nerve
The glossopharyngeal nerve is mixed, attached to the lateral medulla (posterior to the olive), and exits through the jugular foramen. Its sensory component provides taste and general sensation from the posterior one-third of the tongue, sensation from the pharynx and middle ear, and input from the carotid body (chemoreceptors detecting O2, CO2, and pH) and carotid sinus (baroreceptors monitoring blood pressure). Its motor component innervates the stylopharyngeus muscle, which elevates the pharynx during swallowing and speech. Its parasympathetic component stimulates the parotid salivary gland via the otic ganglion. Clinical testing involves the gag reflex (CN IX is the afferent limb, CN X is the efferent limb) and testing taste on the posterior tongue. Damage produces loss of the gag reflex (afferent limb), loss of taste on the posterior tongue, and difficulty swallowing.
CN X — Vagus Nerve
The vagus nerve is mixed, attached to the lateral medulla (posterior to the olive, inferior to CN IX), and exits through the jugular foramen. It is the most widely distributed cranial nerve, aptly named the "wanderer." Its sensory component provides sensation from the pharynx, larynx, thoracic and abdominal viscera (to the splenic flexure of the colon), a small area of the external ear, and taste from the epiglottis. Its motor component innervates the muscles of the pharynx (swallowing) and larynx (vocalization), with the recurrent laryngeal nerve as a major branch. Its parasympathetic component makes it the major parasympathetic nerve of the body: it slows heart rate, stimulates GI peristalsis and secretion, constricts bronchioles, and stimulates pancreatic and gallbladder secretion.
Clinical tests include the gag reflex (efferent limb), asking the patient to say "Ah" and observing uvula deviation, and assessing voice quality for hoarseness. Damage causes uvula deviation away from the lesion side, hoarseness or loss of voice (if the recurrent laryngeal nerve is damaged), dysphagia (difficulty swallowing), tachycardia (loss of parasympathetic cardiac slowing), and GI motility problems.
CN XI — Spinal Accessory Nerve
The spinal accessory nerve is motor, with its attachment at both the medulla and the upper cervical spinal cord (C1-C5). The spinal root ascends through the foramen magnum to join the cranial root before exiting the skull via the jugular foramen. It innervates the sternocleidomastoid (SCM), which turns the head to the contralateral side, and the trapezius, which elevates and retracts the scapula and shrugs the shoulder. Clinical testing asks the patient to shrug the shoulders against resistance (trapezius) and turn the head against resistance (SCM). Damage causes weakness in turning the head to the contralateral side, shoulder droop, and inability to shrug the shoulder on the affected side.
CN XII — Hypoglossal Nerve
The hypoglossal nerve is motor, attached to the anterior medulla (between the pyramid and olive), and exits through the hypoglossal canal. It innervates the intrinsic and extrinsic muscles of the tongue (except the palatoglossus, which is innervated by CN X) and controls tongue movements for speech, chewing, and swallowing. Clinical testing asks the patient to protrude the tongue. Damage causes the tongue to deviate toward the affected side upon protrusion (because the unaffected genioglossus pushes the tongue to the paralyzed side), along with ipsilateral tongue atrophy and fasciculations.
<image>A summary table of all twelve cranial nerves organized in rows. Each row contains: nerve number and name, type (sensory/motor/mixed indicated by color), brainstem attachment point, skull foramen, key functions (with small icons — an eye for vision, a nose for smell, an ear for hearing, a tongue for taste, a muscle for motor), and the clinical test used. Alongside the table, a lateral view of a face shows the sensory distribution of the three divisions of the trigeminal nerve (V1 ophthalmic in blue, V2 maxillary in green, V3 mandibular in yellow) and the motor territory of the facial nerve (CN VII) on the other side of the face, with arrows showing the forehead receiving bilateral cortical input to explain why upper motor neuron lesions spare the forehead.</image>
III. Clinical Correlations and Cranial Nerve Lesion Patterns
Bell Palsy vs. Stroke
Bell palsy (LMN lesion of CN VII) causes paralysis of the entire ipsilateral face, including both upper and lower face, with the forehead affected. A stroke (UMN lesion) affects only the contralateral lower face, with the forehead spared due to bilateral cortical innervation of the upper face. This distinction is a critical diagnostic tool.
Pupil-Involving CN III Palsy
When an aneurysm (such as of the posterior communicating artery) compresses CN III, the parasympathetic fibers on the outside of the nerve are affected first, resulting in a "blown pupil" — a fixed, dilated pupil on the ipsilateral side. This is a neurosurgical emergency if associated with declining consciousness.
Cranial Nerve Examination
Systematic testing of all twelve cranial nerves is a standard component of the neurological exam. Abnormal findings help localize lesions in the brainstem, skull base, or along the peripheral course of the nerves.
