CN Exam 2

How do cranial nerves emerge from the cranial cavity?

Via foramina or fissures in the cranium

What covers cranial nerves as they exit the cranium?

Tubular sheaths derived from the cranial meninges

How many pairs of cranial nerves are there and how are they numbered?

12 pairs, numbered I-XII from rostral to caudal

What do cranial nerve names generally reflect?

Their general distribution or function

What are the three general functional types of cranial nerves?

Sensory, motor, or both (mixed)

What do cranial nerves innervate?

Muscles and/or glands

What are the two directions of cranial nerve fiber conduction called?

Efferent (motor, away from CNS) and afferent (sensory, toward CNS)

How many main functional components can cranial nerve fibers be grouped into?

Five main functional components

Define somatic motor function of cranial nerves.

Motor fibers to voluntary (skeletal) muscles

Define visceral motor function of cranial nerves.

Motor fibers to involuntary muscles or glands

Define somatic sensory function of cranial nerves.

Sensory fibers transmitting general sensation

Define visceral sensory function of cranial nerves.

Sensory fibers conveying sensation from the viscera

Define special sensory function of cranial nerves.

Sensory fibers transmitting unique sensations (vision, hearing, taste, smell, balance)

Which cranial nerve passes through the cribriform foramina?

CN I (olfactory nerve)

Which cranial nerve passes through the optic canal?

CN II (optic nerve)

Which cranial nerves pass through the superior orbital fissure?

CN III (oculomotor), CN IV (trochlear), CN V1 (ophthalmic), CN VI (abducens)

Which cranial nerve passes through the foramen rotundum?

CN V2 (maxillary nerve)

Which cranial nerve passes through the foramen ovale?

CN V3 (mandibular nerve)

Which cranial nerves pass through the internal acoustic meatus?

CN VII (facial) and CN VIII (vestibulocochlear)

Which cranial nerves pass through the jugular foramen?

CN IX (glossopharyngeal), CN X (vagus), CN XI (spinal accessory)

Which cranial nerve passes through the hypoglossal canal?

CN XII (hypoglossal nerve)

What is the mnemonic for the names of cranial nerves I-XII?

On, Ocassion, Our, Trusty, Truck, Acts, Funny, Very, Good, Vehicle, Any, How

What is the full sentence version of the cranial nerve names mnemonic?

"Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven"

What is the mnemonic for cranial nerve function (Sensory/Motor/Both)?

Some, Say, Marry, Money, But, My, Brother, Says, Big, Brains, Matter, More

In the function mnemonic, what does each word represent for CN I-XII?

Some(I)=Sensory, Say(II)=Sensory, Marry(III)=Motor, Money(IV)=Motor, But(V)=Both, My(VI)=Motor, Brother(VII)=Both, Says(VIII)=Sensory, Big(IX)=Both, Brains(X)=Both, Matter(XI)=Motor, More(XII)=Motor

Where does the spinal accessory nerve (CN XI) originate from?

It originates from the upper spinal cord

What is the function of the olfactory nerve (CN I)?

Special sensory - smell

Injury to what structures can cause anosmia?

Nasal mucosa, olfactory nerve fibers, olfactory bulbs, or olfactory tracts, and/or aging

What is anosmia?

Loss of the sense of smell

What can temporal lobe lesions along the CN I pathway cause?

Olfactory hallucinations

What causes olfactory hallucinations?

A lesion that irritates the lateral olfactory area, causing temporal lobe epilepsy or uncinate fits

What are olfactory hallucinations characterized by?

Imaginary disagreeable odors

What movements accompany olfactory hallucinations (due to temporal lobe injury)?

Involuntary movements of the lips and tongue

What is the chief complaint of most people with anosmia?

Loss or alteration of taste

What do clinical studies reveal about the taste complaints in anosmia?

Dysfunction is actually in the olfactory system; most people confuse taste with flavor

What causes transitory olfactory impairment?

Viral or allergic rhinitis (e.g., COVID-19), causing inflammation of the nasal mucous membrane

Why does loss of olfactory fibers occur with aging?

Progressive reduction in the number of olfactory receptor neurons in the olfactory epithelium, reducing acuity of smell

What is the function of the optic nerve (CN II)?

Vision (special sensory)

Where does the optic nerve (CN II) originate from anatomically?

The optic disk

Why are optic nerves susceptible to demyelinating diseases like MS?

Because they are actually CNS tracts (an extension of the brain), not true peripheral nerves

What is optic neuritis?

A lesion of the optic nerve causing diminution of visual acuity; patient can't see clearly and glasses don't help

What are the causes of optic neuritis?

Inflammatory, degenerative, demyelinating, or toxic disorders

What toxic substances can cause optic neuritis?

Methyl and ethyl (non-edible) alcohol, tobacco, lead, mercury

In poor countries, why might toxic methanol-related optic neuritis occur?

People who can't afford drinkable alcohol may be sold inedible/methanol alcohol instead

What does the optic disc look like on ophthalmoscopic exam in optic neuritis?

Pale and smaller

What would a large, swollen optic disc suggest instead?

An increase in intracranial pressure

Does multiple sclerosis (a CNS demyelinating disease) usually affect the PNS?

No, MS usually does not affect other nerves of the peripheral nervous system

What visual field defect results from complete section of the optic nerve?

Blindness in the temporal and nasal visual fields (monocular blindness) of the ipsilateral eye

What visual field defect results from complete section of the optic chiasm?

Reduced peripheral vision resulting in bitemporal hemianopsia (can't see to the sides)

What is the most common cause of bitemporal hemianopsia (optic chiasm lesion)?

A pituitary tumor

Explain the mechanism by which a pituitary tumor causes optic chiasm compression.

Enlarged pituitary gland compresses the sella turcica, which compresses the optic chiasm/nerve

What can bilateral (twin) aneurysms of the internal carotid arteries cause?

Binasal hemianopsia

At the optic chiasm, which retinal fibers cross to the opposite side?

The nasal retinal fibers cross over (temporal fibers stay ipsilateral)

Which visual field does the nasal part of the retina detect?

The temporal (lateral) visual field

Which visual field does the temporal part of the retina detect?

The nasal (central) visual field

What visual field defect results from complete section of the right optic tract?

Eliminates vision from the left temporal and right nasal visual fields

What is the result of a lesion of the optic tract?

Contralateral homonymous hemianopsia

In what patient population is homonymous hemianopsia commonly seen?

Stroke patients

What happens to the visual image as it travels through the visual pathway to the cortex?

It is inverted; the occipital cortex reinverts the images

What are the three cranial nerves responsible for extraocular eye movement?

CN III (oculomotor), CN IV (trochlear), CN VI (abducens)

What does CN III (oculomotor nerve) innervate?

Most extraocular muscles (superior, inferior, medial rectus, inferior oblique), levator palpebrae superioris, and pupillary constriction/accommodation (parasympathetic)

Besides motor fibers, what else does CN III carry?

Sensory (autonomic/parasympathetic) fibers

What can cause a lesion or compression of the oculomotor nerve (CN III)?

Aneurysm of the posterior cerebral or superior cerebellar arteries

What are the first symptoms of oculomotor (CN III) palsy from compression?

Dilation of the pupil and slowed pupillary constriction in response to light

Why are the autonomic/pupillary fibers of CN III affected first during compression?

Because the autonomic fibers are superficial within the nerve

What is the first sign of CN III compression?

Ipsilateral slowness of the pupillary response to light, followed by progressive dilation on the injured side

What condition often compresses CN III against the crest of the petrous temporal bone?

An extradural hematoma

What does CN IV (trochlear nerve) innervate?

The superior oblique muscle

Is the trochlear nerve commonly injured alone?

No, it is rarely injured alone

What is the characteristic sign of trochlear nerve (CN IV) injury?

Diplopia (double vision) from paralysis of the superior oblique

When does diplopia from CN IV injury typically occur?

When looking down

What eye movement does the superior oblique assist with?

It assists the inferior rectus in depressing the pupil (eye)

What is the primary rotational action of the superior oblique on the eyeball?

Intorsion (inward rotation)

What happens when the superior oblique is paralyzed?

The inferior oblique produces unopposed extorsion (outward rotation)

What eye movement is impaired with trochlear nerve (CN IV) lesions?

Inability to turn the affected eyeball inferomedially (down and in); eyes point up and in

Why is the trochlear nerve especially vulnerable to severe head injuries?

Due to its long intracranial course

What does CN VI (abducens nerve) innervate?

The lateral rectus muscle

What does compression of the abducens nerve (CN VI) cause?

Paralysis of the lateral rectus, causing medial deviation (strabismus) of the affected eye

Why does the eye deviate medially with abducens nerve (CN VI) palsy?

Because the action of the medial rectus is unopposed

What is a patient unable to do with complete CN VI paralysis?

Abduct the eye laterally to the primary position

Can CN VI (abducens) paralysis produce diplopia?

Yes

What can cause paralysis of CN VI (abducens nerve)?

Aneurysm of the cerebral arterial circle (Circle of Willis), pressure from atherosclerotic internal carotid artery in the cavernous sinus, septic thrombosis of the cavernous sinus (from nasal/paranasal sinus infection), or a brain tumor

What are the three branches of the trigeminal nerve (CN V)?

V1 (ophthalmic), V2 (maxillary), V3 (mandibular)

Where do all three branches of the trigeminal nerve (CN V) originate?

The trigeminal ganglion

What area does CN V1 (ophthalmic branch) supply somatic sensory innervation to?

The forehead, scalp, and upper face/nose region

What area does CN V2 (maxillary branch) supply somatic sensory innervation to?

The mid-face (cheek, upper lip, nasal cavity region)

What type(s) of fibers does CN V3 (mandibular branch) contain?

Both somatic sensory (lower face/jaw) and somatic motor fibers

What does the somatic motor portion of CN V3 innervate?

Muscles of mastication (temporalis, masseter)

What does injury to CN V (trigeminal nerve) cause?

Paralysis of muscles of mastication, loss of facial sensation, and loss of corneal and sneezing reflexes

Which branches of the trigeminal nerve are commonly anesthetized during dental procedures?

Branches of the maxillary nerve (V2) and the inferior alveolar nerve (branch of V3/mandibular)

Which branch of the trigeminal nerve is most commonly affected in injury?

The second (maxillary, V2) branch

An isolated lesion of the spinal trigeminal tract is associated with which disease?

Multiple sclerosis (MS)

What happens to the mandible with CN V injury?

It deviates toward the side of the lesion

What sensations are lost in the face with CN V injury?

Soft tactile, thermal, or painful sensations (e.g., cannot feel soft cotton)

Which branch of CN V controls the corneal reflex?

The ophthalmic (V1) branch

What is the corneal reflex?

Blinking in response to the cornea being touched

What reflex is lost along with the corneal reflex in CN V injury, and why?

The sneezing reflex (stimulated by irritants that clear the respiratory tract, also involves coughing), due to damage of the maxillary branch

Which part of the eye is the most pain-sensitive?

The cornea

What is the overall function of CN VII (facial nerve)?

Innervates muscles of facial expression (motor), taste to anterior 2/3 of tongue (special sensory), and secretomotor to lacrimal, submandibular, and sublingual glands (visceral motor)

Which cranial nerve innervates the tympanic membrane (nerve to stapedius)?

CN VII (facial nerve)