Cranial Nerves

Question 1

Name the twelve cranial nerves.

Answer 1

• On Old Olympus’ Towering Top, A Fin And* German Viewed A Hop
• olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), auditory* (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), hypoglossal (XII)
• *auditory = vestibulocochlear

Question 2

Where is the nucleus of each cranial nerve found?

Answer 2

• cranial nerves I and II are in the forebrain
• midbrain: nuclei of cranial nerves III and IV
• pons: nuclei of cranial nerves V, VI, VII, and VIII
• medulla: nuclei of cranial nerves IX, X, XI (central portion), and XII
• spinal cord: nuclei of cranial nerve XI (spinal portion)

Question 3

What is unique about cranial nerve I?

Answer 3

• (the olfactory nerve)
• it is the only cranial nerve that does not communicate with the thalamus (so there is no thalamic relay to the cortex with this nerve)

Question 4

What are the functions of cranial nerves I, II, III, and IV?

Answer 4

• I (olfactory): smell (SENSORY)
• II (optic): sight (SENSORY)
• III (oculomotor): eye movement (MR, IR, SR, IO), pupillary constriction, accommodation, eyelid opening (MOTOR)
• IV (trochlear): eye movement (SO) (MOTOR)

Question 5

What are the functions of cranial nerves V, VI, VII, and VIII?

Answer 5

• V (trigeminal): mastication, facial sensation, somatosensation from anterior 2/3 of tongue (BOTH)
• VI (abducens): eye movement (LR) (MOTOR)
• VII (facial): facial movement, taste from anterior 2/3 of tongue, lacrimation, salivation (submandibular and sublingual), eyelid closing, stapedius muscle of ear (BOTH)
• VIII (vestibulocochlear): hearing, balance (SENSORY)

Question 6

What are the functions of cranial nerves IX, X, XI, and XII?

Answer 6

• IX (glossopharyngeal): taste and somatosensation from posterior 1/3 of tongue, swallowing, salivation (parotid), monitoring carotid body and sinus chemoreceptors and baroreceptors, stylopharyngeus muscle (BOTH)
• X (vagus): taste from epiglottic region, swallowing, uvula, soft palate, coughing, thoracoabdominal viscera, monitoring aortic arch chemoreceptors and baroreceptors (BOTH)
• XI (accessory): head turning, shoulder shrugging (MOTOR)
• XII (hypoglossal): tongue movement (MOTOR)

Question 7

Which cranial nerves are motor nerves? Which are sensory nerves? Which are both?

Answer 7

• Some Say Marry Money, But My Brother Says Big Brains Matter Most
• sensory: cranial nerves I, II, VIII
• motor: cranial nerves III, IV, VI, XI, XII
• both: cranial nerves V, VII, IX, X

Question 8

Which cranial nerves supply autonomic innervation? Which are parasympathetic and which are sympathetic?

Answer 8

• cranial nerves III, VII, IX, and X
• they are all parasympathetic (sympathetic innervation of head and neck comes from ascending fibers originating from T1/T2)

Question 9

Where do taste afferent fibers terminate? What else terminates here?

Answer 9

• taste afferents terminate in the medulla, at the nucleus solitarius
• visceral afferents also terminate here

Question 10

Cranial Nerve III

Answer 10

• oculomotor nerve; motor and parasympathetic
• contains the majority of somatic fibers for the extraocular muscles (SR, IR, MR, and IO)
• contains the preganglionic parasympathetic fibers innervating the smooth muscle of the eye (the sphincter pupillae muscle of the iris)
• elevates, depresses, and adducts the eyeball; manages the pupillary light and accommodation reflexes
• passes through the superior orbital fissure

Question 11

Cranial Nerve IV

Answer 11

• trochlear nerve; strictly motor
• contains the somatic fibers for the SO extraocular muscle
• depresses, abducts, and intorts the eyeball
• the only CN that emerges from the dorsal aspect of the brainstem
• passes through the superior orbital fissure

Question 12

Cranial Nerve VI

Answer 12

• abducens nerve; strictly motor
• contains the somatic fibers for the LR extraocular muscle
• abducts the eye
• passes through the superior orbital fissure

Question 13

What happens with a CN III lesion? a CN IV lesion? CN VI?

Answer 13

• CN III (oculomotor): ptosis, dilation of pupil, loss of light and accommodation reflexes, eye looks down and out
• CN IV (trochlear): eye moves upward, diplopia
• CN VI (abducens): diplopia, inability to abduct eye (can’t look out)

Question 14

Cranial Nerve V

Answer 14

• trigeminal nerve; motor and sensory
• contains sensory afferents from its ophthalmic, maxillary, and mandibular branches
• contains somatic fibers involved in mastication and tensor tympani
• main sensory nerve for the head, involved in corneal reflex (sensory part), and jaw jerk reflex

Question 15

Cranial Nerve VII

Answer 15

• facial nerve; motor, sensory, and parasympathetic
• lateral root contains sensory fibers to anterior 2/3 of tongue + outer ear and parasympathetic fibers to submandibular and sublingual salivary glands
• medial root: contains somatic fibers involved in facial expression (corneal reflex - motor part) and to the stapedius
• passes through the internal acoustic meatus

Question 16

Cranial Nerve VIII

Answer 16

• vestibulocochlear nerve; strictly sensory
• 2 parts: vestibular nerve (balance) and cochlear nerve (auditory stimuli)
• pass through the internal acoustic meatus

Question 17

Cranial Nerve IX

Answer 17

• glossopharyngeal nerve; motor, sensory, and parasympathetic
• contains sensory afferents from pharynx, posterior 1/3 of tongue, middle ear, and the chemoreceptors located in the carotid body and baroreceptors in the carotid sinus
• contains motor fibers to the stylopharyngeus muscle (for swallowing)
• contains parasympathetic fibers to parotid salivary glands

Question 18

Cranial Nerve X

Answer 18

• vagus nerve; motor, sensory, and parasympathetic
• contains sensory afferents from pharynx, larynx, esophagus, tympanic membrane, chemoreceptors in aortic bodies and baroreceptors in aortic arch, and thoracic and abdominal viscera
• contains motor fibers to soft palate, pharynx, larynx, and upper esophagus (these arise from nucleus ambiguus)
• contains parasympathetic fibers to viscera

Question 19

Cranial nerve XI

Answer 19

• accessory nerve; strictly motor
• cranial component carries motor fibers to soft palate, pharynx, and larynx (these arise from nucleus ambiguus)
• spinal component carries motor fibers to sternomastoid and trapezius (these arise from neurons in the anterior horns of cervical vertebrae)

Question 20

Cranial Nerve XII

Answer 20

• hypoglossal nerve; strictly motor

- contains motor fibers to the tongue

Question 21

What is the nucleus solitarius? The nucleus ambiguus?

Answer 21

• these are vagal nuclei
• nucleus solitarius deals with all visceral sensory information (taste, baroreceptors, gut distension, etc.); CNs VII, IX, and X project here
• nucleus ambiguus deals with the motor innervation of the pharynx, larynx, and upper esophagus (swallowing); CNs IX, X, and XI project here

Question 22

What is cavernous sinus syndrome? Which cranial nerves can be affected?

Answer 22

• a S.O.L. of the cavernous sinus (mass effect, fistula, thrombosis, etc.) that results in ophthalmoplegia (weak eye muscles) and decreased corneal and maxillary sensation (visual acuity is unaffected)
• this is due to the structures that pass through this sinus being compressed: CNs III, IV, V (V1 and V2), and VI

Question 23

What are common cranial nerve lesions? What are the clinical findings of each?

Answer 23

• CN V (trigeminal): jaw deviates TOWARDS the lesion because of unopposed force from the opposite pterygoid muscle
• CN X (vagus): uvula deviates AWAY from lesion
• CN XI (accessory): weakness when turning head to opposite side (this involves the ipsilateral sternocleidomastoid) and shoulder droop of the affected side
• CN XII (hypoglossal): tongue deviates TOWARDS lesion due to weakened tongue muscles on the affected side (the tongue is pushed, not pulled, out)

Question 24

Explain the major clinical difference between a pseudobulbar palsy and a bulbar palsy.

Answer 24

• pseudobulbar palsy (lesion of the corticobulbar tract; UMN lesion): results in contralateral facial paralysis, with SPARING of the forehead
• bulbar palsy (lesion of the facial nucleus or of CN VII; LMN lesion): results in contralateral facial paralysis without sparing of the forehead
• this occurs because the forehead actually has bilateral corticobulbar (UMN) innervation!