18 - Cranial Nerves IV

Question 1

CN VII

Answer 1

Facial nerve

Question 2

Five functional components of CN VII

Answer 2

• Motor
• Taste
• Parasympathetics
• General sensation
• Visceral sensation

Question 3

SVE components (motor)

Answer 3

SVE – branchiomotor (motor) to the muscles of facial expression, platysma,
posterior belly of the digastric, stylohyoid, stapedius, auricularis
and occipitalis muscles

Question 4

SVA components (taste)

Answer 4

SVA – taste from anterior 2/3 of the tongue

Question 5

GVE components (parasympathetics)

Answer 5

GVE – parasympathetic, secretomotor to the lacrimal, nasal, palatine,
submandibular and sublingual glands

Question 6

GSA components (general sensation)

Answer 6

GSA – general sensation from skin in posterior ear, pinna, external auditory meatus

Question 7

GVA components (visceral sensation)

Answer 7

GVA – visceral sensation from nasal cavity, soft palate, adjacent pharyngeal wall

Question 8

Does the facial nerve carry other CN’s fibers?

Answer 8

No - The facial nerve carries its own motor, parasympathetic and sensory fibers.

Question 9

Two roots of CN VII

Answer 9

• Facial nerve proper

- Nervus intermedius

Question 10

Facial nerve proper (root of CN VII)

Answer 10

Facial nerve proper (motor root, SVE) – consists of the axons of nerve cells whose cell bodies are located in the facial (motor) nucleus

Question 11

Nervus intermedius (root of CN VII)

Answer 11

Nervus intermedius (consists of the central processes of SVA, GSA, and GVA neurons whose cell bodies reside in the geniculate ganglion) and GVE fibers arising from the superior salivatory nucleus

Question 12

3 ganglia of CN VII

Answer 12

1 - Sensory = geniculate ganglion
2 - Parasympathetic = pterygopalatine ganglion
3 - Submandibular ganglion

Question 13

Geniculate ganglion

Answer 13

geniculate ganglion (sensory)

• contains cell bodies of pseudounipolar neurons (GVA, SVA, GSA)
• no synapses

Question 14

Pterygopalatine ganglion

Answer 14

Pterygopalatine ganglion (parasympathetic)
- contains cell bodies of postganglionic parasympathetic (GVE) neurons, whose axons terminate in glands

Question 15

Which glands does the pterygopalatine ganglion terminate in?

Answer 15

• lacrimal gland (tear formation), provides moisture to and cleans eye
• nasal mucous membrane (nasal secretions), moisture to nasal mucosa
• minor salivary glands in palate (salivary secretion)

Question 16

“hay fever ganglion”

Answer 16

The pterygopalatine ganglion is sometimes referred to as the “hay fever ganglion”
(because it causes watery eyes and runny nose with allergies)

Question 17

Submandibular ganglion

Answer 17

• contains cell bodies of postganglionic parasympathetic (GVE) neurons, whose axons terminate in the
  • submandibular gland (salivation)
  • sublingual gland (salivation)

Question 18

Three nuclei of CN VII

Answer 18

• Facial nuclei
• Superior salivatory nucleus
• Solitary nucleus

Question 19

Facial nucleus (motor)

Answer 19

contains cell bodies of motor neurons (LMN’s)

Question 20

Superior salivatory nucleus (parasympathetic)

Answer 20

Contains the cell bodies of preganglionic parasympathetic neurons whose axons synapse in the pterygopalatine or
submandibular ganglia

Question 21

Solitary nucleus (sensory)

Answer 21

The facial nerve shares this nucleus with the glossopharyngeal and vagus nerves

Receives the central processes of SVA (taste) and GVA neurons

Question 22

Two central pathways associated with facial nerve

Answer 22

• Solitary tract

- Spinal tract of trigeminal nerve

Question 23

Solitary tract

Answer 23

Carries central processes of SVA and GVA neurons to the solitary nucleus

The solitary nucleus processes taste sensation. It projects to the hypothalamus which mediates the visceral responses to unpleasant sensation (i.e. vomiting)

Question 24

Spinal tract of the trigeminal nerve

Answer 24

Carries central processes of GSA neurons to the spinal nucleus of the trigeminal nerve

Question 25

Does the facial nerve have a pain nucleus?

Answer 25

The facial nerve does not have a “pain nucleus” of its own, so it sends its fibers relaying nociception from the facial nerve territory to the spinal tract of V that terminates in the pain nucleus of V (spinal nucleus of V).

Question 26

Which nerve DOES have a pain nucleus?

Answer 26

The trigeminal nerve is the only cranial nerve that has a “pain nucleus”.
It not only processes nociception from its own territory (coming from the orofacial region), but ALSO processes nociception from the facial, glossopharyngeal and vagus nerves’ territories. These 3 nerves send their nociceptive fibers to the spinal nucleus of V for further processing by the trigeminal sensory system.

Question 27

Describe the pattern of termination of the corticonuclear (corticobulbar)
fibers (upper motoneurons, UMN’s) in the facial motor nucleus.

Answer 27

Upper half of facial motor nucleus = receives bilateral corticonuclear projections (UMN projections)

Lower half of facial motor nucleus = receives ONLY contralateral corticonuclear projections (UMN projections)

Question 28

Describe the pattern of innervation of the muscles of facial expression by the
lower motoneurons (LMN’s) of the facial motor nucleus.

Answer 28

The LMN’s whose cell bodies reside in the upper half of the facial motor nucleus, innervate the muscles of the upper half of the face

The LMN’s whose cell bodies reside in the lower half of the facial motor nucleus, innervate the muscles of the lower half of the face

Question 29

Corneal blink reflex

Answer 29

1 - Afferent (sensory) limb of reflex arc: ophthalmic division of V

2 - Efferent (motor) limb of reflex arc: facial nerve to orbicularis oculi

3 - Direct corneal reflex – response on side stimulated

4 - Consensual corneal reflex – response on the other side

Question 30

Bell’s Palsy

Answer 30

A disorder that results from a LMN lesion

The entire half of the face is usually paralyzed, ipsilateral to the lesion.

Question 31

If a patient has the following deficits, where is the lesion?

i. dry eye
ii. hyperacusis
iii. diminished taste sensation
iv. weakness/paralysis in one side of face

Answer 31

With these deficits, the lesion is located in the facial canal

Question 32

Why is this the result of a lesion in the facial canal?

Answer 32

• The greater petrosal nerve (GVE) is affected (causing dry eye)
• The nerve to the stapedius (SVE) is affected (causing hyperacusis = acute sense of hearing, sounds are unpleasantly loud)
• The geniculate ganglion was affected (since the patient experienced pain behind the ear, GSA).

• The chorda tympani nerve is affected
(since the patient has loss of taste (SVA) in the anterior 2/3 of the tongue)

Question 33

Explain the role of the stapedius muscle in hearing

Answer 33

The stapedius muscle attaches to the stapes and when it contracts, it normally retracts the stapes from the oval window, preventing it from oscillating on the oval membrane, inhibiting the transmission of excess sound waves to the cochlea. When the stapedius is paralyzed, the stapes oscillates at the slightest sound, nonstop.

Question 34

If the lesion is located distal to the facial canal (after it gives off the above branches), then the patient will have…

Answer 34

Paralysis in only the ipsilateral half of the face.

Question 35

Chronic facial palsy

Answer 35

Note: if the facial palsy is chronic, slow and progressive, this suggests a slow-growing tumor affecting the facial nerve branches.

Question 36

What may cause the patient’s symptoms in Bell’s palsy

Answer 36

Viral infection of the facial nerve’s connective tissue coverings causes inflammation (neuritis), edema, and consequent swelling of the nerve, compressing the enclosed nerve fibers in the narrow boundaries of its
course in the petrous portion of the temporal bone (facial canal) and stylomastoid foramen.

This results in ischemia and compromised conduction of the facial nerve fibers.

Question 37

In Bell’s palsy the degree of facial nerve injury ranges from mild to severe and patients experience one of the following:

Answer 37

Either…
- Pure demyelinating conduction block (neuropraxia)

OR

• Axonal damage with Wallerian degeneration

Question 38

Pure demyelinating conduction block (neuropraxia)

Answer 38

Pure demyelinating conduction block (neuropraxia)

i. includes up to 90% of Bell’s palsy cases
ii. nerve cell axons are not lost
iii. recovery is quick and complete

Question 39

Axonal damage with Wallerian degeneration

Answer 39

Axonal damage with Wallerian degeneration

i. includes about 10% of cases
ii. nerve cell axons are lost
iii. recovery is slow and incomplete

Question 40

What percent of Bell’s palsy patients make a full recovery?

Answer 40

Bell’s palsy is by definition idiopathic.

About 80% of affected individuals make a full recovery.

Question 41

“Crocodile Tears”

Answer 41

Occurs when there is a lesion to the facial nerve proximal to the geniculate ganglion

Question 42

Symptoms of “Crocodile Tears”

Answer 42

During axon regeneration, some of the preganglionic parasympathetic fibers may take the wrong turn, and instead of joining the chorda tympani to head for the submandibular ganglion to synapse there, they join the greater petrosal nerve to terminate in the pterygopalatine ganglion. Thus during eating, instead of salivating, tear formation occurs, referred to as “crocodile tears”.