18 - Cranial Nerves IV Flashcards
CN VII
Facial nerve
Five functional components of CN VII
- Motor
- Taste
- Parasympathetics
- General sensation
- Visceral sensation
SVE components (motor)
SVE – branchiomotor (motor) to the muscles of facial expression, platysma,
posterior belly of the digastric, stylohyoid, stapedius, auricularis
and occipitalis muscles
SVA components (taste)
SVA – taste from anterior 2/3 of the tongue
GVE components (parasympathetics)
GVE – parasympathetic, secretomotor to the lacrimal, nasal, palatine,
submandibular and sublingual glands
GSA components (general sensation)
GSA – general sensation from skin in posterior ear, pinna, external auditory meatus
GVA components (visceral sensation)
GVA – visceral sensation from nasal cavity, soft palate, adjacent pharyngeal wall
Does the facial nerve carry other CN’s fibers?
No - The facial nerve carries its own motor, parasympathetic and sensory fibers.
Two roots of CN VII
- Facial nerve proper
- Nervus intermedius
Facial nerve proper (root of CN VII)
Facial nerve proper (motor root, SVE) – consists of the axons of nerve cells whose cell bodies are located in the facial (motor) nucleus
Nervus intermedius (root of CN VII)
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
3 ganglia of CN VII
1 - Sensory = geniculate ganglion
2 - Parasympathetic = pterygopalatine ganglion
3 - Submandibular ganglion
Geniculate ganglion
geniculate ganglion (sensory)
- contains cell bodies of pseudounipolar neurons (GVA, SVA, GSA)
- no synapses
Pterygopalatine ganglion
Pterygopalatine ganglion (parasympathetic) - contains cell bodies of postganglionic parasympathetic (GVE) neurons, whose axons terminate in glands
Which glands does the pterygopalatine ganglion terminate in?
- 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)
“hay fever ganglion”
The pterygopalatine ganglion is sometimes referred to as the “hay fever ganglion”
(because it causes watery eyes and runny nose with allergies)
Submandibular ganglion
- contains cell bodies of postganglionic parasympathetic (GVE) neurons, whose axons terminate in the
- submandibular gland (salivation)
- sublingual gland (salivation)
Three nuclei of CN VII
- Facial nuclei
- Superior salivatory nucleus
- Solitary nucleus
Facial nucleus (motor)
contains cell bodies of motor neurons (LMN’s)
Superior salivatory nucleus (parasympathetic)
Contains the cell bodies of preganglionic parasympathetic neurons whose axons synapse in the pterygopalatine or
submandibular ganglia
Solitary nucleus (sensory)
The facial nerve shares this nucleus with the glossopharyngeal and vagus nerves
Receives the central processes of SVA (taste) and GVA neurons
Two central pathways associated with facial nerve
- Solitary tract
- Spinal tract of trigeminal nerve
Solitary tract
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)
Spinal tract of the trigeminal nerve
Carries central processes of GSA neurons to the spinal nucleus of the trigeminal nerve
Does the facial nerve have a pain nucleus?
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).
Which nerve DOES have a pain nucleus?
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.
Describe the pattern of termination of the corticonuclear (corticobulbar)
fibers (upper motoneurons, UMN’s) in the facial motor nucleus.
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)
Describe the pattern of innervation of the muscles of facial expression by the lower motoneurons (LMN’s) of the facial motor nucleus.
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
Corneal blink reflex
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
Bell’s Palsy
A disorder that results from a LMN lesion
The entire half of the face is usually paralyzed, ipsilateral to the lesion.
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
With these deficits, the lesion is located in the facial canal
Why is this the result of a lesion in the facial canal?
- 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)
Explain the role of the stapedius muscle in hearing
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.
If the lesion is located distal to the facial canal (after it gives off the above branches), then the patient will have…
Paralysis in only the ipsilateral half of the face.
Chronic facial palsy
Note: if the facial palsy is chronic, slow and progressive, this suggests a slow-growing tumor affecting the facial nerve branches.
What may cause the patient’s symptoms in Bell’s palsy
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.
In Bell’s palsy the degree of facial nerve injury ranges from mild to severe and patients experience one of the following:
Either…
- Pure demyelinating conduction block (neuropraxia)
OR
- Axonal damage with Wallerian degeneration
Pure demyelinating conduction block (neuropraxia)
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
Axonal damage with Wallerian degeneration
Axonal damage with Wallerian degeneration
i. includes about 10% of cases
ii. nerve cell axons are lost
iii. recovery is slow and incomplete
What percent of Bell’s palsy patients make a full recovery?
Bell’s palsy is by definition idiopathic.
About 80% of affected individuals make a full recovery.
“Crocodile Tears”
Occurs when there is a lesion to the facial nerve proximal to the geniculate ganglion
Symptoms of “Crocodile Tears”
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”.
