Cn Viii Anatomy, Physiology, And Pathology

Front 1

neuron

Back 1

nerve cell

Front 2

receptor cell

Back 2

receives stimulation from sensory cell
stimulates another neuron

Front 3

interneuron

Back 3

relay bipolar
communicates between 2 neurons
receives stimulation from one and excites the next

Front 4

what do interneurons do in the pns

Back 4

provide connection between sensory and motor neurons
and interneuron to interneuron

Front 5

motor neuron

Back 5

receives stimulation from neuron
stimulates a muscle
efferent stimulation

Front 6

what are the 3 parts of a neuron?

Back 6

cell body
dendrites
axon

Front 7

cell body

Back 7

soma
contains nucleus
surrounded by cell membrane
has many dendrites

Front 8

dendrites

Back 8

branched projections of a neuron
conduct electrical stimulation received from other cells to cell body
brings information to cell

Front 9

axon

Back 9

nerve fiber
long slender projection
conducts electrical impulses away from cell body
ends in presynaptic/axon terminals
transmit info from cell

Front 10

axons 2 coverings

Back 10

neurilemma
myelin sheath

Front 11

neurilemma

Back 11

first outer covering of axon
aids in regeneration of axon after damage

Front 12

myelin sheath

Back 12

fatty substance
insulates axon
protection

Front 13

nodes of ranvier

Back 13

spaces on axon without myelin sheath
speed transmission of neural impulses

Front 14

ganglion/ganglia

Back 14

collection of cell bodies in periphery

Front 15

where are spiral ganglion located?

Back 15

modiolus

Front 16

modiolus

Back 16

central
cone shaped
perforated
bony core
encompasses nerve fibers from hair cells and blood vessels

Front 17

nucleus

Back 17

collection of cell bodies in cns

Front 18

nerve

Back 18

a collection of axons in the pns

Front 19

where is the cn viii located?

Back 19

internal auditory meatus

Front 20

tract

Back 20

collection of axons in the cns

Front 21

synapse

Back 21

chemical event- the release of a neurotransmitter

Front 22

what triggers the release of a neurotransmitter

Back 22

arrival of a nerve impulse (action potential)

Front 23

how does action potential travel?

Back 23

down axon to presynaptic terminal (spike)

Front 24

where does the neurotransmitter go after released?

Back 24

synaptic cleft

Front 25

what is a synaptic cleft?

Back 25

tiny space between 2 nerve cells where the neurotransmitter passes

Front 26

what happens to neurotransmitter after it reaches synaptic cleft?

Back 26

it is absorbed by the post-synaptic membrane

Front 27

what are the 2 neurotransmitters in the auditory system?

Back 27

glutamate and acetylcholine

Front 28

glutamate

Back 28

afferent neurons- leads to cortex (sensory)

Front 29

acetylcholine

Back 29

efferent neurons- leads away from cortex (motor)

Front 30

cranial nerve VIII

Back 30

auditory nerve

Front 31

where is CN VIII located?

Back 31

connects cochlea to brainstem

Front 32

what does CN VIII do?

Back 32

relays info about
intensity
frequency
timing
of sound after cochlea has completed initial processing of incoming stimulus

Front 33

2 divisions of CN VIII

Back 33

auditory branch
vestibular branch

Front 34

auditory branch

Back 34

to cochlea

Front 35

vestibular branch

Back 35

to semicircular canals, utricle, saccule

Front 36

how long is the CN VIII

Back 36

ranges in length from 22-26 mm

Front 37

where do the branches meet?

Back 37

join in the internal auditory meatus and runs parallel with vestibular branch and facial nerve CN VII

Front 38

where does the auditory nerve originate?

Back 38

fibers originate at the base of the cochlear hair cells- terminal buttons, it is a fragile connection

Front 39

how do the auditory nerve fibers connect to the hair cells?

Back 39

habenula perforata

Front 40

what is the habenula perforata?

Back 40

bony structure of osseous spiral lamina
small opening where auditory fibers pass
afferent and efferent nerve fibers

Front 41

where are dendrites located?

Back 41

under hair cells

Front 42

how many dendritic fibers pass through each habenula perforata?

Back 42

about 30

Front 43

what do the cell bodies form? where are they located?

Back 43

spiral ganglion in modiolus
in the periphery

Front 44

where do the axon terminals synapse occur?

Back 44

cochlear nucleus, first synapse point in auditory brainstem

Front 45

2 distinct kinds of fibers in the auditory system

Back 45

type 1- afferent
type 2- efferent

Front 46

type 1 fiber in the auditory system

Back 46

afferent
90-95% of all auditory nerve fibers
80-85% of these are from the inner hair cells

Front 47

type 2 fiber in the auditory system

Back 47

efferent
5-10% of auditory nerve fibers

Front 48

inner hair cells- what type of fibers? ratio? how does it look? how many contacts?

Back 48

largely innervated by type 1 fibers
each cell is innervated by many different afferent fibers
"many-to-one"
gives radical appearance
10-30 afferent contacts

Front 49

outer hair cells- what type of fibers? ratio?

Back 49

primarily innervated by type 2 fibers
more outer hair cells than type 2 fibers
each nerve fiber goes to many outer hair cells
"one-to-many"
each outer hair cell has multiple fiber connections

Front 50

origin of efferent innervation

Back 50

superior olivary compleax in brainstem

Front 51

what are the efferent innervation fibers called?

Back 51

olivocochlear bundle(OCB)

Front 52

where do OCB fibers go?

Back 52

most cross to the opposite (contralateral) side- crossed
small number stay on same (ipsilateral) side- uncrossed

Front 53

where do efferent to outer hair cells terminate?

Back 53

directly on outer hair cells

Front 54

where do efferent to inner hair cells terminate?

Back 54

on afferent fiber

Front 55

what fibers make up the core of the afferent branch of CN VIII

Back 55

fibers from apical region of cochlea

Front 56

what fibers wrap around these core fibers?

Back 56

fibers from the medial region of cochlea

Front 57

what fibers make up the surface?

Back 57

fibers from the basal region of the cochlea

Front 58

SO where are the highest frequencies represented on CN VIII?

Back 58

on the circumference

Front 59

what does the composition of the afferent branch of the CN VIII do?

Back 59

preserves the tonotopic organization of the basilar membrane

Front 60

when a cell is at resting potential, does the neuron have a charge?

Back 60

yes, voltage stored in cell

Front 61

source of neurons charge

Back 61

-properties of cell membrane under rest
-sodium potassium pump
-since membrane is more permeable to potassium than sodium, it pumps out sodium
-the resulting distribution of ions creates a charge

Front 62

what is the cells resting potential

Back 62

-80mV (millivolts)

Front 63

what happens to the action potential when a neurotransmitter is received by dendrites

Back 63

action potential will propagate down the receiving neuron

Front 64

how does the cell membrane change when a neurotransmitter is received by dendrites?

Back 64

the cell membrane becomes permeable to sodium and pumps out potassium

Front 65

how does the cells charge change when a neurotransmitter is received by dendrites?

Back 65

the charge becomes depolarized, it goes from -80 mV to 40 mV

Front 66

where does the depolarization process go?

Back 66

it travels down the neuron

Front 67

what happens when the charge reaches 40 mV?

Back 67

there is a rapid reversal, the charge goes back to resting state of -80mV

Front 68

what happens when action potential reaches the end of an axon?

Back 68

it causes a synapse with the next neurons dendrites, this leads to sequential excitation from one neuron to the next

Front 69

what type of an event is action potential?

Back 69

all or nothing

Front 70

what happens if the synapse is weak?

Back 70

the neuron returns to resting state without depolarizing

Front 71

how many action potential spikes are there per second?

Back 71

the maximum is about 500 spikes/second

Front 72

refractory period

Back 72

time needed for neuron to recover before it can fire again, lasts about 1 ms

Front 73

if the refractory period is 1 ms then theoretically the max spike rate is...

Back 73

1000 spikes/sec but this has never been observed

Front 74

2 types of neural activities

Back 74

evoked potentials and single-unit recordings

Front 75

evoked potentials

Back 75

observe the simultaneous discharge of many neural units from a distance (ECochG, ABR, whole nerve action potential)

Front 76

single unit recordings

Back 76

measure response of one neuron at a time, tiny electrode not usually done on humans

Front 77

what is a spontaneous rate?

Back 77

neurons always fire even without acoustic stimulation

Front 78

why does this happen?

Back 78

currently unsure

Front 79

spontaneous rates tend to be...

Back 79

low or high
low= less than 20/sec
high= 60-80/sec

Front 80

you must know the spontaneous rate to determine what?

Back 80

when nerve is responding to stimulus

Front 81

what is the neural response to an acoustic stimulus?

Back 81

increase in spike rate (firing rate) above spontaneous rate, greater stimulus intensity the greater the spike rate

Front 82

what changes during a stimulus response?

Back 82

spike rate, size and shape of spike do not

Front 83

PST histogram

Back 83

post-stimulus time histogram
a way of recording neural activity
records spike rate as a function of time (spike rate/time)

Front 84

in a PST histogram describe what happens before tone, tone on, tone off

Back 84

before tone- spontaneous rate
tone on- slight delay, large increase in spike rate, decreases slightly, then sustained through duration of tone
tone off- sharp decline, refractory period which is less than spontaneous rate, then back to spontaneous rate

Front 85

this shows that we are most sensitive to a stimulus when?

Back 85

the beginning

Front 86

PST histograms reflect what type of neuron activity?

Back 86

afferent neuron activity, and therefore primarily the inner hair cells

Front 87

how to measure a neurons threshold across frequencies

Back 87

-find spontaneous rate
-start with low level signal
-change level until there is an increase in spike rate (>SR)
-look for 5-30 spike increase over SR
-perform same analysis and plot results across frequencies

Front 88

what is the result of this measurement?

Back 88

a tuning curve, an audigram for a neuron

Front 89

each neuron has one frequency to which it is most sensitive (has lowest threshold) what is this called?

Back 89

characteristic frequency

Front 90

tuning curves for neurons with low characteristic frequencies are...

Back 90

more symmetric

Front 91

tuning curves for neurons with high characteristic frequencies are...

Back 91

more asymetric, sharply tuned tip, broadly tuned tail

Front 92

for a single neuron, what happens as intensity increases

Back 92

spike rate increases

Front 93

what is the dynamic range for intensity for a single neuron?

Back 93

about 20-50 dB, after 50 spike rate plateaus

Front 94

what happens when there are multiple neurons?

Back 94

at first only those tuned to that characteristic frequency will fire, but then as intensity increases more and more surrounding neurons will fire

Front 95

what is that effect called?

Back 95

spread of excitation

Front 96

acoustic tumor of the auditory nerve- incidence? age?

Back 96

tumor on CN VIII nerve
8.7 persons per 1000

Front 97

onset of acoustic tumor of the auditory nerve

Back 97

age 30-50
may take several years to recognize depending on symptoms

Front 98

what are the acoustic tumor of the auditory nerve symptoms a result of?

Back 98

the tumor is pressing on anatomical regions within the auditory and surrounding systems

Front 99

acoustic tumors of the auditory nerve account for what percent of intracranial tumors? cerebellopontine (within the brainstem) tumors?

Back 99

intracranial tumors- 8-10%
cerebellopontine tumors- 78%

Front 100

where is the auditory nerve unmyelinated?

Back 100

region between endings on hair cells and the habenula perforata

Front 101

when does the auditory nerve become myelinated?

Back 101

as it passes through the internal auditory meatus

Front 102

schwann cells

Back 102

once auditory nerve is myelinated

Front 103

what happens to the nerves as they leave the internal auditory meatus?

Back 103

the auditory, facial, and vestibular nerves go through a recess known as the CPA in the brainstem

Front 104

vestibular schwannoma

Back 104

tumors most often arise from vestibular nerve in the internal auditory meatus (>90%) and encroaches on the auditory nerve

Front 105

cause of vestibular schwannoma

Back 105

-neoplasm arises from schwann sheath cells to form neurilemma
-sudden growth of schwann sheath cells becomes encapsulated which leads to a tumor

Front 106

describe the tumors that result from vestibular schwannoma

Back 106

benign
start small
usually slow growing
small- round and firm
large- lobulated

Front 107

why can vestibular schwannoma tumors be especially dangerous

Back 107

they are very close to the brainstem

Front 108

where can vestibular schwannoma develop outside the internal auditory meatus

Back 108

into the cerebellopontine angle (CPA)
into cochlea

Front 109

what happens if the tumor grows into the cerebellopontine angle?

Back 109

can erode bone
push on brainstem and cerebellum

Front 110

what happens if the tumor grows into the cochlea?

Back 110

-destroy vestibular and auditory nerve fibers
-push on cochlear branch of labyrinthine artery and cut off blood supply which leads to hair cell death and sudden hl
-destroy structures of cochlea
-push on facial nerve and cause paresis/paralysis

Front 111

because most tumors affecting the auditory nerve approach the outside of the nerve fist, the _____ fibers are initially compromised, resulting in a ____ sersorineural hearing loss.

Back 111

-basal
-high frequency

Front 112

auditory symptoms of vestibular schwannoma

Back 112

usually occur first
75% high frequency hearing loss is first symptom
usually gradual, abrupt in about 40%
tinnitus in about 80%
possible aural fullness
difficulty sustaining perception of continuous tone

Front 113

audiogram configuration of someone with vestibular schwannoma

Back 113

variable in degree
sloping
high frequency
sensorineural
speech recognition poorer than predicted

Front 114

vestibular symptoms of vestibular schwannoma

Back 114

-disequilibrium- constant and prolonged
-vertigo
-nausea
-nystagmus
-headaches (pressure)- schwannoma has invaded frontal/occipital region

Front 115

cerebellar symptoms of vestibular schwannoma

Back 115

uncoordinated movement- drunk like, involves lower body
tumor pushing on cerebellum

Front 116

involvement CN V for vestibular schwannoma

Back 116

trigeminal- tingling on face

Front 117

involvement CN VII for vestibular schwannoma

Back 117

facial- facial paralysis (partial or complete)

Front 118

involvement CN IX and X for vestibular schwannoma

Back 118

glossopharyngeal and vagus- late is disease pharyngeal and laryngeal problems, dysphonia

Front 119

why is diagnosis for vestibular schwannoma difficult?

Back 119

small tumors are usually asymptomatic
first symptom is usually hl

Front 120

how do they diagnose vestibular schwannoma?

Back 120

case history
audiologic evaluation (ABR)
otologic evaluation (by otologist)
neurologic evaluation
definitive diagnosis- radiological imaging exam

Front 121

red flags for vestibular schwannoma in audiologic tests

Back 121

-pure tone thresholds- unilateral/asymmetrical SNHL (90-95% of cases)
-speech recognition-poorer than predicted for HL
-tone decay

Front 122

red flags for vestibular schwannoma in electrophysiologic test

Back 122

-acoustic reflex threshold elevated or absent regardless of pure tone average thresholds- about 84% sensitivity
-reflex decay
-auditory brainstem response (ABR) absolute and interwave latencies

Front 123

auditory brainstem response for some one with vestibular schwannoma

Back 123

-may only see wave 1
-wave V may be delayed
- about 92% sensitivity

Front 124

what do they do at a otoneurologic exam when looking for vestibular schwannoma?

Back 124

-physical exam- review audiological findings
-doctor order MRI which is definitive
-CT if cant have MRI

Front 125

MRI

Back 125

magnetic resonance imaging

Front 126

CT

Back 126

computed tomography

Front 127

3 stages of treatment for vestibular schwannoma

Back 127

monitor- periodic audiological assessments, MRIs
gamma knife irradiation- ~25% increasingly used
surgery- ~50% several strategies

Front 128

surgical approach for vestibular schwannoma is determined by? whos role is this?

Back 128

size and location of tumor
audiologist's

Front 129

what are the 3 types of surgical approaches for vestibular schwannoma?

Back 129

-middle fossa approach
-translabyrinthine approach
-suboccipital/retrosigmoid approach

Front 130

which is the least invasive approach?

Back 130

middle fossa approach

Front 131

describe the middle fossa surgical approach (tumors, where is surgery?, how does it affect hearing?)

Back 131

-tumor is within internal auditory canal
-tumor is less than 3/4 inch
- surgical incision near squamous portion of temporal bone to access internal auditory meatus (don't need to go through labyrinth)
- hearing preservation possible (~60%)

Front 132

describe the translabyrinthine approach (tumors, where is surgery? how does it affect hearing?)

Back 132

-tumors outside of the internal auditory canal esp. if no hearing on that side
-surgical incision behind pinna
-mastoidectomy and labyrinthectomy
- 100% sacrifice of vestibular and auditory function on that side

Front 133

describe the suboccipital/retrosigmoid approach (tumors, where is surgery? how does it affect hearing?)

Back 133

-very large tumors
-gain access to all cranial nerves, expose brainstem structures, internal auditory meatus
- possible to preserve hearing (~40%) increased risk secondary to increased exposure

Front 134

what type of disorder is neurofibromatosis type 2?

Back 134

-genetic disorder
-autosomal dominant (chromosome 22)
-defective tumor suppressing genes
-50% cases inherited, 50% are de novo (new)

Front 135

incidence of neurofibromatosis type 2? when does it appear?

Back 135

1/33,000-50,000
appears in teens/young adulthood

Front 136

diagnosis of neurofibromatosis type 2

Back 136

-bilateral CN VIII tumors
-other tumors of the CNS can develop as well, additional phenotypic (observable) features

Front 137

symptoms of neurofibromatosis type 2

Back 137

-same as vestibular schwannoma but bilateral
-tumors tend to grow aster and be more aggressive

Front 138

treatment for neurofibromatosis type 2

Back 138

-depends on individual
-monitor one or both tumors
- surgical decompression
-surgical removal
-complete removal of CN VIII bilaterally in most cases

Front 139

surgical removal for neurofibromatosis type 2

Back 139

- harder than traditional vestibular schwannoma bc tumor is more integrated among nerve fibers
- likely to recur

Front 140

removal of CN VIII bilaterally results

Back 140

-will destroy hearing
- auditory brainstem implant (ABI)

Front 141

ABI

Back 141

auditory brainstem implant
cypass CN VIII
stimulates the cochlear nucleus

Front 142

ABI v cochlear implant

Back 142

cochlear implants bypass hair cells and have electrode array

Front 143

what is presbycusis?

Back 143

hl due to aging

Front 144

4 general changes in the auditory system associated with age

Back 144

(Not My First Concert- neurons, muscles, fluids, cartilage)
- loss of neurons (not replaced)
- changes in composition of intercellular fluids (endolymph and perilymph)
-changes in composition of muscles/ligaments (stapedius/tensor tympani)
-changes in cartilage (pinna and outer 1/3 of canal)

Front 145

age and incidence of presbycusis

Back 145

-30-40% people over 65 have significant age related hearing impairment
-50% over 80
- 2nd most common chronic health condition people report

Front 146

why is it difficult to measure pure presbycusis

Back 146

- noise effects- cumulative over time
- disease- meniere's otosclerosis, progressive genetic disorders, ototoxicity
- some age related declines in all populations, just more in westernized societies

Front 147

4 auditory system changes with age

Back 147

-conductive mechanism
-cochlear function
-CN VIII function
-CANS processing

Front 148

effect of presbycusis on hearing process

Back 148

-elevated thresholds
-poor speech understanding

Front 149

how does presbycusis affect speech understanding

Back 149

-phonemic regression- decrease in intelligibility of speech out of proportion to pure tone hearing loss associated with aging
-poorer speech recognition than predicted by audiogram

Front 150

issues the older population faces

Back 150

-comorbidity- presence of more than one disorder or disease
-HL may be superimposed on other age related communication issues- changes in speech comprehension and processing

Front 151

5 types of presbycusis

Back 151

middle ear/conductive
sensory
neural
strial/metabolic
central
typically don't occur in isolation, occur together

Front 152

physical effects of middle ear/conductive presbycusis

Back 152

-tympanic membrane thickens, loosens, looses elasticity
-ossicles become fixated
- arthritic changes to joints/ligaments
-cartilage softens leads to collapsing canals
not a large effect so there is not much, if any of a conductive component in presbycusis

Front 153

sensory presbycusis- physical effects, onset, audiogram, speech

Back 153

-changes in inner ear: atrophy of organ of corti, degeneration of neurons in organ of corti/spiral ganglion
-gradual onset
-audiogram- sloping/sharply sloping SNHL
-speech discrimination will depend on amount of impairment

Front 154

neural presbycusis- physical effects, onset, HL, audiogram, speech

Back 154

-loss of neurons innervating cochlea
-hair cells intact
-gradually cumulative over time
-starts in young adults
- mild HL
-gradually sloping or flat
-phonemic regression is a big problem

Front 155

strial/metabolic presbycusis- physical effects, HL, audiogram, speech, treatment

Back 155

-atrophy of stria vascularis
-mild to moderate HL
-flat SNHL
good speech recognition
-hearing aids very helpful

Front 156

central presbycusis- physical effects, hl, speech

Back 156

- degeneration of neural tissue throughout CANS
-slight HL
-behavioral thresholds attributed to peripheral fx
- speech recognition in quiet is usually good
- degraded speech tasks very difficult
- discrimination judgments are poorer (need 2x as long to detect difference)

Front 157

challenge for researcher in presbycusis is...

Back 157

determining underlying bases for pathology (genetic, environmental, additive, etc.)

Front 158

for clinicians, findings on a thorough audiologic evaluation of someone with presbycusis will reflect the sum of...

Back 158

various presbycusis types

Front 159

vestibular changes with aging

Back 159

-deterioration of sensory end organs for balance
-cristae in ampullae- rotary movement
-macule in saccule and utricle- linear acceleration

Front 160

what do these vestibular changes cause?

Back 160

disequilibrium
dizziness
falling easily
cerebral changes
comorbidity