Physiology of the Auditory and Vestibular Systems

Question 1

What is the makeup of endolymph?

Answer 1

high K

low Na

found in scala media

Question 2

What is the makeup of perilymph

Answer 2

low K

high Na

found in scala vestibuli and scala tympani

bathes basal end of cochlear hair cells

Question 3

How do sound waves go through the cochlea?

Answer 3

tympanic membrane –> malleus –> incus –> stapes in oval window –> scala vestibuli –> cochlear duct –> basilar membrame –> scala tympani –> round window

Question 4

What is the basilar membrane like in proximal and distal parts and how does this affect the frequency of waves that go through it?

Answer 4

near oval window: narrow and stiff –> high frequency waves go here

near helicotrema: wide and flexible –> low-frequency waves go here

Question 5

What types of cells are hair cells?

Answer 5

specialized epithelial cells = mechanoreceptors

receive afferent and efferent input, but themselves are NOT neuronal

Question 6

Where are stereocilia on hair cells?

Answer 6

on the apical surface

stiff, graded in size, rich in actin

neural synapses on basal side

Question 7

What is a kinocilium?

Answer 7

the tallest cilia

stereocilia bend in direction of of kinocilium –> depolarizes cells

Question 8

How do hair cells depolarize and signal?

Answer 8

stereocilia are deflected in direction of the kinocilium –> K+ ions enter cell and depolarize it –> voltage-gated Ca channels open –> vesicles w/ glutamate are released –> post synaptic afferent neuron

Question 9

What 2 things drive the depolarization of hair cells?

Answer 9

endolymph is high in potassium = concentration gradient

large electrical gradient = scala media has high positive charge

Question 10

Where are the stereocilia of hair cells located?

Where is the rest of the cell located?

Answer 10

stereocilia in endolymph

rest of cell is in perilymph

Question 11

What connects stereocilia?

Answer 11

topconnector chains

if one hair is deflected –> they all move

Question 12

What is the difference between excitation and inhibition hair cell signalling?

Answer 12

excitation: cilia deflect toward kinocilium –> depolarization –> increase in APs

resting = basal tone of APs

inhibition: cilia deflect away from kinocilium –> hyperpolarization –> decrease in AP freqeuncy

Question 13

What lets K+ into hair cells?

Answer 13

TRPA1 channel

Question 14

What NT do hair cells release?

Answer 14

glutamate

Question 15

What is the stria vascularis?

Answer 15

specialized epithelium of scala media

pump K into endolymph to maintain electrochemical properties of the endolymph

Question 16

What is the composition of hair cells on the basilar membrane?

Answer 16

one row of inner hair cells = primary source of auditory info

3 rows of outer hair cells = amplifier, contractile bc of prestin proteins –> boost mechanical vibrations of basilar membrane; susceptible to injury

Question 17

What is the difference in innervation of inner and outer hair cells?

Answer 17

outer: get both efferent and afferent I from type II affererents

inner: just afferents from type I

Question 18

Which hair cells bend first?

Answer 18

outer first –> then inner

Question 19

How is the cochlear nucleus divided?

Answer 19

dorsal cochlear nuclei: integrates acoustic info w/ somatosensory info to localize

ventral cochlear nuclei: begins processing temporal and spectral features of sound

Question 20

What is the significance of the superior olivary complex?

Answer 20

first site in the brainstem where info from both ears converges = binaural processing

MSO = primary nucleus

receives excitatory (glutamate or aspartate) projections from cochlear nuclei

Question 21

What does the medial superior olivary nucleus (MSO) do?

Answer 21

generates a map of the interaural TIME differences to help localize sound

responds strongest when the 2 inputs arrive simultaneously, which occurs when 2 sides compensate for microsecond differences in the time of arrival of the sound at the 2 ears

Question 22

What does the lateral superior olivary nucleus (LSO) do?

Answer 22

generates a map of the interaural INTENSITY differences to help localize the source of a sound

side of sound localization –> excites ipsi LSO –> receives inhibition from contra side; but net excitation > inhibition

Question 23

What do the inferior and superior colliculi do in hearing?

Answer 23

IC: suppresses info related to echoes, which would interfere w/ localization and arrives at final est of localization along horizon

SC: takes location data from IC –> adds vertical dimension = spatial map

Question 24

What does the MGN of the thalamus do in hearing?

Answer 24

relay station (from inf colliculus)

tonotopic map is maintained

processing features of speech inflections

Question 25

Where is the primary auditory cortex (AI)?

Answer 25

post part of sup temporal gyrus

essential in conscious perception of sound

higher order processing

tonotopic map is maintained

Question 26

Where are low and high frequencies located in the primary auditory cortex?

Answer 26

anterior = low freq (apex of cochlea)

posterior = high freq (base of cochlea)

Question 27

What does the auditory association complex do?

Answer 27

broca’s and wernicke’s, others

less specifically organized in tonotopic arrangement than primary

thought to respond to more complex sounds, IDing a sound, speech

Question 28

What are olivocochlear efferents?

Answer 28

originat in Superior olivary complex

Medial OC –> innervate outer hair cells

Lateral –> inner hair cells

LIMO

Question 29

What is the function of olivocochlear efferents?

Answer 29

shifts responses to higher sound levels

decrease adaptation

reduce the response to noise

may protect hair cells from damage

Question 30

What are otoacoustic emissions?

Answer 30

ear can emit sound, but usually inaudible

spontaneous OAE: in 1/3 of normal ppl

Evoked: used to test for hearing loss (no emissions evoked if damage present)

evoke on newborns

Question 31

What is sensorineural hearing loss?

Answer 31

caused by damage to hair cells or nerve fibers

noise damage, ototoxic drugs, etc.

Outer HCs more susceptible than inner

base (high freq) more susceptible

Question 32

What is the difference in effect of damage to inner vs outer hair cells?

Answer 32

injury to outer –> decrease in sensitivity and broader tuning

injury to inner –> cuts off auditory input to CNS

Question 33

What is a cochlear prosthesis?

Answer 33

multiple electrode array threaded through cochlea to stimulate surviving nerve fibers

Question 34

What motion do the semicircular canals detect?

Answer 34

rotational acceleration

Question 35

What motion do the otolith organs detect?

Answer 35

utricle: linear acceleration forward and backward
saccule: linear acceleration up and down

Question 36

What does each semicircular canal detect?

Answer 36

horizontal: rotation in horizontal plane
posterior: rotation in saggital plane backward (falling back)
anterior: rotation in saggital plane forward

Question 37

What muscle is activated when the horizontal canal is firing?

Answer 37

medial rectus m is active and lateral rectus inhibited (in eye ipsi to rotation)

Question 38

What muscles are activated whn the posterior canal is firing?

Answer 38

falling back

superior oblique active and inferior oblique inhibited

(eyes ove down to focus on a point as you fall back)

Question 39

What muscles are activated when the anterior canal is firing?

Answer 39

falling forward

superior rectus activated

inferior rectus inhibited

Question 40

How does head rotation affect the hair cells?

Answer 40

turn away from ear –> hair cells pushed away from king bc of endolymph pressure –> inhibitory signal

turn head toward ear –> hair cells pushed toward king –> excitatory signal