B7.041 Prework: Hearing Loss

Question 1

description of CN VIII

Answer 1

auditory/vestibular nerve

• sensory neurons in vestibular ganglion (balance) or auditory ganglion (hearing)
• axons synapse in the brain stem (vestibular or cochlear nuclei)

Question 2

auditory system

Answer 2

peripheral components: external, middle, and inner ear
provides info about sound in the environment
hair cells are sensory transducers
afferent information from CN VIII synapses on cochlear nuclei in the brainstem
information reaches cortex

Question 3

vestibular system

Answer 3

peripheral components within the inner ear
provides info about motion and gravity
hair cells are sensory transducers
afferent information from CN VIII synapses on vestibular nuclei in brainstem
info does not reach cortex; interactions with motor system

Question 4

external ear

Answer 4

external auditory meatus

separated from middle ear by tympanic membrane

Question 5

middle ear

Answer 5

between tympanic membrane and round/oval window
filled with air (equilibrates with environment via eustachian tube)
contains malleus, incus, and stapes bones

Question 6

inner ear

Answer 6

filled with perilymph
contains cochlea, semicircular canals, saccule, and utricle
canals filled with endolymph and surrounded by perilymph
connected to middle ear by stapes

Question 7

how do hair cells transduce energy

Answer 7

movement of stereocilia
toward tall side= depolarization and increased impulse frequency
toward short side= hyperpolarization and decreased impulse frequency

Question 8

vestibular hair cells

Answer 8

located in inner ear but do not detect sound
2 locations
1. macular organs: saccule, utricle, detect linear acceleration (gravity)
2. semicircular canals: ampullae, detect rotational acceleration

Question 9

macular organs

Answer 9

area that contains hair cells laid out in flat sheets
detect gravity (linear acceleration)

Question 10

otoliths

Answer 10

calcium carbonate crystals embedded in glycoprotein matrix on top of hair cell stereocilia
shift with gravity and bend stereocilia

Question 11

movement detection in the ampullae

Answer 11

hair cells located in ampullae on a ridge of tissue called crista ampullaris
stereocilia embedded in the cupula, a tall, glycoprotein matrix mass
cupula is deflected by fluid flowing through the canal when the head turns

Question 12

functions of middle ear

Answer 12

impedence matching
pressure equalization
gain control of vibrations reaching inner ear

Question 13

impedence matching

Answer 13

ossicular chain and the size difference between the tympanic membrane and oval window prevents loss of pressure at oval window

Question 14

pressure equalization in middle ear

Answer 14

eustachian tube connects the middle ear and the nasopharynx

Question 15

how does the middle ear gain control of vibrations reaching the inner ear

Answer 15

two muscles (tensor tympani and stapedius) can change the stiffness of the tympanic membrane or dampen the movement of the stapes
these actions increase the dynamic range of the inner ear and protect cochlear hair cells from loud sounds

Question 16

function of cochlear compartments

Answer 16

allows auditory hair cells to be stimulated by sound waves vibrating fluid in cochlea

Question 17

path through cochlea

Answer 17

input through scala vestibuli
output from scala tympani
basilar membrane (bottom) and vestibular membrane (top) separate the cochlear duct filled with endolymph from the perilymph canals

Question 18

inner hair cells of cochlea

Answer 18

closer to middle of cochlea

primary sensory transduces or auditory system that sends signals to the CNS

Question 19

outer hair cells of cochlea

Answer 19

change properties of basilar membrane to create more/less movement to stimulate inner hair cells

Question 20

what technically causes hair cells in the cochlea to vibrate

Answer 20

sitting on top of floppy basilar membrane

movement with fluid vibration causes tectorial membrane to move across tops of hair cells, vibrating them

Question 21

sequence of sound waves entering the ear

Answer 21

1. sound waves enter external auditory canal; amplified 1-4 kHz
2. tympanic membrane vibrates
3. ossicles vibrate and increase force
4. oval window vibrates and sets up motion in perilymph of scala vestibuli
5. vestibular membrane transfers vibration to scala media
6. basilar membrane vibrates
7. hair cells move relative to tectorial membrane
8. stereocilia deflect toward long side
9. channels open and K+ enters through stereocilia tips
10. hair cell membrane depolarizes
11. voltage gates Ca2+ channels open and Ca2+ enters hair cell
12. neurotransmitter released
13. afferent nerves respond with action potential headed for the brainstem

Question 22

how is sound info organized in the auditory system

Answer 22

frequency of sound (tonotopic)

movement of basilar membrane is not uniform along its length, displacement in a given region depends on sound frequency

Question 23

orientation of basilar membrane

Answer 23

varies in width and stiffness from base to apex
allows different frequencies of sound to stimulate specific locations along it (gets thinner and floppier toward apex, lower frequencies heard here…high frequencies at base)

Question 24

function of outer hair cells

Answer 24

innervated by efferent fibers from neurons in superior olivary nucleus in the pons
in response to low intensity sounds, OHC movement adds energy to basilar membrane vibration and amplifies vibration
OHCs also sharpen frequency of tuning of basilar membrane

Question 25

otoacoustic emissions

Answer 25

sounds that originate in the ear

produces by OHC movement of basilar membrane

Question 26

auditory input to central pathways

Answer 26

CN VIII and cochlear nucleus receive monoaural input from ipsilateral ear
all other nuclei above the level of the cochlear nucleus receive input from both ears
auditory nuclei/cortex exhibit tonotopic organization

Question 27

where is the auditory cortex

Answer 27

lower bank of lateral sulcus