Anatomy and Physiology of the Ear

Question 1

where do sound waves enter? what does this cause at the end of the canal to vibrate?

Answer 1

external auditory canal; tympanic membrane

Question 2

within 4 cm or so occupied by the outer and middle ears, three distinct physical principles operate to magnify the weak vibrations in the ear, what are they? what is the result?

Answer 2

organ pipe increase air pressure
bone lever system can triple it
arrangement of the eardrum and oval window can increase it 30 fold

the result of the three is amplification of sound more than 800 times before it sets the inner ear in motion, like whispering, weak vibrations are amplified

Question 3

what is the Tendon of stapedius muscle and the tensor tympani innervated by and their function?

Answer 3

the stapedius muscle attached to the stapes and innervated by the facial nerve

tensor tympani muscle attached to the malleus and innervated by trigeminal

both attempt to dampen loud noises before they are transferred to the inner ear

Question 4

***what is the path of sound waves through the ear?

Answer 4

air waves hit the pinna
pushed to the external auditory canal
rattles the TM
malleus, incus, stapes
oval window
cochlea
perilymph in the scala vestibule
all the way to the helicotrema
scala tympani
back to the round window in the mid ear
vibrations dampen eustachian tube
nasopharynx

Question 5

***how is sound transmitted to the brain? (11 steps)

Answer 5

1. Pinna reflects sound & affects ability to locate sounds
2. Outer & middle ear turns air vibration into bone vibration
3. Inner ear turns bone vibration into fluid vibration
4. Cochlea turns fluid vibration into tissue movement
5. Different parts of the cochlea resonate to different frequencies of sound
6. The basilar membrane of the cochlea contains ciliated, mechanosensitive, receptor cells (which also resonate)
7. The receptor cells release neurotransmitter on 1° auditory neurons
8. 1° auditory neurons of the spiral ganglion project in the 8th cranial nerve to the medulla
9. The place coding is preserved throughout the CNS (tonotopy)
10. High & low frequency sounds are localized in the horizontal plane by interaural intensity & timing differences, respectively
11. Sound is localized in the vertical plane, and back/front ambiguity is resolved by spectral differences

Question 6

what is significant about the vibrating basilar membrane found in the cochlear duct?

Answer 6

the membrane is narrow and taut at the end near the stapes and wider and more pliant at the other and so hydraulic pressure waves in cochlea induce wave lie ripple in the basilar membrane which travels from taut(beginning) to loose end (apex)

high tones produce the greatest crests where the membrane is tight (beginning), low tones where the wall is slack (end)

the crests are important because it determines which nerve fiber will send signals to the brain

Question 7

***why can’t elderly people hear high frequency sounds as well?

Answer 7

because the basilar membrane isn’t as taut at the beginning

Question 8

T/F, the basilar membrane also picks up vibrations in the skull from such sources as teeth clicking

Answer 8

T

Question 9

what part of the outer ear collects and focuses sound waves into the external auditory canal?

Answer 9

pinna and tragus which then transmit sound through the external auditory canal which ends up at the tympanic membrane

Question 10

what bounds the middle ear? function of the middle ear?

Answer 10

TM and the oval window; it transfers vibrations from the TM to the oval window using three bones or ossicles known as the malleus, incus and stapes

Question 11

what is the auditory portion of the inner ear? innervation?

Answer 11

cochlea innervated by auditory/cochlear nerve (branch of CN VIII)

Question 12

what are the two membranes that divide the cochlea into 4 fluid filled compartments?

Answer 12

reissner’s membrane separating scala vestibuli from middle compartment (scala media)

basilar membrane separating the scala media from the scala tympani

Question 13

***the scala has what type of lamina that meets with this type of ganglion eventually innervated by this nerve?

Answer 13

spiral lamina to spiral ganglion to cochlear portion of CN8

Question 14

what is the function of the organ of corti?

Answer 14

contains the cells responsible for hearing

Question 15

capillaries nearest to the organ of corti end at the wall of cochlea and nutrients on their way out are carried to and from capillaries by?

Answer 15

endolymph fluid that bathes the organ

Question 16

the basilar membrane supports the organ which contains a mass of cells almost touching the branch endings of the _____?

Answer 16

auditory nerve

these cells embed in a thick overhang sheet called the tectorial membrane

as the basilar membrane bellies, it pushes and pulls the complex of tissues above it and the hair cells of the organ of Corti ride with the basilar membrane

Question 17

what is the tectorial membrane?

Answer 17

the hairs located on here are transducers

Question 18

how do the pressure waves travel through the cochlea?

Answer 18

the pressure waves exert energy that begin at the oval window and end at the round window so pressure can dissipate, note that pressure is applied to the oval window at stapes and all parts of cochlea

Question 19

what is the most important membrane found in the cochlea duct?

Answer 19

vibrating basilar membrane which separates sound by frequency in between scala tympani and media where the organ of corti is found

Question 20

where do the sensations in both auditory/vestibular systems begin?

Answer 20

inner ear and both use hair cells (mechanoreceptors specialized to detect minuscule movement along one particular axis)

Question 21

the hair cells found in the auditory and vestibular systems are contained where?what is the stimulus for the hair cell

Answer 21

membranous labyrinth; bending of the hair, vibrating endolymph

Question 22

the auditory portion of the labyrinth is what part of the structure?

Answer 22

spiraling cochlea

Question 23

the vestibular portion of the labyrinth contains two structures called?

Answer 23

the otolith organs (gravity and head tilt)

semicircular canals (head rotation)

Question 24

what does the hair bundle of the auditory/vestibular labyrinth consist of?

Answer 24

1 large kinocilium and stereocilia

Question 25

***what happens when the bending hair bundles toward the kinocilium?

Answer 25

it excites the cell and causes AP (opens K+ channels that depolarize the membrane that allow for Ca2+ influx facilitating an AP to the nerve cell fibers

Question 26

***what happens when the bending hair bundles away from the kinocilium?

Answer 26

cell is hyper polarized and K+ channels are closed

Question 27

the primary afferents for hearing are innervated by what nerve?

Answer 27

CN VIII

Question 28

the primary afferents for hearing convey information about what?

Answer 28

sound frequency - through location on basilar membrane of hair cells they contact
sound loudness - via amount of NT released and so the afferents respond to amount of NT by firing at a higher frequency

Question 29

what is the arrangement of neurons along the auditory pathway?

Answer 29

tonotopic

Question 30

how is sound information from each ear transmitted?

Answer 30

bilaterally through the thalamus to each auditory cortex

Question 31

the receptive fields of neurons in the auditory cortex are _____? why is this important?

Answer 31

binaural; sound localization

Question 32

what is the effect if there is damage to the cortex of one hemisphere?

Answer 32

it doesn’t cause hearing loss but it does for sound localization

Question 33

what is the sound detection in the horizontal plane for high intensity?

Answer 33

high intensity is (2-20kHz) and so the CNS detects the difference in INTENSITY between the 2 ears

Question 34

what is the sound detection in the horizontal plane for low intensity?

Answer 34

low intensity is (<2kHz) sounds and so the CNS detects the SOUND DELAY between 2 ears

Question 35

what is the CNS processing of sounds?

Answer 35

cochlea to cochlear nerve to cochlea nucleus to ipsi/contralaterally to medial superior olivary nucleus to ipsi/contralaterally to cortex (both ears do this)

Question 36

what is the effect when there is a delay pending what side heard first?

Answer 36

this gives the brain a chance to process and localize what side the noise came