Hearing

Question 1

Outer Ear

Answer 1

funnels sound waves to the eardrum

Question 2

Pinna

Answer 2

Collects sound

Question 3

Tympanic membrane or eardrum

Answer 3

Separates outer and middle ear; vibrates in response to sound wave

Question 4

Middle ear

Answer 4

Air-filled cavity; connected to the nasopharynx by the auditory

Question 5

(Eustachian) tube

Answer 5

Equalizes pressure
Contains ossicles (malleus, incus, stapes)

Question 6

malleus, incus, stapes

Answer 6

The smallest bones in the body

The ossicles conduct vibrations of the tympanic membrane through the middle ear to the cochlea (the inner ear).

Question 7

Cochlear

Answer 7

Is coiled like snail shell
Contains:
Basilar and Reissner’s membranes
Scala vestibuli
Scala media
Scala tympani

Question 8

Basilar and Reissner’s membrane divide the tube into

Answer 8

Divide the tube (Cochlear) into 3 chambers called Scalae:
Scala vestibuli
Scala media
Scala tympani

Atop the basilar membrane lies the hair cell receptor organ for hearing (Organ of Corti)

Question 9

Scala vestibuli (dorsal)

Answer 9

Is filled with perilymph and connects at the large end to the oval window

Question 10

Scala media (middle, flexible)

Answer 10

The cochlear duct; contains endolymph

Question 11

Scala (ventral)

Answer 11

Contains perilymph and connects at its large end to the round window

Question 12

Number of cochlear turns

Answer 12

Guinea pig 4.0
Cow3.5
Dog 3.25
Cat 3.0
Human 2.75

Question 13

Apex

Answer 13

At the apex, the two perilymphs communicate via a small opening, the helicotrema

Question 14

Apex

Answer 14

At the apex, the two perilymphs communicate via a small opening, the helicotrema

Question 15

Organ of Corti

Answer 15

4 rows of hair cells (1 inner, 3 outer) throughout entire scala media

Most (~95%) of cochlear nerve endings synapse with inner cells (more important functionally than the outer cells)

The stereovilli of hair cells project into a collagen-containing tectorial membrane that is tightly attached to the basilar membrane on one edge

Question 16

Stria vascularis

Answer 16

Produces endolymph

Question 17

Transduction

Answer 17

The organ of Corti’s hair cells mediate transduction of sound waves into action potentials.
The hair cell receptors are similar in structure and function to those in the vestibular system.
The cells synapse on sensory neurons that form the cochlear portion of CN-VIII. The cell bodies of these neurons reside in the spiral ganglion.
Sound-induced bending of cilia changes the frequency of action potentials in CN-VIII fibers (N.B. the high [K+] in endolymph, role of K-channels in initiating depolarization)

Displacement of the stapes & the oval window moves fluid in the scala vestibuli, causing opposite fluid movement in the s. tympani and thus an opposite displacement of the r. window; the deflected pressure pushes the stapes/r.window towards the middle ear, t. vestibuli pressure falls and hence b. membrane is pushed up

Question 18

Steps in Transduction

Answer 18

1.Upward bowing of basilar membrane: tilts hair bundles toward longer cilia, opening channels.

2.Depolarization of inner hair cells causes enhanced transmitter release.

3. In outer hair cells, depolarization causes contraction of prestin, a motor protein
4. Downward movement of b. membrane tilts cilia away from longer ones, closing channels.

Question 19

Sound Transmission

Answer 19

A, Ossicular movement displaces the oval window of the fluid-filled inner ear, resulting in a traveling wave on the basilar memb

B, Basilar membrane is narrow and stiff at the base and wider and more flexible farther from the base.

C, As the frequency of a sound decreases, the region of maximum displacement of the basilar membrane, produced by the traveling wave, is located progressively farther from the base.
A region near the base (oval window) is significantly displaced by a high-frequency sound, and as the sound frequency decreases, the region of significant displacement is located progressively closer to the apex.

Question 20

High frequency

Answer 20

High frequency sounds are most likely to affect hair cells and nerves near the base

Question 21

Low frequency

Answer 21

Low frequency activates hair cells and neurons that are located progressively closer to the apex.

Question 22

Tonotopic Organization

Answer 22

Tonotopic organization refers to the orderly relationship between the frequency of a sound and the region of the cochlea that is activated by that frequency

Question 23

Auditory pathways

Answer 23

Action potentials travel along cochlear branch of CN-VIII to cochlear nuclei (medulla).

Then to the superior olivary nucleus (pontomedullary border), inferior colliculus (the midbrain), medial geniculate nucleus (thalamus), and finally to the auditory cortex (temporal lobe).

Conscious perception of sound occurs in the cerebral cortex.

Question 24

Deafness

Answer 24

Conduction deafness

Nerve deafness

In animals, deafness is usually congenital; some drugs are also ototoxic.

Question 25

Conduction deafness

Answer 25

Loss of sound transmission in outer or inner ear

Question 26

Nerve deafness

Answer 26

Malfunction of the cochlear hair cells or CN-VIII fibers

Cochlear implants for problems distal to CN-VIII (animals as models)

In 2009, 25,000 US children received implants