Chapter 13 - The Auditory System

Question 1

The external ear consists of four structures. Which?

Answer 1

1. Pinna
2. Concha
3. Auditory meatus
4. Tympanic membrane

Question 2

What is the most important function of the auditory meatus?

Answer 2

It selectively boosts the sound pressure 30- to 100-fold for frequencies around 3 kHz via passive resonance effects.

Question 3

What is the major function of the middle ear?

Answer 3

The major function of the inner ear is to match relatively low-impedance airborne sounds to the higher-impedance fluid of the inner ear. The term “impedance” in this context describes a medium’s resistance to movement.

Question 4

How much is the auditory signal strengthened in the middle ear?

Answer 4

The middle ear ensures transmission of the sound energy across the air-fluid boundary by boosting the pressure measured at the tympanic membrane almost 200-fold by the time it reaches the inner ear.

Question 5

How does the middle ear achieve its pressure amplification?

Answer 5

Two mechanical processes:

1. The first and major boost is achieved by focusing the force impinging on the relatively large-diameter tympanic membrane onto the much smaller-diameter oval window.
2. A second and related process relies on the mechanical advantage gained by the lever action of the ossicles.

Question 6

Mention a clinical test to determine whether hearing loss is due to conductive problems or to damage either to the hair cells of the inner ear or to the auditory nerve itself.

Answer 6

A Weber test uses a tuning fork placed against the scalp, which causes vibrations in the skull itself. The ossicles will conduct these vibrations onto the oval window despite tympanic membrane damage.

Question 7

What is the oval window?

Answer 7

An area of the cochlea where the bone is absent, and in which the stapes projects the vibrations to the cochlea.

Question 8

What is the round window?

Answer 8

An area of the cochlea where the bone is absent, and in which the sound vibrations exit the tube.

Question 9

What is the cochlear partition?

Answer 9

A flexible structure that bisects the cochlea from its basal end almost to its apical end.

Question 10

What are the names of the different fluid environments of the cochlea?

Answer 10

1. Scala vestibuli
2. Scala tympani
3. Scala media

Question 11

What is the scala vestibuli?

Answer 11

The scala vestibuli is the fluid environment in which the sound waves first enter. It becomes the scala tympani at the apical end. It contains perilymph. (Low K+ and high Na+, 0mV)

Question 12

What is the scala tympani?

Answer 12

The scala tympani is the fluid environment in which the sound waves travel from the apex of the cochlea toward the round window. The scala tympani contains perilymph. (Low K+ and high Na+, 0mV).

Question 13

What is the scala media?

Answer 13

The scala media is the fluid environment within the cochlear partition. It contains endolymph. (High K+, +80mV)

Question 14

Frequency tuning within the inner ear is attributable to…

Answer 14

It is attributable in part to the geometry of the basilar membrane, which is wider and more flexible at the apical end and narrower and stiffer at the basal end.

Question 15

Two membranes are embedded in the cochlear partition. Which?

Answer 15

1. Basilar membrane

2. Tectorial membrane

Question 16

The points responding to high frequencies are at the … of the basilar membrane.

Answer 16

base.

Question 17

The points responding to low frequencies are at the … o the basilar membrane.

Answer 17

Apex

Question 18

The cochlear hair cells are arranged in rows. How?

Answer 18

One row of inner hair cells and three rows of outer hair cells.

Question 19

Which row of hair cells are the actual sensory receptors?

Answer 19

The inner hair cells are the sensory receptors, and 95% of the fibers of the auditory nerve that project to the brain arise from this subpopulation.

Question 20

Current evidence tells us what about the outer hair cells?

Answer 20

Current evidence suggests the the outer hair cells play an important role in modulating basilar membrane motions and the they function as an important component of the cochlear amplifier.

Question 21

Describe the hair cell.

Answer 21

The hair cell is a flask-shaped epithelial cell named for the bundle of hairlike processes that protrude from its apical end into the scala media. Each hair bundle contains anywhere from 30 to a few hundred stereo cilia, with one larger kinocilium.

Question 22

The hair cell has a resting potential of..

Answer 22

Between -45 to -60 mV relative to the fluid that bathes the basal end of the cell.

Question 23

What happens in the hair cell when the hair bundle is displaced in the direction of the tallest stereocilium?

Answer 23

It causes more transduction channels to open, which causes an influx of K+ from the K+-rich endolymph.

Question 24

Depolarisation in the hair cell due to K+-influx causes..

Answer 24

It causes the opening of voltage-gated calcium channels, and calcium ions causes the release of neurotransmitter onto the auditory nerve endings. It also causes K+ channels located on the basal surface of the hair cell to open onto the perilymph. This causes K+ efflux and thus repolarisation.

Question 25

What is meant by: the receptor potential of the hair cell is biphasic?

Answer 25

It means that the hair cell can both become more likely to fire (depolarised) and less likely to fire (hyperpolarised), depending on the direction of the force. This is because some transduction channels are open at rest.

Question 26

Von Békésy’s model of cochlear mechanics was a passive one (the cochlea was taken post mortem), resting on the premise that the basilar membrane acts like a series of linked resonators, much as a concatenated set of tuning forks. More recent studies indicate that normal hearing depends on the activity of a biological amplifier within the cochlea. The rational for this is based on three observations:

Answer 26

1. The tuning of the auditory periphery, whether measured at the basilar membrane or recorded as the electrical activity of auditory nerve fibers, is too sharp to be explained by passive mechanisms alone.
2. At very low sound intensities, the basilar membrane vibrates 100-fold more than would be predicted by linear extrapolation from the motion measured at high intensities.
3. The ear can generate sounds under certain conditions.

Question 27

How are outer hair cells related to the cochlear amplifier?

Answer 27

1. The high sensitivity of auditory nerve tuning curves is lost when the outer hair cells are selectively inactivated.
2. Mutant mice lacking inner hair cells, although deaf, nonetheless produce otoacoustic emissions.
3. Isolated outer hair cells contract and expand in response to small electrical currents, thus providing a potential source of energy to drive an active process within the cochlea.

Question 28

The auditory nerve exits the ear and enters which major division of the central nervous system?

Answer 28

The brainstem

Question 29

The auditory nerve enters the brainstem, where it branches to innervate …

Answer 29

The three divisions of the cochlear nucleus.

Question 30

The auditory nerve enters the brainstem, where it branches to innervate the three divisions of the cochlear nucleus. The neurons in these nuclei give rise to several different pathways. These pathways bring auditory information to the following structures (5):

Answer 30

1. Superior olive (MSO) in the pons
2. Nucleus of the lateral lemniscus of the Pons-midbrain junction
3. Inferior colliculus of the Caudal midbrain
4. The medial geniculate complex of the thalamus
5. The primary auditory cortex in the dorsal temporal lobe.

Question 31

The ability to localise the horizontal position of sound sources is computed by the ..

Answer 31

medial superior olive (MSO) in the pons.

Question 32

How is horisontal sound localisation computed for sounds below 3kHz?

Answer 32

A given MSO neuron responds most strongly when the two inputs from the two ears arrive simultaneously, as occurs when the contralateral and ipsilateral inputs precisely compensate (via their different lengths) for differences in the time of arrival of a sound at the two ears. This is a model, not evidence.

Question 33

How is horisontal sound localisation computed for sounds above 3kHz?

Answer 33

The lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB) are involved in computing the differences in intensity of the sound, as sounds of these frequencies in theory should be high enough so that the human head begins to act as an acoustical obstacle.

Question 34

What is the role of the nuclei of the lateral lemniscus in auditory processing?

Answer 34

Their role is not yet known. Some cells in the lateral lemniscus nuclei signal the onset of sound, regardless of intensity or frequency.

Question 35

What is the role of the inferior colliculus in auditory processing?

Answer 35

1. Experiments in the barn owl, animal that is extraordinarily proficient at localising sounds, indicate that neurons in the inferior colliculus create a topographical representation of auditory space.
2. Many neurons in the inferior colliculus respond only to frequency-modulated sounds, while others respond only to sounds of specific durations. It might therefore be important in our ability to process sounds with complex temporal patterns.

Question 36

What is the role of the medial geniculate complex (MGC), in the thalamus, in auditory processing?

Answer 36

1. In bats it is the first station in the auditory pathway that shows a selectivity for combinations of frequencies.

Question 37

Divide the auditory cortex.

Answer 37

The primary auditory cortex (A1) and the belt areas.

Question 38

How is sound information organised in, and what are the functions of the auditory cortex?

Answer 38

The primary auditory cortex, which is organised tonotopically, supports basic auditory functions, such as frequency discrimination and sound localisation, and also plays an important role in processing of intraspecific communication sounds. In non-human primates.

Question 39

Which part of the auditory cortex do you know anything about..?

Answer 39

Wernickes area is considered a part of the belt areas of the auditory cortex.