Auditory I-III

Question 1

How does sound work?

Answer 1

Sound radiates from vibrating sources (a tuning fork or the vocal cords of the larynx) as a series of pressure waves of alternating compression and rarefaction of air molecules.

Question 2

What is rarefraction?

Answer 2

Decreased air density (pressure)

Question 3

What is compression?

Answer 3

Increased air density (pressure)

Question 4

What is the intensity of a sound?

Answer 4

Loudness (the higher the intensity, the louder a sound is)

Increases when the air is compressed more forcefully during the peak compression in each cycle, resulting in increased density of air

Question 5

What is the unit of intensity of a sound?

Answer 5

dB SPL (decibels Sound Pressure Level)

0 dB SPL= threshold
20dB SPL= 10 times the pressure of threshold
40 dB SPL= 100 times the pressure of threshold
etc.

Question 6

What is frequency (in relation to sound)

Answer 6

Pitch

Number of times per second that a sound wave reaches the peak of rarefaction (or compression)

Question 7

What are the units of frequency?

Answer 7

Hertz (Hz; cylces/sec)

Question 8

How do audiologists quantify hearing loss?

Answer 8

For each ear, check different frequencies to see the lowest intensity (dB SPL) that a person can detect (threshold)

Question 9

Presbycusis

Answer 9

Common in elderly

Loss of high-frequency hearing

Especially problematic for the perception of speech since fricative consonants (such as t, p, s, f) are distinguished by high frequency components that fall in the upper end of the human audiogram.

Question 10

Transmission of sounds through the ear

Answer 10

Mechanical

Sound pressure waves reach the middle ear
Pressure changes move the tympanic membrane
Tympanic membrane pushes against the ossicles.

Question 11

External ear components

Answer 11

pinna and external auditory meatus (ear canal) bounded by the tympanic membrane

Question 12

Middle ear components

Answer 12

cavity containing the ossicular chain or 3 middle ear bones: malleus, incus and stapes

Question 13

Inner ear components

Answer 13

cochlea and the semicircular canals

Question 14

Impedance mismatch

Answer 14

Fluid is much more resistant to movement than air; in mechanical terms, water is said to have a high impedance, and air a low impedance.

Most of the sound’s energy (>99.9%) reaching an air-water interface is reflected back, and and

Question 15

How does the ear fix impedance mismatch

Answer 15

The middle ear!

The middle ear bones or ossicles, the malleus, incus and stapes, translate the airborne pressure waves into motion of the fluid of the inner ear.

P=F/A

The area of the foot of the stapes is much smaller than the tympanic membrane, so the smaller area means larger pressure amplitude.

The orientation of the middle ear bones confers a levering action resulting in a larger force (a gain of about 1.3:1)

Question 16

Conductive hearing loss

Answer 16

Mechanical transmission of sound energy through the middle ear is degraded.

Causes:

1) filling of the middle ear with fluid during otitis media (i.e., ear infection)
2) otosclerosis, in which arthritic bone growth impedes the movement of the ossicles
3) malformations of the ear canal (atresia), including “swimmer’s” and “cauliflower” ear
4) perforation/rupture of the tympanic membrane
5) interruption of the ossicular chain
6) static pressure in middle ear

Question 17

Sensorineural hearing loss

Answer 17

Occurs from damage to or the loss of hair cells and or nerve fibers

Causes:

1) excessively loud sounds
2) exposure to ototoxic drugs (diuretics, aminoglygocide antibiotics, aspirin, cancer therapy drugs)
3) age (presbycusis)

Question 18

How can you test if hearing loss is sensorineural or conductive?

Answer 18

Compare the audibility of a 512 Hz tuning fork held in the air or pressed against the skull.

In conductive hearing loss fork against bone is effective at presenting sound by bone conduction, thus overcoming the conductive loss that pertains to air-borne sound.

Question 19

3 compartments of the cochlea

Answer 19

scala vestibuli, scala media and scala tympani

fluid-filled, membranous compartments

Question 20

Basilar membrane

Answer 20

separates the scala media and the scala tympani

Question 21

Organ of Corti

Answer 21

Sits on top of the basilar membrane, within the scala media

Contains the inner hair cells that transduce sound into electrical signals

Question 22

Helicotrema

Answer 22

A hole in the BM located at the apex of the cochlea that connects the fluid filled scala vestibuli to the scala tympani. Relieves pressure.

Question 23

Tonotopic arrangement or tonotopic map of basilar membrane

Answer 23

Mechanical properties of the BM vary along the length of the cochlea. As a result the compression of the oval window sets a traveling wave that will reach a maximum amplitude at a certain location along the length of the BM.

At the base of the cochlea (the end near the oval and round windows), the BM is thinner, narrower and more rigid (inner hair cells here vibrate best to high frequency sounds), while at the apex the BM is more flexible, wider and thicker (inner hair cells here vibrate best to low-frequency sounds).

Question 24

What is the primary stimulus attribute that is mapped along the cochlea

Answer 24

Frequency because each IHC will respond best to a certain frequency determined by the mechanical properties of the BM at that particular location

Question 25

Inner hair cell

Answer 25

Convert mechanical vibration into membrane potential changes

Stereocilia project out from apical surface of inner hair cells

Question 26

Activation of hair cells

Answer 26

When the stereocilia bundle is pushed in the direction toward the longest stereocilia, the membrane potential depolarizes; when the bundle is pushed in the direction toward the shortest stereocilia, the potential hyperpolarizes.

Bending of the stereocilia results in altered gating of transduction channels located near the tips of the individual hairs. The transduction channel is a non-specific cation channel that is voltage-insensitive.

Question 27

Endolymph

Answer 27

K+-rich fluid that fills the scala media and bathes the stereocilia on the apical end of hair cells

Question 28

Perilymph

Answer 28

A fluid with ionic composition similar to blood (high Na, low K+) that fills the scala vestibuli and scala tympani and bathes the basal end of the cell

Question 29

Endocochlear potential

Answer 29

The positive potential inside the scala media due to active pumping of K+into the endolymph by the stria vascularis

Magnitude of +80 mV (endolymph positive with respect to perilymph).

Provides further driving force for influx of K+ ions into the cell. In essence the endocochlear potential is added to the cell’s membrane potential so that the total driving potential across the stereocilia membrane is a whopping –130 mV (the membrane potential, -50 mV, minus the endocochlear potential, +80 mV)

Question 30

What ion underlies hair cell depolarization?

Answer 30

K+ influx into hair cells through open mechanotransduction channels

Question 31

What does A collapse of the endocochlear potential cause?

Answer 31

sensorineural deafness because of the loss in driving force for transduction

Question 32

Tip links

Answer 32

Tiny, thread-like, connections between adjacent stereocilia.

Bending the stereocilia bundle in the direction of the longer stereocilia causes tip-links to pull on the tops of the stereocilia, mechanically opening the channels causing a depolarization of the hair cells.

Question 33

How does the motion of the BM translate into displacement of the hair bundles?

Answer 33

BM and tectorial membrane are attached at different points on the cochlear wall and have different pivot points. Up-down motions of both membranes occur as the folding of a parallelogram, resulting in lateral shearing forces between basilar and tectorial membranes.

Hair cells are attached to both the basilar and tectorial membranes, so the ciliary bundles are pushed from side to side as a consequence of the shearing force.

Can reliably signal a bundle displacement of only a few angstroms!

Question 34

Auditory nerve fibers

Answer 34

(Part of the cranial VIIIth nerve)

Cell bodies are located in the spiral ganglion

Send an axon centrally to end on cells in the cochlear nucleus of the brainstem

Basal end the hair cell is contacted by auditory nerve fibers

Question 35

What happens once a hair cell is depolarized?

Answer 35

When the hair cell is depolarized by opening of the transducer channel, voltage dependent calcium channels in the basolateral membrane are opened and synaptic vesicles release glutamate causing excitation of the afferent axon. If the afferent is depolarized enough, an action potential is initiated.

Question 36

Step by step of sound to action potential to brainstem happens

Answer 36

1. Airborne pressure waves in the external ear canal set up vibrations of the eardrum
2. Eardrum vibrations move the 3 ossicles
3. Vibrations of the stapes on oval window set up traveling waves in the cochlear fluids
4. These fluid waves cause a vertical displacements of the basilar and tectorial membranes
5. The relative shearing force between membranes bends the ciliary bundles of the hair cells
6. Ciliary bending leads to depolarization and hyperpolarization of the membrane potential
7. Causes increased and decreased rates of transmitter release, respectively
8. Transmitter (aspartate or glutamate) causes depolarization of the afferent auditory nerve fiber
9. Results in action potentials that are sent to second order neurons in the brainstem.

Question 37

Type I vs Type II auditory nerve fibers (ANF)

Answer 37

Type I ANFs innervate the IHCs and are myelinated (95%)

Type II innervate the OHCs and are not myelinated (5%)

Question 38

Outer hair cells

Answer 38

The “cochlear amplifier”

Have stereocilia that possess mechanotransducing channels

Poorly innervated by afferent ANFs, so NOT transducers. Instead, the efferent innervation from the central auditory system act upon OHCs to amplify the movements of the BM.

OHCs respond to changes in voltage with a change in length – they are “electromotile”.

Because the OHCs are attached to the BM, the change in length of the OHC pulls the BM toward or away from the tectorial membrane, and thus changes the mechanical frequency selectivity of the BM. The OHCs are though to contribute up to 50 dB of the cochlea’s sensitivity to sound!

Question 39

What is the clinical significance of outer hair cells (OHCs)?

Answer 39

Because of their susceptibility to damage by ototoxic antibiotics and prolonged exposure to loud sounds

Streptomycin and gentamycin, can block the transduction channel of the OHCs and, with prolonged action, can kill them resulting in deafness.

Question 40

Frequency tuning curve

Answer 40

The sound response of single ANFs

Number of action potentials fired per sec is plotted as a function of sound frequency

The peak of this function is the “characteristic frequency” of sound to which that fiber is maximally sensitive; higher and lower frequencies results in fewer action potentials per second.

Question 41

Response properties of ANFs

Answer 41

Frequency of sound is partly encoded by the place along the cochlea where the afferent fiber innervates an IHC.

And within a neuron’s frequency tuning curve, sound intensity is encoded via increases in the rate at which the neuron fires action potentials as the SPL is increased

Question 42

Phase locking

Answer 42

Temporal coding of timing

We use the temporal pattern of action potentials in ANFs to determine the “pitch” of sounds with frequencies below ~1 kHz.

Neurons tend to fire action potentials only at particular phases (i.e., compression or rarefaction) of the ongoing sound waveform

Results directly from the oscillating membrane potential of the IHCs (Fig 6) and the tendency for IHCs to release excitatory neurotransmitter only during the depolarizing phase

Question 43

Pathway of ANF

Answer 43

Bifurcate upon entering the brainstem: one branch innervates the ventral cochlear nucleus (VCN), and the other the dorsal cochlear nucleus (DCN).

Some axons from cells in the cochlear nucleus cross the midline to the opposite side of the brain in the dorsal acoustic stria (from DCN) and trapezoid body (from VCN). These tracts regroup as the lateral lemniscus and ascend to the inferior colliculus of the midbrain.

Along the way, many axons terminate in various nuclear complexes in the pons: the several nuclei comprising the superior olivary complex and the nuclei of the lateral lemniscus.

Question 44

Where are the ventral cochlear nucleus and the dorsal cochlear nucleus found?

Answer 44

The dorsal and lateral aspects of the inferior cerebellar peduncle

Question 45

Menier’s disease

Answer 45

VERY COMMON

Associated with endolymphatic hydrops- a type of swelling of the endolymph compartment of the inner ear.

Clinical Presentation (high yield!!!) involves multiple episodes of definitive vertigo lasting 20 minutes or longer, hearing loss, tinnitus, and a feeling of aural fullness. A variety of audiometric configurations of hearing loss on audiology may be seen, but a rising, or come and go, pattern is most common. We don’t know the exact cause, and time course is unpredictable. Treat with low salt diet and diuretics early on.