Hearing (Dr. Gale)

Question 1

What is sound ?

Answer 1

It is the varying distribution of air molecules in space over time about a fixed point in space.
Molecules affect neighboring molecules giving rise to propagation of the sound via successive compressions and dilatations of air layers.

Question 2

How is pitch measured ?

What about loudness ?

Answer 2

By frequency (in Hz) and loudness by amplitude (usually in mV).

Question 3

What is the human hearing range and in what interval is the human ear most sensitive ?

Answer 3

The human hearing range is 20-20000Hz.

The human ear is most sensitive to 1000-4000Hz.

Question 4

What is sound pressure level ?

Answer 4

Sound pressure level = 20log(Pt/Pr) dB
Pt = test pressure
Pr = reference pressure = 20μPa = threshold (just audible) sound pressure)

Question 5

How does sounds pressure level vary w/ respect to Pt ?

Answer 5

For every time Pt=10Pr, sound pressure level increase by 20.

Question 6

What is the sounds pressure of rustling leaves ?
A normal converstation ?
A loud rock group ?

Answer 6

Rustling leaves = 25dB
Normal conversation = 70dB
Loud rock group = 130dB

Question 7

What are the three main divisions of the ear ?

Answer 7

The outer ear = pina + auditory canal
The middle ear = tympanic membrane (or eardrum), ossicles, middle ear muscles + eustachian tube
Inner ear = vestibular labyrinth, oval window, cochlea + auditory vestibular nerve

Question 8

What is the role of the outer ear ?

Answer 8

It gathers the sound and thus transforms the sounds pressure at the tympanic membrane, amplifies it (by 10-15dB for 1.5-7Hz) and filters it, thus providing aid in sound localization.

Question 9

What are the 3 ossicles ?

Answer 9

The malleus, incus and stapes.

Question 10

What are the ossicles’ roles ?

How is this done and what is the system’s efficiency ?

Answer 10

Lever action and pressure amplification.
The difference in area between the tympanic membrane and the oval window compensates for impedance mismatch between air and cochlear fluid.
This provides up to 30dB increase in sound at 2.5kHz.

Question 11

How can the ossicles prevent ear damage ?

Answer 11

By reflex contraction, a mechanism that reduces ossicle mvnt. This only works however when sounds get progressively louder and will not help against an abrupt loud noise e.g. car honk

Question 12

How do we call middle ear damage compared to inner ear damage ?

Answer 12

Middle ear damage is termed conductive deafness whereas inner ear damage is termed nerve deafness.

Question 13

Describe the basic anatomy of the cochlea.

Answer 13

The cochlea is a coiled structure. In humans, it is 1cm wide, 5cm high and if uncoiled would measure about 3.5cm. It connects to the stapes at the oval window.

Question 14

What are the 3 compartments (or scalae) of the cochlea ?

Answer 14

• a basilar membrane (BM) that runs along its full length
• an oval window: when the footplate vibrates, the cochlear fluid is set into motion
• a round window - fcts as the pressure relief port for the fluid set into motion initially by the mvnt of the stapes in the oval window

Question 15

What does the cochlea do ?

Answer 15

It has 3 main fcts: it splits complex sounds (like speech) into simple components, it amplifies the signal and it converts or traduces mechanical vibrations into APs i.e. sensory transduction

Question 16

How does the cochlea separate frequencies ?

Answer 16

The BM w/in the cochlea is “tuned”: it goes from narrow, thick and stiff at the base to wider, thiner more flexible at the apex (the “center” of the coil).
Thus, frequencies around 20kHz are detected at the base and frequencies around 20Hz are detected at the apex.

Question 17

What is the Traveling Wave and who discovered it ?

Answer 17

The stapes vibration displaces he cochlear fluids and causes the BM to vibrate up and down. This was discovered by George von Békésy, for which he won the Nobel Prize in medicine or physiology in 1961. (“for his discoveries of the physical mechanism of stimulation within the cochlea”).

Question 18

Where are the sensory hair cells situated w/in the cochlea ?

Answer 18

In the organ of Corti.

Question 19

Consider a horizontal (or axial) cross section of the cochlea.
What are the distinct regions you can observe ?

Answer 19

The scala vestibuli and scala tympani, contaning the perilymph, the scala media, containing the endolymph, as well as the tectorial membrane and the organ of Corti (thus the HCs).
The BM separates the scala media from the scale tympani, and Reissner’s membrane separates the scale vestibuli form the scala media.

Question 20

How many Auditory Nerve Fibres (ANFs) are there in humans ?

What types are they ?

Answer 20

About 30 000.
95% of these are type I, myelinated, synapse w/ IHCs, up to 20 different afferents per IHC, 1 IHC per afferent
5% are type II, unmyelinated, synapse w/ OHCs, 1 afferent for 5-100 OHCs
The cell bodies of all these neurons are in the spiral ganglion

Question 21

Are there only sensory fibres in the inner ear ?

Answer 21

No, there are also efferent fibres that synapses w/ OHCs and w/ afferent fibre terminals.

Question 22

How many IHCs and OHCs are there in the human cochlea ?

Answer 22

About 3500 IHCs and 12000 OHCs.

Question 23

What is Place Coding ?

Answer 23

This principle is based on the fact that there is a place on the BM at which each freq is processed. IHCs receptor potentials are driven by BM mvnts, and type I ANF are connected to IHCs. Thus, ANF have the same tuning as the part of the BM to which they are connected, which allows the tonotopic map to be carried into the central auditory system.

Question 24

How are the ANF tuned ?

Answer 24

ANF are sharply tuned: each fibre shows a preference for a specific frequency (shown by lowering the amplitude of signal and seeing to which freq the ANF responds).

Question 25

What is phase-locking ?

How is this limited ?

Answer 25

An ANF can be phase locked w/ the IHC to which it is attached when its APs follow the IHC receptor potentials.
This ability to phase lock starts declining at 2kHz and disappears above 3kHz.

Question 26

How is the central auditory cortex mapped ?

Answer 26

Selective innervation patterns of afferent neurons provide “labelled lines” for a tonotopic map in the brain, w/ the part responding to the apex of the cochlea anterior, and the part responding to the base of the cochlea posterior.

Question 27

How does the brain detected increased SPLs ?

Answer 27

By the increasing firing rate of IHCs and thus ANFs.

Question 28

How can the cochlea amplify the signal it receives ?

Answer 28

OHCs aplify BM vibrations, enhancing sensitivity and freq selectivity
RPs drive length changes in OHCs
This active process is made evident in otoacoustic emmission.