Audition

Fox, part of Chapter 10 and one figure in chapter 8

Physics of Sound

(not all of this is in the book)
Intensity dB = 20 log (pressure 1/pressure2)
standard is 0.0002 dynes/cm2
Threshold amplitude of vibration is 10 to the -11 m (10 pico meters)

waves of compressions and rarefactions of air (must have medium) described by sine wave
Frequency Hz cycles per sec
vibration - 20 - 20,000 Hz, above which is ultrasound .
Audibility curve - Intensity [dB] vs log (freq) [Hz] very sensitive

Ear

Fig. 10.18
Ear structure
pinna, eardrum=tympanic membrane, ossicles, cochlea, part of nerve VIII = cochlear nerve

Fig. 10.19
hammer, anvil, stirrup=malleus, incus, stapes - to match impedance of air -> fluid
Eustachian tube
oval window is "inner ear drum"
20:1 "amplification" tympanic to oval

Fig. 10.20
Since the cochlea is wound like a snail, a section through it shows repeated structures

Fig. 10.22
higher magnification, most importantly basilar and tectorial membrane
also inner hair cells and outer hair cells

Auditory transduction

hair cells on basilar and tectorial membranes
3,500 inner hair cells
many more outer hair cells
Bend as basilar membrane vibrates relative to tectorial membrane

(repeated from vestibular apparatus lecture)
kinocilium (real cilium, missing in post-natal human hair cells)
plus about 30 stereoocilia
mechanoreception assisted by tip links - depolarization if move toward kinocilium
hyperpolarize if in opposite direction
Threshold displacement is about 0.3 nm

go back to Fig. 10.20
perilymph is fluid of scala vestibuli and scala tympani is like CSF - bathes baso-lateral hair cell
High K+ in endolymph of scala media (bathing hairs)
stria vascularis (endothelium lining scala media) pumps ions to produce this unusual extracellular fluid
thus when channels open, K+ comes into cell
endocochlear potential endolymph 80 mV more + than perilymph

Frequency discrimination

At about 1000 Hz, you can tell the difference of a few Hz. This is explained by Helmholtz's place theory as modified by lateral inhibition as described in Bekesy's (1961) Nobel Prize winning work on "physical mechanism of stimulation within the coclea."

At low frequencies, frequency discrimination is better explained by Rutherford's telephone theory. Here, frequencies to both ears can cause neural impulses that stay true to the frequency so that beats can be from neural comparison from the two ears.

Fig. 10.21
Vibration of basilar membrane is mapped by tonotopy
fluid vibration at oval window through helicotrema
Low frequencies vibrate mostly near helicotrema
High frequencies vibrate mostly near oval window
But the localization is crude
Lateral nhibition refines localization on the way to brain so that cells in brain respond to only a few frequencies

Fig. 10.25
Frequency map on cortex

Frequency discrimination at low frequencies
there was another theory, Rutherford's "telephone" theory
phase-locking gives volley principle up to 4 kHz

Projection

Auditory nerve to dorsal and ventral cochlear nucleus - no crossing
Several synapses on the way to the Thalamus

Fig. 10.24
Thalamus to auditory cortex

Auditory localization

difference in time of arrival and intensity (in big headed animals) [human 700 micro sceond difference]
(speed of sound 1087 ft (331 m) / s in air)
Localization up and down does not rely on 2 ears,may relate to pinna
small-headed animals are extraordinary

Exam questions fron 2004 - 2008 that apply to this outline

Helmholtz's place theory for frequency discrimination refers to place on what membrane?

basilar

Because the perilymph cannot be compressed, pressure applied at the oval window is released where?

round window

If the pressure is 0.002 dynes/cm2, 10 times the standard for audition (0.0002 dynes/cm2), how many dB is the sound?

20

The stapes drives vibrations to what structure?

oval window

If two audio oscillators set to about 1000, playing through loud speakers, give 5 beats per second, what would you perceive if you heard one then the other sequentially?

you would hear distinctly different pitches

In addition to the vestibular sense, which utilizes hair cells where "hair" refers to stereocilia?

audition

For people, in addition to intensity of sound for one ear vs the other, what contributes to auditory localization?

time of arrival

What does "tonotopic" refer to with respect to organization of the cortical projection for hearing?

different frequencies project to different places in an organized way

A log unit is an order of magnitude, i.e. 10 x. How many dB per log unit?

20

Tip links between stereocilia contribute to channels responsible for what kind of stimulation?

mechanosensation, or hearing, possibly K+

If a certain sound is 20 dB louder than another sound, how many times as loud is it?

10 x (one log unit)

Why is it reasonable to propose that a sound might arrive at the two human ears at different times?

speed of sound is finite (slow) and head is big

Why was it useful to hold the tuning fork and loud speaker to one ear simultaneously before demonstrating frequency discrimination by holding them to one ear sequentially?

beats prove that two stimuli differ by only a few Hz

"Hairs" on hair cells bend when the basilar membrane moves with respect to what other membrane?

tectorial

A young human can hear frequencies from about 20 Hz up to about (what)?

20,000

In what way is K+ particularly relevant to auditory transduction?

K+-rich extracellular endolymph in scala media (secreted by stria vascularis) makes it so that, when channels open, K+ comes into cells

Helmholtz proposed that different frequencies stimulated different places along the basilar membrane. In what way(s) was his place theory confirmed or contradicted?

true, but localization is crude, lateral inhibition corrects for this

The vestibular apparatus shares a nerve to the brain with what special sense?

auditory

"There is tonotopic localization in the auditory cortex." Explain.

different frequencies at different locations in an order

"The audibility curve extends from about 20 to 20,000 Hz." What would be the most obvious difference (from this statement) among the people in the room where your physiology course was taught.

your professor would have an age related loss at higher frequencies

Why is the round window useful, in fact necessary?

to release pressures applied to oval window since fluid cannot be compressed

What is compared that would allow you to tell which side of your head a sound is coming from?

inputs from the two ears

How is it that K+ moves in, rather than out, through channels in auditory hair cells?

an unusual extracellular fluid (endolymph) is high in K+

About how many Hz is the just noticeable difference at 1000 Hz?

2

Twenty times the log of one pressure divided by 0.0002 dynes/cm2 tells us what property of sound?

intensity

There is a cut-off of (what? - give number and units) between "sound" (that humans can hear) and ultrasound (such as dogs and bats can hear).

20,000 Hz

"Helmholtz was correct in general but wrong in the details" (about frequency discrimination). How so?

Bekesy demonstrated that different freqencies stimulated different places, but more crudely than Helmholtz imagined. (need latreral inhibition to process)

The sound may arrive at one ear 700 microseconds before the other ear. Answer either (1) How can this be? Or (2) Why is that useful?

(sound has a finite (slow) speed considering the size of the head

Movement of the organ of Corti relative to (what adjacent membrane?) causes the "cilia" of auditory hair cells to bend.

tectorial

K+ comes in through the auditory cell channel. Now, wait a doggone minute. How did that happen?

b/c K+ is high, which is unusual, in the extracellular fluid (endolymph)

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