4.2 Audition

Question 1

What is detected:Sound waves (2)

Answer 1

<ul> <li>When sound is produced, it causes relative compression of the air, with a sinusoidal change in air pressure</li> <li>This wave travels at the speed of sound, and we can detect variations in air</li></ul>

Question 2

Structure (1)Outer ear

Answer 2

Consists of pinna, concha, and auditory meatus

Question 3

Function (2)Outer ear

Answer 3

<ul> <li>Auditory canal propagates to tympanic membrane at middle ear</li> <li>Shaped to attenuate and amplify certain frequencies of sound (Preferential 2000-3000Hz)</li></ul>

Question 4

Structure (2)Middle ear

Answer 4

“<ul> <li>Consists of membranes and bones</li> <li>Tympanic membrane are attached to ossicles, 3 bones (Malleus, incus, stapes), which are a system of levers which move when tympanic membrane vibrates</li></ul><div><img></img><br></br></div>”

Question 5

Function (2)Middle ear

Answer 5

<ul> <li>Transfer vibrations efficiently from tympanic membrane (air) to the oval window of the cochlea (liquid) by amplifying pressure</li> <li>Tympanic membrane absorbs energy from soundwaves and vibrates, transfers to ossicles and then to oval window</li></ul>

Question 6

Rationale (1)Middle ear

Answer 6

It is difficult to transfer energy from 1 medium to another without energy loss, but this system in the middle ear manages to do efficiently (Tympanic membrane and ossicles decrease ROM, but increases force i.e., stapes moving less with a lot more force)

Question 7

Inner earStructure (2)

Answer 7

<ul> <li>Consists of the cochlea and semicircular canals (both fluid filled)</li> <li>Stapes contacts the oval window of the cochlea</li></ul>

Question 8

Structure?Cochlea

Answer 8

<ul> <li> <div>The cochlea is a tube of fluid which spirals 3.5 times around the malleolus bone</div> </li> <li> <div>3 liquid filled chambers</div> <ul> <li>Scala Vestibuli</li> <li>Scala Media high K+ concentration (cochlear duct), containing the basilar membrane</li> <li>Scala Tympani low K+ concentration</li> </ul><div><br></br></div> <div><br></br></div> </li></ul>

Question 9

Organ of Corti. Where is it located? And what does it have?

Answer 9

<ul> <li>Sits on top of the basilar membrane with many mechanically supporting cells (includes sensory receptor cells)</li></ul>

Question 10

What do inner and outer hair cells have? <br></br><br></br>They have ____, which are ____________ and where ______. Its ___________ contacts the __________

Answer 10

<ul> <li>Inner and outer hair cells have stereocilia, which are membrane covered microtubular structures and where transduction occurs</li> <li>Its cilia from hair cells contacts tectorial membrane</li></ul>

Question 11

Inner (2) vs outer (2) hair cells

Answer 11

<ul> <li>Inner hair cells? (2) <ul> <li>A single row of inner hair cells (sensory) is on innermost part of Organ of Corti</li> <li>Auditory nerve comprised of 95% afferent fibres from inner hair cells (inner to brain)</li> </ul> </li> <li>Outer hair cells? (2) <ul> <li>3 rows of outer hair cells (non-sensory but motor) are on the outermost part of Organ of Corti</li> <li>Auditory nerve comprised of 5% efferent fibres to outer hair cells (brain to outer)</li> </ul> </li></ul>

Question 12

When stapes is vibrated, where does it go to?

Answer 12

<ul> <li>Vibration from the stapes is transferred into the fluid of scala tympani, then transferred into scala vestibule</li> <li>This causes vibration in Scala Media, and the movement of the basilar membrane</li></ul>

Question 13

What is Spectral Decomposition? (i.e., How is freuqency of sound detected?)

Answer 13

Frequency of sound is detected by location of vibration on the basilar membrane, NOT vibration frequency (2)<br></br><div><br></br></div><div>Location of vibration along the basilar membrane depends on the frequency of incoming sound (2)<br></br></div>

Question 14

Location of vibration along the basilar membrane depends on the frequency of incoming sound. How are the various frequencies represented on the basilar membrane? (2)

Answer 14

<ul> <li>High frequencies = Basilar membrane vibration at the base (where basilar membrane is short, narrow, thick)</li> <li>Low frequencies = Basilar membrane vibration at the apex (where basilar membrane is long, wider, thin)</li></ul>

Question 15

Frequency of sound is detected by location of vibration on the basilar membrane, NOT vibration frequency (2). Neuronal Firing

Answer 15

<ul> <li>However, at <1000Hz, neurons fire at same AP as vibrations (bonus)</li><li>1000Hz, neurons cannot fire APs at the rate</li></ul>

Question 16

How do hair cell mechanics initiate auditory transduction (2)

Answer 16

“<ul> <li>The basilar membrane and tectorial membrane have different ‘pivot points’ so that there is relative tangential motion between them</li> <li>Travelling wave is translated into shearing motion between the basilar membrane and the tectorial membrane (2 membranes shearing relative to one another), bending stereocilia protruding from hair cells<br></br><br></br><img></img><br></br></li></ul>”

Question 17

Hair cell mechanics: Mechanoelectrical transduction

Answer 17

“<ul> <li> <div>When K+ channels are mechanically opened, K+ flows into cells, depolarising them, and go out of the cells in basal region due to concentration gradient</div> <ul> <li>Apical surface of hair cells: High K+ concentration</li> <li>Basal surface of hair cells: Low K+ concentration</li> </ul> </li> <li> <div>This depolarization also casues Ca2+ to enter ICF</div> <div><br></br></div> </li><li><div>Thus, glutamate release, exciting an auditory afferent<br></br><br></br><img></img><br></br></div></li></ul>”

Question 18

In audition, why is the primary ion K+?

Answer 18

“K+ moves into inner hair cells because there is a high ECF [K+] in the scala media relative to inner hair cell ICF<br></br><img></img><br></br>”

Question 19

Response tuning of auditory nerve fibres (2)

Answer 19

“<ul> <li>Broad tuning curve</li> <li>Collective behaviour of all hair cells allow us to have the resolution we have for hearing<br></br><br></br><img></img><br></br></li></ul>”

Question 20

What lets us detect pitch, loudness, tone (In terms of auditory function)

Answer 20

<ul> <li>Pitch: Auditory nerve fibres</li> <li>Loudness: Amplitude of Vibration</li> <li>Tone: Spectral decomposition</li></ul>

Question 21

Central auditory pathways (1)

Answer 21

“Auditory pathway is essentially bilateral beyond the cochlear nucleus.<br></br><br></br><img></img><br></br>”

Question 22

Localising soundsMethod 1 (2)

Answer 22

“<ul> <li> <div>Method 1 (2)</div> <div>Arrival Times</div> <ul> <li>Sounds reaching the two ears from a distant source differ in time of arrival. These differences increase as the sound source moves further from the midline and allow the sound source to be localised in space.</li> <li>Different arrival times of sounds can be detected by neurons<br></br><br></br><img></img><br></br></li> </ul> <div><br></br></div> </li></ul>”

Question 23

Localising soundsMethod 2 (4)

Answer 23

“<div>Sound Intensity</div> <ul> <li>Sound intensity one 1 side of the head will be different to the other side of the head (Head blocks sound)</li> <li>More useful for high frequency sounds</li> <li>Circuits in the brain stem compare the intensity of sounds arriving from the same source</li> <li>Small inhibitory circuits amplify these intensity differences<br></br><br></br><img></img><br></br></li></ul>”

Question 24

Cortical representation of auditory input is… (4)

Answer 24

“<ul> <li>Tonotopic map of the frequencies in the primary auditory cortex (like a map of the basilar membrane)<br></br></li><li>Cortical representation of auditory input is asymmetric: Degree of asymmetry is on average greater in men than women</li> <li>½ of the map covers 1-2000Hz, ½ the map covers 2000-16000Hz</li> <li>Humans have large cortical representation of sounds between 1000-2000Hz because this corresponds to the frequencies of human speech<br></br><br></br><img></img><br></br></li></ul>”