Lecture 9, 10

Question 1

What is sound?
Amplitude?
Frequency?

Answer 1

Pressure weaves generated by vibrating air molecules
Acoustically complex was forms and inner ear acts like an acoustical prism (decomposes waveforms and gives us a readout), decomposing complex sounds into a myriad of constituent tones

Amplitude is the loudness of sound (Db)

Frequency is pitch or cycles per second (Hz)

Slide 1 lecture 9/10

Question 2

How does sound get amplified in middle ear?

How it moves through inner ear

Answer 2

Sound waves move tympanic membrane -> tympanic membrane moves the ossicles -> ossicles move over window membrane -> oval window motion moves fluid in the cochlea -> fluid movement in the cochlea causes a response in sensory neurons

Slide 2 lecture 10

Question 3

What does the middle ear do?

Answer 3

Middle ear functions to match the low-impedance airborne sounds to the higher-impedance fluid of the inner ear

Boosts pressure measured at tympanic membrane by ~200 fold by the time it reaches the inner ear

Slide 2 lecture 9

Question 4

What is the cochlea?

Answer 4

3 fluid filled chambers:
Scala vestibuli- low K high Na (similar to CSF)
Scala tympani- low K high Na (similar to CSF)
Scala media- low Na high K (like intracellular fluid)

Energy from sonically generated pressure waves is transformed into neural impulses

Slides 3-7 lecture 9/10

Question 5

What is the helicotrema?

Answer 5

Hole in the apex basilar membrane that connects the 2 scalae

Basilar membrane widens towards the apex and the membrane stiffness decreases from base to apex (base 100x stiffer)
Fluid movement bends basilar membrane at base that sets up a wave that travels toward the apex

Slide 4 lecture 9

Question 6

What is tonotopy?

High frequency and low frequency?

Answer 6

Tonotopy- systematic organization of sound frequency within an auditory structure

Low frequency -> low vibration -> little dissipation of energy -> propagation

High frequency -> high vibration -> dissipation of energy -> little propagation

Slide 6 lecture 9

Question 7

How do cochlear implants work?

Answer 7

They take advantage of the tonotopic arrangement of the auditory nerve fibers

Place out of electrodes in the scala tympani
Current is intercepted by nearby cochlear nerve fibers and action potentials sent to the brain

Slide 7

Question 8

What is the organ of Corti?

Answer 8

Slides 8-9 lecture 9/10

Where transduction happens

Auditory receptor cells convert mechanical energy into a change in membrane polarization
Hair cells form synapses with neurons the cell bodies of which are located in the spiral ganglion

Question 9

How does transduction happen in hair cells?

Answer 9

Critical event in transduction of sound into a neural signal is the bending of the stereocilia
Bend in one direction= depolarize
Other direction = hyperpolarize

A stiff filament tip link connects each channel to upper wall of adjacent cilium
Increased tension = open
Straight = partially open
Tension relief = close

Tip links pull on proteins

Slides 10-11 lecture 9/10

Question 10

What is the innervation of hair cells?

Ratios of cells, etc

Answer 10

3 outer hair cells : 1 inner hair cell
1 inner hair cell : 10 spiral ganglion cells
1 spiral ganglion cell : many outer hair cells

> 95% of the fibers in the auditory nerve heading to the brain comes from the inner hair cells

Slide 12 lecture 9/10

Question 11

How do outer cells amplify signals?

Answer 11

Sounds changes receptor potential and length of outer hair cells
Cochlear amplifier increases the peak movement of the basilar membrane 100 fold

Descending efferent input from brain can regulate outer hair cell shape and effect inner hair cell response and auditory sensitivity

Slide 15 lecture 9/10

Question 12

What are the 4 central auditory processes?

Answer 12

Auditory receptors in cochlea ->
Brain stem neurons ->
MGN ->
Auditory cortex

Slide 14-15 lecture 9/10

Question 13

How do spiral ganglion cells only respond to certain frequencies?

Answer 13

Spiral ganglion cells receive input from a single inner hair cells at a particular location on the basilar membrane so they fire APs only in response to sound within a limited range of frequency

Intense sound- basilar membrane vibrates with greater amplitude -> increased hair cell activation -> nerve fibers fire APs at greater rates

Slide 16 lecture 9/10

Question 14

What are the 2 ways frequency is represented in the CNS?

Answer 14

1. Tonotopy- systematic organization of characteristic frequency within an auditory structure
2. Phase locking in the response of nerve fibers

Different frequencies are represented at very low frequencies by phase locking; intermediate frequencies phase locking and tonotopy; at high frequencies tonotopy only

Slide 17 lecture 9/10

Question 15

What is the mechanism of sound localization?

Answer 15

The auditory brainstem nuclei mediate sound localization

2 different strategies used to localize horizontal position of sound sources

Slide 18 lecture 9/10

Question 16

What computes the location of a sound by interaural time differences (<3 kHz)?
What are the 5 steps?

Answer 16

The medial superior olive computes the location of a sound by interaural time differences (<3 kHz)

1. Sound reaches left ear first
2. AP begins traveling toward MSO
3. Sounds reaches right ear a little later
4. AP from right ear travels toward MSO
5. APs converge on an MSO neuron that responds most strongly of their arrival is coincident

Slide 19-20 lecture 9/10

Question 17

What computes the location of a sound by interaural intensity differences?
What are the 4 steps?

Answer 17

The lateral superior olive computes the location of a sound by interaural intensity differences

1. Stronger stimulus to left ear excites left LSO
2. Stimulus also inhibits right LSO via MNTB interneuron
3. Excitation from left side is greater than inhibition from right side, resulting in net excitation to higher centres
4. Inhibition from left side is greater than excitation from right side, resulting in net inhibition on right and no signal to higher centres

Slide 21 lecture 9/10

Question 18

What is the medial geniculate nucleus?

Answer 18

Medial geniculate nucleus in thalamus is relay for all ascending auditory information destined for the cortex
Neurons in this complex receive convergent inputs from spectrally and temporally separate pathways
Tonotopy of lower brainstem is maintained

Slide 22

Question 19

What is the inferior colliculus?

Answer 19

Perception of auditory space is somehow synthesized by circuitry of the inferior colliculus in the midbrain
Neurons in the auditory space map in the colliculus respond best to sounds originating in a specific region of space and have both a preferred elevation and horizontal location

Slide 22

Question 20

What is the auditory cortex?

Answer 20

Ultimate target if the afferent auditory information

Primary auditory cortex (A1) is locates on the superior temporal gyrus in the temporal lobe and receives point to point input from the thalamus, so it contains precise tonotopic map

Slide 23

Question 21

What is the place code?

Answer 21

Basilar membrane establishes a place code in which different locations of membrane are maximally deformed at different sound frequencies

Place code is when stimulating different places stimulates pitch

Slide 6 lecture 10