Lecture 16: Hearing 2

Question 1

What are the various windows of the cochlea?

Answer 1

Oval window where the stapes footplate inserts into

Round window, covered in a membrane that acts as a pressure relief valve.

Question 2

What are the various fluids and compartments of the cochlear?

Answer 2

Scala vestibuli (Perilymph, Na heavy)

Scala media (Endolymph, K heavy)

Scala tympani (Perilymph, K heavy)

Question 3

What is the organ of corti?

Answer 3

Refers to the transduction structure of the cochlear.

• Tectorial membrane interacts with the stereocilia which sit on the basilar membrane

Question 4

What are the stereocilia on the basilar membrane?

Answer 4

IHC = Main transducers  (5000)
OHC = Amplify sound (15000)

Question 5

Describe the movement of sound through the ear:

Answer 5

The pinna collects sound and sends it through the external ear, vibrational waves cause the tympanic membrane to oscillate which is conducted through the ossicular tree. The stapes footplate oscillates in the oval window causing the perilymph of the scala vestibuli to transmit the waves and this travels down and around into the scala tympani and on to the round window where the pressure is essentially released. During this journey the vibrational energy causes the tectorial membrane to wave and interact with the stereocilia. Sounds transduction occurs at this point.

Question 6

What does movement of the stapes said to do?

Answer 6

Movement of stapes in oval window sets up travelling wave on organ of corti and basilar membrane

Question 7

Describe how the travelling waves dont activate the entire basilar membrane

Answer 7

The structure of the basilar membrane has physical properties that mean that depending on the Hz of the sound, the resonance will be greatest with the corresponding basilar membrane for that Hz.

Cochlea spatially tuned to different frequencies = Tonotopicity.

Base = High Hz
Apex = Low Hz

Decreased stiffness and increased mass towards the apex.

Question 8

Describe the mechanoelectrical transduction mediated by the hair cells:

Answer 8

The stereocilia of the hair cells are have linkages between the gated K ion channels, these are ratcheted tight by actin+myosin and this means as the hairs are moved radially by the tactorial membrane the voltage gated ion channels are opened and K influxes causing the cells to depolarize and neurotransmitter is released.

The hair cells return to position and the channels are closed. This is the cycle of depolarisation:repolarisation

NB K effluxes naturally and the cells repolarize. There are no ion pumps.

Question 9

What is a significant cause of hearing and balance disorders?

Answer 9

Abnormal fluid homeostasis is significant cause of hearing and balance disorders. i.e Menieres disease

Question 10

What innervates the hair cells?

Answer 10

Afferent nerve cells: Spiral ganglion cells

Type 1 innervate IHC (90-95% of nerve fibres)

Type 2 innervate OHC (5-10%)

Question 11

What do IHC do?

Answer 11

The dominant innervation of IHC indicates main sensory cell input to CNS

Question 12

What do OHC do?

Answer 12

Responsible for amplifying sound and tuning at low levels

• OHC are electromotile, contain prestin protein in cell membrane that allow them to contract and amplify the sound / tuning

Question 13

Do hair cells regenerate?

Answer 13

no

Question 14

Describe the auditory pathway:

Answer 14

Cochlea
Cochlear nucleus
Sup. olivary complex
Inferior colliculus
Medial geniculate body
Primary auditory complex

Question 15

What does the cochlear nucleus do?

Answer 15

Relay nucleus

Somatosensory integration

Question 16

What does the SOC do?

Answer 16

• Binaural integration

- Sound localization and detection speech in noise

Question 17

What is the function of the inf. colliculus

Answer 17

Sensory integration esp. for earing and vision enhances sound localisation

Question 18

What is the function of the medial geniculate body of thalamus?

Answer 18

Radiates to auditory cortex in Heschels gyrus of the temporal lobe

Question 19

Describe the Hz (Pitch) detection coding in the auditory nerve:

Answer 19

Frequency coding in the cochlea involves 2 mechanisms

• Spatial (Place) : Cochlea is tonotopically organized; Hz detected by spatial representation along length
• Time locking to sound periodicity: low Hz detected by temporal firing of neurons in time with frequency. (i.e the rate of neural firing is equal to the Hz of sound… doesnt work for higher pitches)

Question 20

How are the auditory nerve fibers tuned?

Answer 20

Auditory nerve fibres are sharply tuned to different Hz defined by location along the cochlea

Question 21

Describe what is meant by temporal coding for low Hz:

Answer 21

Hz analysis determined by neural response locked to stimulus cycle (phase locked). Because of nerve refractory period only useful for low Hz.

Hz extended by population response (Summation)

Question 22

How many neurons do each hair cell have?

Answer 22

Each hair cell have many neurons. This means that as one is in refractory the other can fire.

Question 23

How are cells in the primary auditory cortex mapped?

Answer 23

Tonotopically

Question 24

How is loudness coded?

Answer 24

Rate increases at tonotopic site, activity spreads along cochlea with intensity.

Question 25

Describe the binaural auditory process:

Answer 25

“Coincidence detector”

• Cells in sup. olive receive simultaneous inputs from both ears, time delay determines excitatory response and thus localization