Lecture 11 - How do we hear?

Question 1

What type of technique has been used to understand a large part of how the sensory neurons in the brain function?

Answer 1

Extracellular microelectrode recordring

Question 2

How do sound waves affect air pressure?

Question 3

What are three characteristics of sound waves that reflect our perception of sound?

Answer 3

Amplitude is perceived as volume.
Frequency is perceived as pitch.
Complexity is perceived as timbre.

Question 4

Explain how sound waves are processed in the middle and inner ear and translated into neural signals the brain can use and interpret as sound?

Question 5

What makes up the middle ear?

Answer 5

The middle ear is made up of the tympanic membrane and the ossciles.

Question 6

What makes up the inner ear?

Answer 6

The inner ear consists of the cochlear, which in turn contains the basilar membrane, which is the structural basis for the hair cells and tectorial membrane.

Question 7

Which of the ossicles pushes on the oval window of the cochlear?

Answer 7

The stapes.

Question 8

What are ossicles?

Answer 8

The tiny bones of the middle ear.

Question 9

What are the two soft membrane spots on the cochlear called and how do they aid the processing of sound to neural signals?

Answer 9

They are known as the oval window and the round window.

Question 10

What is the membrane in the cochlear called that is the first membrane to translate physical movement into a neural signal?

Answer 10

The tectorial membrane.
The tectorial membrane is moved by the movement of the fluid in the cochlear . It is contact with the sterocilia of the hair cells. The stereocila are linked together in such a way that if they are moved one way they will close off an ion channel, but if they move another way ion channels will be opened. If they are opened then they cause the release of neurotransmitters from the hair cells, which in turn causes action potentials in the spiral ganglion cells.

Question 11

What is the difference in inner and outer hair cells?

Answer 11

Multiple outer hair cells contribute to one spiral ganglion cells.
Multiple spiral ganglion cells receive information from one spiral inner hair cells.

Question 12

What are spiral ganglion cells?

Answer 12

Spiral ganglion cells are the primary neurons in the inner ear that convey sound signals to the brain.

Question 13

How is the basilar membrane coded?

Answer 13

By frequency.
Areas of the tympanic membrane are most responsive to different frequencies of sound. This means that hair cells along the basilar membrane will only move and cause the generation of action potentials in spiral ganglion cells if the sound frequency enters the ear that corresponds to their location along the basilar membrane.

Question 14

How do the hair cells and spiral ganglion cells convey pitch and volume to the brain?

Answer 14

Pitch/frequency is conveyed to the brain via the position of the hair cells and spiral ganglion cells along the basilar membrane.
Volume is conveyed by the number of action potentials sent by the spiral ganglion cells. The number of action potentials is determined by the force of the sound waves received.

Question 15

Do the signals received from one ear get processed by one or both sides of the brain?

Answer 15

The signals that are generated in one ear get processed by both sides of the brain.

Question 16

The destination of signals generated in the auditory nerves is the primary auditory cortex.
How is the primary auditory cortex organised?

Answer 16

The primary auditory cortex is organised in a tonotopic manner. This means that the neurons are organised to represent different frequencies of sound, just as the basilar membrane is organised in a tonotopic manner.

Question 17

Can people with damage to their spiral ganglion cells have their hearing (partially) restored by a cochlear implant?

Answer 17

No. Cochlear implants work by stimulating spiral ganglion cells and causing them to generate action potentials which can be send down the auditory nerve to the primary auditory cortex.
If there is damage to the spiral ganglion cells then a cochlear implant will not be appropriate/work. Cochlear implants are only appropriate for those who have damage to their hair cells.

Question 18

How does a cochlear implants work?

Question 19

For complex sounds a lot of electrodes are required for cochlear implants.

Question 20

The range of frequencies required for understanding speech is quite low. What does this mean for the number of electrodes required for a cochlear implant for understanding conversations?

Question 21

How will an older person with congenital hearing loss respond differently to a cochlear implant compared to a younger person with congenital loss of hearing?

Question 22

Cochlear implants have improved. What does this mean for the range and complexity of sound people are able to hear with these implants?

Question 23

What is the tympanic membrane? What is its common name? What is it’s function?

Answer 23

The tympanic membrane is the membrane separating outer ear from middle ear and is part of the middle ear. It is known as the Ear Drum and translates changes in air pressure/sound into movement that moves the ossicles, which in turn move the oval window, which in turn moves the cochlear fluid which causes the round window to bulge out.

Question 24

How is frequency mapped on the basilar membrane?

Answer 24

High frequencies are mapped to/represented at the base of the basilar membrane and lower frequencies are mapped to/represented at the apex of the basilar membrane.

Question 25

The physical qualities of sound are amplitude/intensity, frequency and complexity of the sound wave. These qualities are perceived by us as….?

Answer 25

Volume, pitch, and timbre respectively.

Question 26

The middle ear consists of what structures?

Answer 26

The tympanic membrane/ear drum, the ossicles (maleus, incus and stapes).

Question 27

The inner ear consists of what structures?

Answer 27

The cochlear which is made up of a rigid membrane with two some parts of the membrane known as the oval and the round windows. The oval windows is depressed by the stapes in response to sound waves that are carried from the tympanic membrane to the maleus to the incus to the stapes.
The movement of the fluid within the cochlear moves the tectorial membrane which in turn moves the stereocilia of the hair cells which in turn