Lecture 22: Structure and function of the ear - How do we hear?

Question 1

Q: What is the primary function of the ear in hearing?

Answer 1

A: The ear captures sound waves and transmits them to the brain, where they are interpreted as pitch, loudness, and location.

Question 2

Q: What are the three main parts of the ear involved in hearing?

Answer 2

A: The outer ear, middle ear, and inner ear.

Question 3

Q: How does the middle ear amplify sound?

Answer 3

A: Through the ossicles (tiny bones: malleus, incus, and stapes) which increase pressure by the surface area ratio (20:1) and lever mechanics.

=> This amplification is vital because the sound must move from air to the fluid-filled inner ear, requiring a pressure boost​.

Question 4

Q: What is the main function of the outer ear?

Answer 4

A: It funnels sound waves into the auditory canal, increases sound pressure at the eardrum, and helps with sound localization.

Question 5

Q: What role does the cochlea play in hearing?

Answer 5

Inner Ear (Cochlea):
A: It converts sound vibrations into electrical signals via hair cells, with different areas of the cochlea responding to different frequencies.

Question 6

Q: What determines the pitch of a sound?

Answer 6

A: The frequency of the sound waves, measured in Hertz (Hz).

Question 7

Q: How do we perceive loudness?

Answer 7

A: Loudness is perceived as sound pressure, measured in decibels (dB), and is amplified by the middle ear.

Question 8

Q: Why is the timing of sound important?

Answer 8

A: It allows the ear to detect the onset, duration, and change in sound waves, which is essential for understanding speech and rhythm.

Question 9

Q: How does the brain localize sound using interaural time differences?

Answer 9

A: By detecting the slight delay in sound arrival between the two ears, the brain can determine the direction of the sound.

Direction: The ear’s ability to locate sound is based on differences in interaural time and intensity. Sounds from the left, for instance, reach the left ear first, with the brain calculating time differences (milliseconds) to locate the source​​.

Question 10

Q: How does the brain use interaural intensity difference for sound localization?

Answer 10

A: Sounds coming from one side are louder in the nearer ear, and the brain uses this intensity difference to localize high-frequency sounds.

Question 11

Q: What is the Doppler Effect?

Answer 11

The Doppler Effect describes how the frequency of sound changes as the source moves relative to the listener. As a sound source approaches, the frequency increases (pitch rises), and as it moves away, the frequency decreases (pitch lowers). This effect is crucial for detecting motion, such as a moving vehicle or a predator​.

Question 12

Q: What is place coding in the cochlea?

Answer 12

A: Different regions of the cochlea respond to different frequencies, helping the brain determine the pitch of a sound

The cochlea, a fluid-filled structure, is lined with hair cells that respond to different frequencies, creating what is known as place coding. High frequencies stimulate the base, and low frequencies stimulate the apex​​.

Question 13

Q: What are the three physical properties of sound? .

Answer 13

A: Frequency (pitch), pressure (loudness), and timing (onset and duration)

Question 14

Q: What is spectral content in sound perception?

Answer 14

A: It refers to the frequency composition of sound, which helps distinguish different tones and pitches.
The frequency composition of sound helps to recognize specific tones and pitches.

Question 15

Q: What is a safe level of sound exposure for 8 hours?

Answer 15

A: 85 decibels (dB) for up to 8 hours. Prolonged exposure to sounds above this can cause hearing damage.

Question 16

How Loud Sounds Cause Hearing Damage

Answer 16

Prolonged exposure to loud sounds can cause hearing damage by:

Damaging hair cells in the cochlea: Hair cells in the inner ear are responsible for converting sound vibrations into electrical signals. Intense sounds (above 85 dB) can overstimulate and damage these hair cells, which do not regenerate, leading to permanent hearing loss.

Stress on auditory nerve fibers: Loud sounds can overstress the auditory nerve, reducing its ability to transmit sound signals to the brain effectively.

Physical damage to structures in the ear: Extremely loud sounds, especially sudden ones (e.g., explosions), can physically damage the eardrum or ossicles, impairing the ear’s ability to amplify sound.

Question 17

external, middle and inner ear features

Answer 17

external : pinna directs sounds waves inot the ear through ear canal and amplifying them and funneling them toward the tympanic membrane (eardrum)

middle : auditory ossicles—malleus, incus, and stapes—are three small bones in the middle ear that transmit air vibrations from the outer ear to the inner ear to be processed as sound

internal: vestibular apparatus, semicircular canals, oval window, nerves, cochlea?

Question 18

Q: How is sound transmitted from the outer ear to the inner ear?

Answer 18

1. Sound waves cause the tympanic membrane to vibrate, transferring energy to the ossicles (malleus, incus, stapes) in the middle ear.
2. The stapes vibrates the oval window, creating waves in the fluid-filled cochlea.
3. These waves cause movement in the flexible membranes in the cochlear duct
   => hair cells bend and ion channels open, triggering the release of neurotransmitters that generate electrical signals, which are then sent to the brain via the cochlear nerve.
4. Neurotransmitter release onto sensory neurones creates AP and travel through cochlear nerve to the brain

Question 19

Q: How do sound waves travel through the cochlear ducts?

Answer 19

A: Fluid waves generated at the oval window travel through the vestibular duct, and pressure is relieved at the round window through the tympanic duct, ensuring the continuous movement of sound through the cochlea.

Question 20

Q: How is excess sound energy dissipated in the ear?

Answer 20

A: Excess energy from the fluid waves in the cochlea is dissipated through the round window, preventing damage to the inner ear structures.

Question 21

Q: How does the middle ear amplify sound?

Answer 21

A: The middle ear amplifies sound by leveraging the size difference between the tympanic membrane and the oval window, increasing sound pressure before it reaches the fluid-filled cochlea.

Question 22

Outer ear structure

Answer 22

Pinna/ Auricle : entire external ear structure, which funnels sound into the ear canal. (protruding ear part, all on the side of the head) ==> Elastic cartilage: This makes up the structure of the pinna (the visible part of the ear).

Concha, = the dent in your ear next to the ear canal

Stratified squamous epithelium: Covers both the pinna and the auditory canal, providing a protective outer layer.
Hair follicles, sebaceous glands, and ceruminous glands: These are found primarily in the auditory canal.
Sebaceous glands produce oils that help moisturize the ear.
Ceruminous glands (modified sweat glands) produce earwax, which protects and cleans the ear canal, and helps to fight off infections.

Question 23

How is intensity coded

Answer 23

1) Firing rate of
neurons (change in
the firing rate of the
neurons most
sensitive to the
stimulus).
2) Number of
activated neurons.
(different sensitivity
of neurons with
equivalent
characteristic
frequencies)

Question 24

Interaural Time Difference (ITD):

Answer 24

Definition: The slight difference in time when a sound reaches one ear compared to the other.
Explanation: If a sound comes from your left, it will reach your left ear slightly earlier than your right ear. The brain uses this time difference to determine the direction of the sound.

Question 25

Interaural Intensity Difference (IID):

Answer 25

Definition: The difference in loudness (intensity) of a sound as it reaches both ears.
Explanation: Sounds from one side will be louder in the closer ear and softer in the far ear. The brain compares these differences to localize high-frequency sounds.

Question 26

Interaural Time Difference (ITD) + Interaural Intensity Difference (IID):

Answer 26

Both ITD and IID are processed by the brainstem to help us pinpoint the location of a sound in space.

ITD:
Sound from the left reaches the left ear first, activating neurons in the left cochlea. It then reaches the right ear, activating neurons slightly later. The brainstem (superior olive) processes this time difference to locate the sound’s direction.
==> For continuous tones, the time delay between ears changes based on frequency. For low frequencies (e.g., 400 Hz), ITD is clear because the wavelength is longer. However, for high frequencies (>1500 Hz), phase ambiguity occurs, making ITD less effective.

IID:
Explanation: For high-frequency sounds, there is a difference in loudness between the ears due to the head casting an “acoustic shadow.”

Diagram: At higher frequencies (e.g., 6000 Hz), sound waves are blocked more by the head, creating a noticeable intensity difference between the ears. At lower frequencies (e.g., 200 Hz), this effect is negligible as low-frequency waves are longer and can bend around the head easily.