53

Question 1

Path of sound from eardrum to cochlea?

Answer 1

Sound waves hit the tympanic membrane (eardrum), causing it to vibrate. The malleus, attached to the center of the eardrum, moves with it. The malleus is connected by ligaments to the incus, which connects to the stapes. The flat end of the stapes (faceplate) rests on the oval window, passing vibrations from malleus → incus → stapes → cochlea (inner ear).

Question 2

Function of tensor tympani muscle?

Answer 2

The tensor tympani pulls the malleus slightly to keep the eardrum taut. A stretched eardrum efficiently transmits vibrations to the ossicles. A relaxed eardrum would reduce hearing efficiency.

Question 3

How do ossicles amplify sound?

Answer 3

Ossicles act as a lever system with a fulcrum near the end of the eardrum. They don’t increase distance but increase force. The stapes moves ¾ as much as the malleus, but the force is increased by 1.3 times. Combined with the 17-fold smaller surface area of the stapes faceplate compared to the eardrum (3.2 mm² vs. 55 mm²), the total force is amplified about 22 times.

Question 4

Why is sound amplification important for cochlea?

Answer 4

Cochlear fluid has higher inertia than air, so more force is needed to move it. The ossicles and eardrum provide impedance matching, transferring 50–75% of sound energy into the cochlea, especially for 300–3000 Hz range where human speech occurs.

Question 5

What happens without ossicles and eardrum?

Answer 5

Sound could still reach the cochlea through air in the middle ear, but much less efficiently. You’d hear things 15–20 decibels quieter—like the difference between normal speech and whispering.

Question 6

What is attenuation reflex?

Answer 6

When loud sounds enter the ear, after 40–80 milliseconds, stapedius and tensor tympani muscles contract. Stapedius pulls stapes outward, tensor tympani pulls malleus inward, stiffening ossicles and reducing movement, especially for frequencies <1000 Hz.

Question 7

Functions of attenuation reflex?

Answer 7

1. Protect cochlea from loud sounds.
2. Reduce background noise to focus on speech (above 1000 Hz).
   Also activated when we speak, so we don’t hear our voice too loudly—controlled by the brain.

Question 8

What is bone conduction of sound?

Answer 8

The cochlea is embedded in the bony labyrinth of the temporal bone. Vibrations from objects (e.g., tuning fork on mastoid process) can reach the cochlear fluid directly via bone conduction. Requires strong vibrations or amplification.

Question 9

Structure of cochlea?

Answer 9

The cochlea is a spiral-shaped structure in the inner ear with 3 parallel fluid-filled tubes: scala vestibuli, scala media, and scala tympani.

Question 10

Membranes in cochlea?

Answer 10

Scala vestibuli and scala media are separated by Reissner’s membrane (vestibular membrane). Scala media and scala tympani are separated by the basilar membrane.

Question 11

What is organ of Corti?

Answer 11

The organ of Corti sits on the basilar membrane inside the cochlea. It contains specialized hair cells that convert sound vibrations into nerve signals sent to the brain, allowing hearing.

Question 12

How does sound enter the cochlea?

Answer 12

Sound enters the inner ear through the oval window, which is covered by the stapes. The stapes pushes on the fluid inside the cochlea, creating pressure waves.

Question 13

Where do pressure waves go after entering the cochlea?

Answer 13

Pressure waves enter the scala vestibuli and easily pass into the scala media because Reissner’s membrane is thin and flexible, allowing them to act like one connected chamber.

Question 14

What causes the basilar membrane to vibrate?

Answer 14

The movement of fluid in the scala vestibuli and scala media causes the basilar membrane to vibrate.

Question 15

What determines which part of the basilar membrane vibrates?

Answer 15

The frequency of the sound determines which part of the basilar membrane vibrates. High frequencies activate the base; low frequencies activate the tip.

Question 16

How is the basilar membrane structured?

Answer 16

It consists of 20,000 to 30,000 basilar fibers. These fibers are short and stiff near the base and become longer and more flexible towards the apex (tip).

Question 17

What kind of sound activates the base of the cochlea?

Answer 17

High-frequency sounds, like a whistle or bird chirp, cause the short, stiff fibers at the base of the cochlea to vibrate.

Question 18

What kind of sound activates the tip of the cochlea?

Answer 18

Low-frequency sounds, like a drum or thunder, cause the long, floppy fibers at the tip (near the helicotrema) to vibrate.

Question 19

What is tonotopic organization?

Answer 19

It is the frequency map of the cochlea: high pitches activate the base and low pitches activate the tip. This spatial arrangement of frequency processing is called tonotopic organization.

Question 20

What creates the traveling wave in the cochlea?

Answer 20

The inward movement of the stapes into the oval window generates a traveling wave in the cochlear fluid.

Question 21

Why does the round window bulge outward?

Answer 21

Because the cochlea is surrounded by bone and fluid is incompressible, the round window bulges outward to accommodate pressure changes caused by the stapes.

Question 22

How does the traveling wave move along the cochlea?

Answer 22

It starts at the base and travels toward the tip. The wave gains strength as it approaches the point that resonates with the sound frequency, then it dies out after that point.

Question 23

Where do different frequency waves die out?

Answer 23

High-frequency waves die out near the base, mid-frequency waves reach halfway, and low-frequency waves travel to the tip before fading.

Question 24

Why does wave speed change along the cochlea?

Answer 24

The wave moves faster at the base and slower toward the apex. This speed difference helps the cochlea distinguish similar high-frequency sounds by spreading them out spatially.

Question 25

What does amplitude of vibration represent?

Answer 25

It represents the strength of movement at each point on the basilar membrane. Amplitude peaks at the point that matches the sound frequency.

Question 26

What is an example of amplitude matching sound frequency?

Answer 26

A sound at 8000 Hz causes the strongest vibration near the base. A sound at 200 Hz causes vibration near the tip of the cochlea, close to the helicotrema.

Question 27

How does the brain determine sound frequency?

Answer 27

The brain uses place coding to detect frequency. It identifies the pitch of a sound based on the location of maximum vibration on the basilar membrane.

Question 28

How does the auditory nerve relay frequency information?

Answer 28

Hair cells at the point of maximum vibration send nerve signals through the auditory nerve to the brain, allowing us to distinguish sounds of different frequencies—even very similar ones.

Question 29

What is the organ of Corti?

Answer 29

It is the part of the inner ear located on the basilar membrane within the scala media. It converts sound vibrations into nerve signals.

Question 30

What triggers the organ of Corti to activate?

Answer 30

As the basilar membrane vibrates, the organ of Corti also moves, which activates hair cells and triggers nerve signaling.

Question 31

How many rows of inner hair cells are there?

Answer 31

There is one row of inner hair cells, totaling about 3,500 cells. Each is around 12 micrometers wide.

Question 32

What is the function of inner hair cells?

Answer 32

Inner hair cells are primarily responsible for sending sound information to the brain. They are the main sensory cells for hearing.

Question 33

How many rows of outer hair cells are there?

Answer 33

There are 3 or 4 rows of outer hair cells, totaling about 12,000 cells. Each is thinner, about 8 micrometers wide.

Question 34

What is the function of outer hair cells?

Answer 34

Outer hair cells help tune and adjust the response of the cochlea to different pitches. They play a supporting role in hearing by enhancing sensitivity and frequency selectivity.

Question 35

How are auditory nerve fibers distributed among hair cells?

Answer 35

Despite being fewer, inner hair cells are connected to 90% to 95% of auditory nerve fibers, showing their critical role in hearing.

Question 36

Where do auditory signals go after leaving hair cells?

Answer 36

The nerve endings connected to hair cells go to the spiral ganglion of Corti, located in the modiolus (center of the cochlea). About 30,000 nerve fibers carry signals through the cochlear nerve into the CNS. They enter the brain at the upper medulla.

Question 37

How do hair cells get excited by sound?

Answer 37

Hair cells have stereocilia (stiff hairs) on top that touch or are embedded in the tectorial membrane. These stereocilia bend when the basilar membrane vibrates, causing excitation or inhibition of the hair cell, depending on the direction of bending.

Question 38

What happens when the basilar membrane vibrates?

Answer 38

The base of hair cells is connected to the reticular lamina, supported by the rods of Corti. When the basilar membrane moves up, the reticular lamina tilts up and in. When it moves down, it tilts down and out. This causes stereocilia to bend back and forth against the tectorial membrane, generating electrical signals in hair cells.

Question 39

What does bending stereocilia do?

Answer 39

Bending stereocilia in one direction excites (depolarizes) the hair cell, while bending in the opposite direction inhibits (hyperpolarizes) it. This back-and-forth motion creates electrical signals that activate auditory nerve fibers at the base of the hair cells.

Question 40

Why are outer hair cells important?

Answer 40

Outer hair cells help control the sensitivity of inner hair cells to different pitches (tuning). Damage to outer hair cells can cause major hearing loss even if inner hair cells are intact. They also receive retrograde fibers from the brain that can change their length or stiffness, helping to fine-tune sound perception.

Question 41

How are electrical signals created in hair cells?

Answer 41

Each hair cell has ~100 stereocilia arranged in step-like rows. Shorter hairs are connected to taller ones by protein links. Bending the hairs toward the tallest opens channels for K⁺ to enter from the scala media, causing depolarization. Then Ca²⁺ enters, amplifying the signal. K⁺ exits via Ca²⁺-activated channels, repolarizing the cell. This is mechanical transduction: converting physical motion into electrical signals.

Question 42

What does a depolarized hair cell do next?

Answer 42

A depolarized hair cell releases glutamate, which excites nearby auditory nerve endings. These nerves carry the signal to the brain, where it is perceived as sound.

Question 43

What is the endocochlear potential?

Answer 43

It is a +80 mV voltage difference between the endolymph (in scala media) and perilymph (in scala vestibuli and tympani), maintained by the stria vascularis which pumps K⁺ into the scala media. This voltage difference makes hair cells highly sensitive to tiny stereocilia movements.

Question 44

What fluids are found in the cochlea?

Answer 44

Endolymph is found in the scala media and is high in K⁺, low in Na⁺, produced by the stria vascularis. Perilymph is found in the scala vestibuli and scala tympani, resembles cerebrospinal fluid, and connects to the brain’s fluid spaces.

Question 45

Why is the endocochlear potential important for hearing?

Answer 45

The stereocilia of hair cells are bathed in endolymph (+80 mV) and their base in perilymph (0 mV). The inside of the hair cell is –70 mV compared to perilymph, or –150 mV compared to endolymph. This large voltage makes the cell very sensitive—small movements in stereocilia cause large electrical responses, allowing even faint sounds to be detected. It ‘supercharges’ the hair cells.

Question 46

How brain tells sound frequency

Answer 46

Different sound frequencies cause specific regions of the basilar membrane in the cochlea to vibrate most. High frequencies stimulate the base, low frequencies stimulate the apex, and middle frequencies stimulate intermediate regions. This pattern is preserved all the way up to the auditory cortex, with each region tuned to specific frequencies. This is known as the place principle.

Question 47

What is the place principle

Answer 47

The place principle is the idea that the brain identifies sound frequency based on where the basilar membrane vibrates most. High-pitched sounds activate the base, low-pitched sounds activate the apex.

Question 48

What happens below 200 Hz

Answer 48

At very low frequencies (under ~200 Hz), vibrations reach the apex and are not localized. In this case, the brain uses the volley principle, where nerve fibers fire in bursts matching the rhythm of the sound. This allows frequency discrimination even if the apex is damaged.

Question 49

What is the volley principle

Answer 49

It’s a mechanism used to identify very low frequencies (under ~1500–2000 Hz). Nerve fibers fire in rhythmic volleys that match the frequency of the sound. For example, a 100 Hz tone causes neurons to fire ~100 times per second. The brain decodes this pattern to determine frequency.

Question 50

How we tell loudness: rate of firing

Answer 50

Louder sounds cause stronger vibration of the basilar membrane, leading to faster firing of hair cell nerves.

Question 51

How we tell loudness: number of fibers

Answer 51

Stronger vibrations spread out, stimulating more hair cells beyond the center of vibration. This activates additional nerve fibers, which signals greater loudness.

Question 52

How we tell loudness: outer hair cells

Answer 52

Outer hair cells are activated mainly by loud sounds. When they respond, it signals to the brain that the sound is loud. Their activation adds extra confirmation of loudness.

Question 53

How loudness is measured in the brain

Answer 53

The brain compresses actual sound intensity using a power law. Loudness perception is roughly proportional to the cube root of the sound energy. For instance, a sound that is a million times stronger may only feel 10,000 times louder.

Question 54

What is the power law of loudness perception

Answer 54

It’s the brain’s method of compressing wide variations in sound energy into a manageable range of perceived loudness. A whisper and a jet engine can be interpreted on the same scale using this law.

Question 55

What is a decibel

Answer 55

A decibel (dB) is a logarithmic unit for measuring sound energy. 1 bel = 10× increase in energy; 1 dB = 0.1 bel = ~1.26× increase in energy. It allows large differences in intensity to be measured on a compact scale. The smallest noticeable difference in loudness is about 1 dB.

Question 56

Hearing sensitivity by frequency

Answer 56

We are most sensitive to sounds between 1000–4000 Hz (especially speech). A 3000 Hz sound can be heard even when it's 70 dB below 1 dyne/cm², but a 100 Hz sound must be 10,000× stronger to be heard.

Question 57

Hearing range

Answer 57

Young, healthy people hear from 20 Hz to 20,000 Hz, with the best sensitivity between 500–5000 Hz. This range shrinks with age, often to 50–8000 Hz or lower.

Question 58

Sound pathway from cochlea to brain

Answer 58

Sound signals begin at the spiral ganglion in the cochlea → travel to dorsal and ventral cochlear nuclei in the upper medulla → mostly cross to opposite side → go to superior olivary nucleus (first place both ears' input is compared) → ascend via lateral lemniscus → may synapse in nucleus of lateral lemniscus → continue to inferior colliculus (midbrain) → then to medial geniculate nucleus (thalamus) → finally to auditory cortex in the temporal lobe.

Question 59

What does superior olivary nucleus do

Answer 59

It compares input from both ears and helps with localizing sound. It’s the first structure in the auditory pathway where this comparison happens.

Question 60

Which side of brain gets sound signals

Answer 60

Both sides of the brain get input from both ears, but most auditory fibers cross over to the opposite side. This crossing happens in the trapezoid body, the commissure between lateral lemnisci, and the commissure between inferior colliculi.

Question 61

What is the trapezoid body

Answer 61

It’s one of the main crossover points where auditory signals from one side of the brainstem cross to the opposite side.

Question 62

What does lateral lemniscus do

Answer 62

It carries sound signals from the superior olivary nucleus to the inferior colliculus. Some fibers also stop at its nucleus on the way up.

Question 63

What does inferior colliculus do

Answer 63

It’s a major sound processing center in the midbrain, receiving input from lower brainstem and sending output to the thalamus.

Question 64

Role of medial geniculate nucleus

Answer 64

It is a relay station in the thalamus that passes auditory signals to the auditory cortex in the temporal lobe.

Question 65

Role of auditory cortex

Answer 65

It processes and interprets sound signals into meaningful perceptions such as speech, music, or environmental sounds.

Question 66

Other brain areas involved in sound

Answer 66

Some sound signals go to the reticular activating system (RAS), which helps wake the brain during loud sounds. Others go to the vermis of the cerebellum, helping prepare muscle coordination for sudden noises.

Question 67

What is RAS’s role in hearing

Answer 67

RAS (reticular activating system) spreads through the brainstem and spinal cord. It helps alert the brain to sudden or loud sounds, allowing quick reaction.

Question 68

Auditory frequency mapping in brain

Answer 68

The auditory system maps sound frequencies spatially, like a piano keyboard, from the cochlear nuclei to the cortex. Different frequencies always activate specific areas along this path, allowing the brain to distinguish pitch.

Question 69

Firing rate in auditory nerve

Answer 69

Auditory nerve fibers entering the cochlear nuclei can fire up to 1000 times per second. This firing rate mainly reflects loudness of the sound.

Question 70

How firing rate relates to frequency

Answer 70

Up to ~2000–4000 Hz, auditory nerve impulses synchronize with the sound wave (but not every wave). This synchronization helps encode frequency early in the pathway.

Question 71

What happens to synchronization in brainstem

Answer 71

In brainstem tracts, synchronization disappears except for sounds below 200 Hz. Above the inferior colliculus, even low-frequency synchronization is mostly lost.

Question 72

Why synchronization is lost in higher centers

Answer 72

Because higher auditory centers analyze and decode the signal rather than pass it unchanged. They separate components like direction, loudness, and pitch for deeper processing.

Question 73

Where do sound signals go first in the brain?

Answer 73

The first place sound signals go in the brain is the auditory cortex, mainly located in the temporal lobe. It also spreads to the insula and part of the parietal lobe. The auditory cortex has two parts: the primary auditory cortex (receives signals from the thalamus and processes basic sound features) and the auditory association cortex (helps understand speech, recognize music, and includes Wernicke’s area for word and meaning comprehension).

Question 74

How does the brain organize sound frequencies?

Answer 74

The auditory cortex organizes sounds by frequency, using tonotopic maps. High-frequency sounds are processed in one part, and low-frequency sounds in another. These maps are arranged like a piano keyboard. Different maps help recognize pitch, detect sound direction, or notice sound changes like tempo.

Question 75

Why are there multiple frequency maps in the brain?

Answer 75

There are multiple frequency maps because the brain analyzes different sound features: pitch recognition, location, and temporal changes like speeding up or slowing down in music.

Question 76

How does the brain sharpen hearing during sound processing?

Answer 76

As sound moves along the auditory pathway, neurons become more specialized and respond to a narrower frequency range. This sharpening happens through lateral inhibition, where activation of one frequency suppresses nearby frequencies, helping focus on specific sounds like adjusting contrast in a blurry photo.

Question 77

What is the role of the auditory cortex in sound pattern recognition?

Answer 77

The auditory cortex recognizes and distinguishes sound patterns like melodies. Animals without it can hear but can’t differentiate patterns. Damage to the cortex impairs pitch recognition and localization, though hearing may still occur. If only the auditory association cortex is damaged, the person can hear but may struggle with interpreting meaning (e.g., speech), especially if Wernicke’s area is affected.

Question 78

How does the brain determine where a sound comes from?

Answer 78

The brain uses two cues: 1) Time difference – small delays in sound arrival between ears, most effective for low-frequency sounds (<3000 Hz); 2) Intensity difference – compares loudness between ears, better for high-frequency sounds as the head blocks these more. The shape of the pinna helps determine vertical and front-back sound direction by altering sound entry.

Question 79

Which brainstem nuclei detect sound direction?

Answer 79

The superior olivary nucleus detects sound direction. The medial superior olivary nucleus detects time differences with specially arranged neurons. The lateral superior olivary nucleus detects intensity differences to determine which side is louder. After processing, signals are sent to the auditory cortex in a separate pathway from pitch processing.

Question 80

How does the brain send signals back to the ear?

Answer 80

The brain sends centrifugal (retrograde) inhibitory signals from the superior olivary nucleus to the cochlea. These signals reduce sensitivity in selected areas, helping focus on specific sounds like one instrument in an orchestra.

Question 81

What causes nerve deafness?

Answer 81

Nerve deafness is caused by damage to the cochlea, auditory nerve, or brain circuits. It leads to permanent hearing loss. Causes include loud sounds, aging, or ototoxic drugs like antibiotics.

Question 82

What causes conduction deafness?

Answer 82

Conduction deafness results from damage to structures transmitting sound (e.g., eardrum, ossicles). Bone conduction may still allow hearing if the cochlea and auditory nerve are intact. It can be caused by infections or otosclerosis. Severe cases (e.g., frozen stapes) may require surgery to restore hearing.

Question 83

How does an audiometer work?

Answer 83

An audiometer tests hearing by emitting pure tones at various frequencies. The volume is increased until the sound is heard. The difference in decibels from the normal threshold shows the level of hearing loss. Results are plotted on an audiogram to visualize hearing across frequencies.

Question 84

How does nerve deafness appear on an audiogram?

Answer 84

Nerve deafness shows hearing loss in both air and bone conduction. High-frequency loss is common, especially from cochlear base damage. Low-frequency deafness can result from loud sound exposure. Drug-related deafness affects all frequencies.

Question 85

How does conduction deafness appear on an audiogram?

Answer 85

Conduction deafness shows normal bone conduction but reduced air conduction, especially at low frequencies. It can be caused by infections or otosclerosis. Severe cases, such as a frozen stapes, may block sound, but surgery (e.g., stapes prosthesis) can restore hearing.