Physiology of Hearing

Question 1

What generates sound waves?

Answer 1

Sound waves are generated by the displacement of air molecules in response to a moving object.

Question 2

How is the pitch of a sound determined?

Answer 2

Human ears can typically detect sounds between 20 Hz and 20,000 Hz.

Question 3

What is the typical range of sounds that human ears can detect?

Answer 3

The loudness or amplitude of a sound is determined by the difference between areas of high and low pressure. It’s measured in decibels (dB).

Question 4

What causes a sound to be louder?

Answer 4

The greater the force placed on an object, the greater the difference between waves of high and low air pressure, producing a louder sound.

Question 5

What is the first step in sound transmission?

Answer 5

The first step in sound transmission is the funneling of sound waves from the cone-shaped auricle into the auditory canal where they strike the tympanic membrane.

Question 6

What happens when sound waves strike the tympanic membrane?

Answer 6

When sound waves strike the tympanic membrane, they cause the membrane to vibrate at the same frequency.

Question 7

How are the vibrations of the tympanic membrane transferred to the inner ear?

Answer 7

The vibration of the tympanic membrane is transferred to the ossicles in the middle ear, which then transfers the vibration to the oval window leading into the inner ear.

Question 8

What is the function of the ossicles in the middle ear?

Answer 8

The ossicles act as a lever system, amplifying the vibration differences between the larger tympanic membrane and the smaller oval window to compensate for the transfer of the sound wave from air into fluid, which requires more energy.

Question 9

How is pitch determined in the inner ear?

Answer 9

Pitch, measured in cycles or vibrations per second (Hz), is determined by which area of the basilar membrane vibrates.

Question 10

What determines the loudness of the sound in the inner ear?

Answer 10

Loudness, measured in decibels (dB), is determined by how much the basilar membrane vibrates at the same area.

Question 11

What is the role of the spiral organ in the processing of sound in the inner ear?

Answer 11

The spiral organ, which rests on the basilar membrane, allows for the transfer of sound wave energy to a region where vibrations can be converted (transduced) into electrical signals that can be sent to the brain for interpretation.

Question 12

What are the two types of hair cells in the spiral organ and what are their functions?

Answer 12

The two types of hair cells within the spiral organ are inner and outer hair cells. Inner hair cells form a single row of cells and are primarily responsible for detecting sound, while outer hair cells are arranged in three rows.

Question 13

What is the function of the tectorial membrane in the spiral organ?

Answer 13

The tectorial membrane is a stiff structure that extends over the hair cells, sandwiching the cells and their stereocilia between the tectorial membrane and the basilar membrane. The stereocilia on the outer hair cells are in contact with the tectorial membrane, and the vibration of the basilar membrane causes the hair cells to move toward and away from the tectorial membrane.

Question 14

What happens when the basilar membrane moves up towards the tectorial membrane?

Answer 14

When the basilar membrane moves up towards the tectorial membrane, it bends the stereocilia towards the tallest stereocilium. This bending opens potassium ion channels and depolarizes the hair cell.

Question 15

What happens when the potassium ion concentration in the endolymph is high?

Answer 15

When the potassium ion concentration in the endolymph is high, ions flow into and depolarize the hair cells.

Question 16

What is the result of a depolarized hair cell?

Answer 16

A depolarized hair cell releases a neurotransmitter, triggering action potentials in the axons of the cochlear division of CN VIII.

Question 17

What is tinnitus and what causes it?

Answer 17

Tinnitus is the sensation of hearing noises in the absence of actual sound. The noise is often described as ringing, buzzing, whistling, or hissing. It is not a disease itself, but a symptom of an underlying problem. The most common cause is hearing loss due to damaged stereocilia of cochlear hair cells.

Question 18

What are the two broad categories of hearing loss?

Answer 18

Hearing loss can be classified into two broad categories: conduction and sensorineural. Conduction hearing loss is due to a problem in the outer or middle ear that prevents sound waves from reaching the inner ear.

Question 19

What are some causes of conduction hearing loss?

Answer 19

Conduction hearing loss can be caused by excessive buildup of ear wax, middle ear infection, perforated tympanic membrane, or fusion of auditory ossicles. Many forms can be corrected, for example, by removing excess ear wax or repairing a perforated tympanic membrane.

Question 20

What is sensorineural hearing loss?

Answer 20

Sensorineural hearing loss is a deficit in the cochlea or any part of the neural pathway. It has two types: sensory hearing loss and neural hearing loss, which differ in terms of cause and treatment.

Question 21

What causes sensory hearing loss and how can it be treated?

Answer 21

Sensory hearing loss is usually due to hair cell dysfunction in the cochlea, preventing the generation of action potentials. It can be caused by exposure to loud sounds, certain medications, or aging, which may damage hair cells or cause loss of stereocilia, especially those in the proximal part of the cochlea responsible for detecting high-frequency sounds. Hearing aids can help mild to moderate cases of sensory hearing loss, and cochlear implants may also be an option.

Question 22

What is neural hearing loss and how is it different from sensory hearing loss in terms of treatment?

Answer 22

Neural hearing loss occurs when signals fail to travel through the cochlear branch of the vestibulocochlear nerve or CNS pathways. It can be caused by strokes or tumors such as acoustic neuroma. Unlike sensory hearing loss, cochlear implants are not effective for treating neural hearing loss.

Question 23

What is a cochlear implant and how does it work?

Answer 23

A cochlear implant is a device consisting of surgically implanted electrodes that bypass damaged hair cells and directly stimulate the cochlear nerve. It does not fully restore hearing but enables a person to perceive enough sound to understand speech or hear warnings such as smoke alarms. The implant consists of an external microphone, usually worn just behind the ear, which sends sound waves to a processor that converts the waves to electronic signals. An external transmitter sends these signals to an internal receiver implanted beneath the skin. The receiver then relays the signals to a long, thin bundle of electrodes threaded through the cochlea, which stimulate the nerve endings of the vestibulocochlear nerve.

Question 24

How does the success of cochlear implants vary among different individuals?

Answer 24

The success of cochlear implants varies. Children who are born deaf or suffer hearing loss shortly after birth usually get implants before age 3, the period of maximal language and speech acquisition, and often learn to speak normally. If a deaf child receives an implant after age 9, it is unlikely that normal speech will develop. Children and adults who lose their hearing after acquiring speech and language often do well with cochlear implants and no longer have to solely use sign language or lip reading. Regardless of age, extensive training is required to use the implant successfully.