Sound conduction and transduction

Question 1

Label the following diagram:

Answer 1

Question 2

What is sound?

Answer 2

Change in air pressure.

Question 3

At what speed do changes in air pressure occur?

Answer 3

343m/sec (767mph)- speed of sound.

Question 4

What is frequency?

Answer 4

Sound causes a change in pressure that is periodic thus consists of compressed and rarefied air.

The frequency of sound is the number of compressed or rarefied patches of air that pass our ears each second.

Question 5

What sound frequencies is the human ear sensitive to?

Answer 5

20-20,000Hz

Question 6

What does sound intensity determine?

Answer 6

The loudness of the sound that we perceive.

Frequency remains unchanged- difference between the pressure compressed and rarefied air regions.

Intensity of the loudest sound (safe) is 1 trillion times greater than the faintest intensity perceived- dynamic range.

Amplification.

Question 7

What structures make up the outer ear?

Answer 7

Pinna (auricle) + external acoustic meatus (external auditory meatus/ auditory canal).

Question 8

What is the function of the outer ear?

Answer 8

Collects and conducts sound waves towards tympanic membrane.

Question 9

What is the middle ear?

Answer 9

Air-filled chamber in bone, lying between tympanic membrane laterally, and oval and round windows medially.

Continuous with nasal cavity- Eustachian tube.

Question 10

What structures make up the inner ear?

Answer 10

Cochlea + organs of balance.

Hair cells transduce mechanical energy of sound into electrical signal in cochlear nerve.

Question 11

What are the mechanisms of sound amplification?

Answer 11

Conduction through middle ear amplifies sound by 30dB.

Achieved by lever system of articulated ossicles and ratio of area of tympanic membrane to oval window.

Question 12

What are the protective mechanisms in the ear against loud noises?

Answer 12

Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles.

Protects against natural sounds but maybe not against man-made sounds.

Auditory tube allows equilibration of air pressure on either side of tympanic membrane.

Question 13

What is conductive hearing loss?

Answer 13

Sound is prevented from reaching the cochlea.

Question 14

What are the causes of conductive hearing loss?

Answer 14

Wax

Otitis media

Otosclerosis of ossicles

Perforated tympanic membrane

Congenital malformations

Question 15

Describe the structure of the cochlea.

Answer 15

Hollow tube in bone, curled into spiral.

Divided longitudinally into 3 compartments, separated by 2 membranes.

Sound wave causes vestibular (Reissner’s) and basilar membranes to vibrate.

Cochlear hair cells are attached to basilar membrane.

Question 16

Describe the structural and functional properties of the basilar membrane in the cochlea of the ear.

Answer 16

Structural properties determine how the membrane responds to sound: wider at apex than at base by 5x, stiffest at base and most flexible at apex.

Movement of stapes causes endolymph to flow causing a travelling wave in the membrane.

The distance the wave travels depends on its frequency.

Different locations of the basilar membrane are maximally deformed at different frequencies creating a placed code.

Question 17

Describe the structural and functional features of the organ of Corti (spiral organ).

Answer 17

Hair cells are surrounded by supporting cells.

Tectorial membrane is gelatinous and does not vibrate with sound.

Spiral ganglion is embedded in modiolus and innervates hair cells.

Stria vascularis secretes endolymph (high in K+, low in Na+).

Question 18

What are the 2 types of hair cells?

Answer 18

Inner hair cell- about 3500 cells arranged in single row, densely innervated by about 10 sensory axons/cell.

Outer hair cell- about 20,000 cells arranged in 3 rows sparsely innervated: one axon innervates several cells.

Both types of hair cell respond to sound but inner hair cells provide information for brain.

Question 19

What is the sound transduction mechanism?

Answer 19

Basilar membrane vibrates in response to sound

Upward movement displaces stereocilia away from modiolus à K+ channels open à K+ enters from endolymph à hair cell depolarises

Downward movement displaces stereocilia towards modiolus.

K+ channels close.

Hair cell hyperpolarises.

Highly sensitive- response to threshold sound requires 0.3nm deflection.

Depends on maintenance of endolymph at +80mV by stria vascularis.

Depolarisation opens Ca2+ channels in body of hair cell.

Glutamate released from base depolarises axon of spiral ganglion cell.

Action potential.

Question 20

What is the mechanisms for differentiation of pitch?

Answer 20

Basilar membrane acts as frequency analyser.

High frequencies vibrate basilar membrane nearer to base.

Low frequencies vibrate membrane nearer to apex.

Perceived pitch of a sound is determined by frequency.

Question 21

What are the three compartments of the inner ear?

Answer 21

Scala vestibuli (contains perilymph fluid)

Scala tympani (contains perilymph fluid)

Scala media (contains endolymph fluid)

Question 22

How is a place code or tonotopic map generated in the ear, and what is its purpose?

Answer 22

Different parts of the basilar membrane are sensitive to different frequencies.

A specific tone (frequency) is associated with a spatial position on the membrane.

This tonotopic arrangement is mirrored in higher levels of auditory processing.

Higher frequency sounds tend to make the basilar membrane towards the base of the cochlear vibrate more, whereas lower frequency sounds make the basilar membrane near the apex vibrate more.

Question 23

What is the organ of Corti?

Answer 23

The sense organ of the cochlea of the inner ear which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve.

Lies on top of the basilar membrane and beneath the tectorial membrane.

Acts as a sensor for when the basilar membrane is deflected by a pressure wave.

Consists of inner and outer hair cells.

Question 24

Describe the central auditory pathway.

Answer 24

Fibres from the organ of Corti project out of the cochlea via the spiral ganglion to form the vestibulocochlear nerve (VIII), which then projects to the ipsilateral cochlear nucleus.

After this point all connections are bilateral.

Fibres then project to the superior olive.

All ascending auditory pathways converge on the inferior colliculus.

Receives input from both cochlea.

Pathways are bidirectional.

The pathway continues to the medial geniculate nucleus and then to the auditory cortex.

There are also collateral pathways to the reticular formation and cerebellum.

Question 25

Which frequencies are spatially organised by tonotopy and which by phase locking?

Answer 25

Very low frequency: phase locking.

Mid-frequency: mix phase locking and tonotopy.

Very high frequency: tonotopy.

Question 26

List some causes of sensorineural deafness.

Answer 26

Presbyacusis.

Exposure to loud noise- 4kHz.

Meniere’s disease.

Toxicity, e.g. some antibiotics.

Hereditary disorders.

Acoustic neuroma.

Viral infection.

Question 27

List some central (rare) causes of sensorineural deafness.

Answer 27

Demyelination in MS.

Injury to central auditory pathway (unlikely to cause serious deafness unless both auditory cortices affected).

Question 28

What are the different types of hearing loss?

Answer 28

Conductive

Sensorineural

Mixed: SNHL and CHL

Auditory neuropathy spectrum disorder (ANSD)

Auditory processing disorder (APD)

Question 29

What is conductive hearing loss (CHL)?

Answer 29

Prevents sound transmission through the external or middle ear.

Question 30

What is sensorineural hearing loss (SNHL)?

Answer 30

Permanent hearing loss as a result of cochlea +/- auditory nerve dysfunction affecting sensitivity to sound as well as quality of sound detection.

Question 31

What is auditory neuropathy spectrum disorder (ANSD)?

Answer 31

Dysfunction in neural transmission of sound from the IHCs to lower brainstem resulting in poor speech discrimination beyond expected.

Question 32

What is auditory processing disorder (APD)?

Answer 32

Dysfunction of the central auditory pathways resulting in difficulty in processing (i.e. interpretation) of sound information despite good peripheral hearing, e.g. understanding speech in background noise, localising sound- requires more extensive testing.

Question 33

List management strategies for hearing loss.

Answer 33

Hearing aids

Hearing tactics

Assistive listening devices

Management of comorbidity (medical, psychological)

Support development of speech, language, learning in children

Question 34

What are the causes of prelingual hearing loss?

Answer 34

Acquired (20-25%):

• Perinatal cause (hypoxic event, kernicterus, ototoxicity)
• Bacterial meningitis
• Congenital infection (CMV, rubella, syphilis)
• Head injury
• Ototoxicity

Unknown (25-50%).

Question 35

List objective hearing tests.

Answer 35

Tympanometry- test of middle ear function.

Otoacoustic emissions- test of outer hair cell function; reflects cochlea function.

Electrocochleography- test of cochlear function and CN VIII.

Auditory brainstem response- test of retrocochlear function; for synchronous firing of auditory nerve fibre/ asymmetrical latencies/ frequency-specific threshold estimation.

Stapedial reflex- test of auditory nerves and part of brainstem function.

Question 36

What membrane separates the scala vestibuli and the scala media?

Answer 36

Reissner’s membrane.

Question 37

What membrane separates the scala tympani and the scala media?

Answer 37

Basilar membrane.

Question 38

What are the differences between perilymph and endolymph?

Answer 38

Perilymph is CSF-like.

Perilymph has low K+ and high Na+.

Endolymph has high K+ and low Na+.

Question 39

At what frequency does noise-induced hearing loss occur?

Answer 39

4000Hz.