Sound Conduction and Transduction

Question 1

What does the outer ear do?

Answer 1

focuses on tympanic membrane that boosts sounds and creates pressure waves

Question 2

What happens in the middle ear?

Answer 2

increases pressure of vibration by:

• Focusing vibrations from the larger tympanic membrane to the smaller oval window
• The incus has a flexible joint with the stapes so the ossicles can use leverage to increase force on the oval window

Question 3

What are the protective mechanisms in the middle ear?

Answer 3

The stapedius and tensor tympani muscles can contract when noise is loud to restrict the movement of the ossicles to protect the inner ear from excessive volumes

Question 4

What are the components of the inner ear?

Answer 4

1. Scala vestibuli – contains perilymph fluid
2. Scala tympani – contains perilymph fluid
3. Scala media – contains endolymph fluid (high potassium concentration)

Question 5

What happens in the inner ear?

Answer 5

cochlea: to transduce vibration into nervous impulses

In doing this, the cochlea produces a frequency/pitch and intensity analysis of the sound

The basilar membrane is sensitive to different frequencies at different points along its length (high proximal, low distal)

Question 6

What are the protective mechanisms in the inner ear?

Answer 6

The auditory tube allows equilibrium of air pressure on either side of the tympanic membrane

Question 7

What is the organ of corti?

Answer 7

collectively describes the hair cells surrounded by supporting cells.

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

The tectorial membrane is gelatinous and does not vibrate with sound.

Question 8

What are causes of middle-ear conductive hearing loss?

Answer 8

wax

Acute otitis media – inflammation of the middle-ear

Otitis media with effusion – inflammation of the middle-ear with fluid accumulation

Chronic otitis media – can be of two forms:
- No cholesteatomatous:
> Without perforation -> retraction of the TM to Pars Tensa or Pars Flaccida
> With perforations -> active or inactive perforation of TM
- Cholesteatoma – destructive and expanding growth consisting of keratinizing squamous epithelium in the middle ear and/or mastoid process.

Otosclerosis – soft, spongy growth of new bone mostly near the oval window

perforated ear drum

congenital malformation

Question 9

What are the two types of hair cells?

Answer 9

1. Inner hair cell – 3,500 cells arranged in a single row densely innervated by ~10 sensory axons/cell
2. Outer hair cell – 20,000 cells arranged in 3 rows sparsely innervated by 1 axon for several cells

Both types of hair cells respond to sound but it’s the inner cells that transmit signals to the brain

Higher amplitudes of sound will cause a greater deflection of stercocilia and K+ channel opening

Question 10

Outline the mechanism of transduction

Answer 10

1. Basilar membrane vibrates to sound
2. Upward movement displaces stercocilia away from modiolus:
   - K+ channels open -> K+ enters from endolymph -> hair cell depolarises
3. Depolarisation opens Ca2+ channels in body of hair cell
4. Glutamate released from base depolarises axon of spiral ganglion cell -> action potential
5. Downward movement displaces stercocilia towards modiolus:
   - K+ channels close -> hair cell hyperpolarises

The mechanism is highly sensitive – threshold sound requires 0.3nm deflection

The system depends upon the maintenance of the endolymph being at +80mV by the stria vascularis

Question 11

What is the central auditory pathway?

Answer 11

Spiral ganglion cells from each cochlea project via CN VIII (vestibulocochlear nerve) to the ipsilateral cochlear nuclei (monoaural neurons)

After that, all connections are bilateral
- Thus deafness in ONE ear MUST be caused only by problems in the cochlear nucleus or CN VIII nerve (rare)

Hearing is TONOTOPICALLY organised in the brain

Question 12

How do we differentiate pitches?

Answer 12

Humans hear between 20 - 20,000Hz – most sensitive between 1000 - 3000Hz

High frequencies vibrate the basilar membrane closer to the base

Low frequencies vibrate the basilar membrane closer to the apex

Question 13

How is loudness measured?

Answer 13

on a logarithmic decibel scale (0dB - 120dB)

The scale is logarithmic because the ears response to sound is NOT linear but logarithmic

120dB is about thunder while 60 is conversational speech

Question 14

What are different types of hearing loss?

Answer 14

conductive = outer or middle ear problem

sensorineural = inner ear problem

mixed

Question 15

What are some causes of outer- ear conductive hearing loss?

Answer 15

Congenital malformations
- Congenital atresia – collapse or closure of the ear canal

Impacted wax

Foreign bodies

External otitis – inflammation of the passage of the outer-ear

• Otorrhea – abnormal fluid
• Pain on mobilisation of the ear and tragus and can lead to systemic symptoms

Exostosis – benign bony growths in the ear canal due to repeated exposure to cold water
- Usually multiple and bilateral -> leads to accumulation of ear wax and external repeat ear infections

Question 16

What are causes of inner-ear sensorineural hearing loss?

Answer 16

Prebyacusis – age-related hearing loss

• Is gradual and symmetric and is due to the aging European population
• It affects frequencies of speech for the 5th decade of life
• Men are 2x more affected and tinnitus is often associated

Sudden hearing loss – greater than 30dB hearing reduction over >3 contiguous frequencies, occurring over a period of 72 hours or less
- Unexplained rapid loss of hearing usually in one ear

Ototoxic drugs – e.g. diuretics, beta-blockers, TCAs, antibiotics etc

Infections – e.g. mumps, measles, chicken pox, influenza and syphilis

Noise-induced hearing loss – two forms:
- Acoustic trauma – brief exposure to very intense sounds and HL may be severe but substantial recovery is common
- Long-term noise exposure – damage results from long-term exposure usually in one frequency
> Common in occupational settings

Question 17

What is tonotopic mapping?

Answer 17

spatial arrangement of where sounds of different frequency are percieved, transmitted or received

tonotopy in cochlea and auditory cortex

low frequency ventrally and high frequency dorsal

Question 18

What is the function of the auditory cortex?

Answer 18

neurons respond to complex sounds

primary auditory cortex
- The primary auditory cortex A1 is located in the superior bank of the
temporal lobe - this is the central area of the AC and it is
tonotopically mapped
- Loudness, rate and frequency modulation
also seem to be mapped in A1

superior auditory cortex
- In the visual system the outputs of the primary visual cortex are
segregated
- We can identify a “What” and “Where” stream in the
auditory system (primates)

Question 19

What are the different neurons in the cochlear nucleus?

Answer 19

t-stellate cells - encode sound frequency and intensity of narrowband stimuli, tonotopic array represents sounds’ spectra

bushy cells - produce more sharply but less temporally precise versions of the cochlear nerve fibres, provide the resolution needed to encode relative time of arrival of inputs to the two ears

Question 20

What does the superior olivary complex do?

Answer 20

compares bilateral activity of cochlear nuclei

medial superior olive - interaural time difference is computed,

• sounds first detected at nearest ear before they reach the other one
• bushy cells carry information about timing of inputs at every cycle
• map of interaural delay can be formed to delay lines (birds)

lateral SO - detects differences in intensity between two ears

• neurons excited by sounds arising from ear in same side, while they are inhibited by opposite sounds
• interaural level difference is computed to localise sounds in horizontal plane

SOC sends feedback to hair cells (MSO to IHCs bilaterally and LSO to OHCs ipsilaterally)
* feedback used to balance responses in two ears and to reduce sinsitivity of cochlea

Question 21

What happens in the inferior and superior colliculus?

Answer 21

inferior
- responses from different frequencies merge
- all ascending
pathways converge
- in mammals: central nucleus, dorsal cortex and
external cortex
- Only central nucleus is tonotopically organised.
- the more we ascend towards the cortex the more neurons become
responsive to complex sounds
- in the IC many carry information
about sound location - Precedence effect

superior
-  auditory and visual maps merge
- neurons are tuned to respond to
stimuli with specific sound directions
- the auditory map here created is fundamental for reflexes in orienting
the head and eyes to acoustic stimuli