HUF 2-53 Physiology of hearing

Question 1

Sound and measurement of its intensity

Answer 1

Sound:

• Oscillation of molecules in an elastic medium
• Propagates as waves
• Air: larger air pressure variation = more energy

SPL = 2log(10) Px/P0
Px: abs. sound pressure
P0: ref. pressure (2x10^-5 Pas)

Question 2

Why use decibel?

Answer 2

• Log scale compresses very large of abs sound intensity of ear
• 6 dB ↑ = doubling in Pas
• 20 dB ↑ = 10 times ↑ in Pas

Question 3

Sensitivity of ear to different freq.

Answer 3

1. Loudness: intensity
2. Pitch: freq.

• Freq. also affects audibility of tone (curves of equal loudness)
• Most sensitive to freq. from 10^2 to 10^3 Hz (range of normal speech)
• Hearing impairment in this range
  => Affects verbal communication

Question 4

Structure of auditory apparatus

Answer 4

1. Outer ear
   - Convoluted pinna, auricle, ext. auditory meatus
   - Complex acoustic cavity efficiently transmit sound pressure at tympanic membrane (~2-5 kHz)
   - Sound collection and localisation
2. Middle ear
   - Tympanic membrane, malleus, incus, stapes
   - Sound amplification (P1A1 = P2A2)
   - Impedance matching: energy from air-borne sound converted to fluid-borne sound in inner ear
3. Inner ear
   - Cochlea: mechanoelectric transduction of sound; preliminary analysis of sound waves (cochlear n.)
   - Vestibule: balance

Question 5

Structure of inner ear

Answer 5

Scala vestibuli -> Scala tympani

• Communicate at helicotrema
• Filled with perilymph (low K+, high Na+)

Scala media
- Filled with endolymph (high K+)
- Mechanosensitive hair cells
=> Organ of Corti (lies on basilar membrane)

Question 6

Transmission of sound into inner ear

Answer 6

Stapes vibrates
=> Pressure changes within cochlea
=> Pressure waves along scala vestibula;
Travelling wave in highly flexible basilar membrane (non-uniform)

• Stiffness of basilar membrane decreases from base to apex
  ∴ Diff. freq. => diff. points of basilar membrane

Question 7

Transduction of sound by Organ of Corti

Answer 7

• 1 row of inner hair cells; 3 rows of outer hair cells (cochlear n.)
• Tectorial membrane on top is in contact with stereocilia of hair cells
• Sound through inner air
  => Relative movement of basilar and tectorial membrane
  => Stretch stereocilia
• Stereocilia bend toward longest one
  => Mechano-sensitive ion channels open
  => Sub-microscopic links (tip links) provide translational mechanical forces among stereocilia
  => K+ influx
  => Depolarisation
  => Ca2+ influx
  => GLU exciting aff. n.; other ions maintain membrane excitability

Question 8

Role of outer hair cells - cochlear amplifies

Answer 8

Tiny motors on outer hair cells

• Right membrane voltage changes
  => Outer hair cells contracts / relaxes
  => Mechanically amplify movement of basilar membrane

Question 9

Ascending pathways of auditory signal

Answer 9

Cochlear nu.
=> Dorsal acoustic stria;
Intermediate acoustic stria;
Trapezoid body (-> superior olivary nu.)
=> Nu. of lateral lemniscus
=> Inferior colliculus
=> Medial geniculate nu.
=> 1° auditory cortex

• SLIM
• Bilateral projection

Question 10

Cues for localising source of sound

Answer 10

• Sound intensity diff. in 2 ears
• Pattern of sound diffraction by outer ear
• Diff. of time of arrival of same sound at 2 ears

Neurons which receive signals from both ears detect simultaneous arrival of signals from L. and R. ears
=> Localisation

• Sup. olivary nu.

Question 11

Coding of sound freq.

Answer 11

Cochlea
=> Chchlear nu.
=> Other brainstem nu.
=> 1° auditory cortex
=> Tonotopic map

• 1° auditory cortex: first stage of cortical processing of sound
• Info relayed to higher auditory areas for further processing

Question 12

2 streams of sound processing

Answer 12

• Tonotopic 1° auditory cortex (core)
  => 2° auditory cortex (Belt and Parabelt regions)
  => Analyse complex sounds:
  Anterior stream: ‘what’ is heard
  Poterior stream: ‘where’ the sound comes from; speech
  => Prefrontal cortex

Question 13

Hearing impairment

Answer 13

Congenital / acquired

1. Conduction deafness: loss / inefficient sound transmission in tympanic membrane-ear bone apparatus
   e. g. middle ear infection; otosclerosis; damage of tympanum
2. Perception deafness
   - damage in inner ear or higher centres
   e. g. noise-induced hair cell loss; brain tumour

Question 14

Standard audiometric test

Answer 14

Audiometry: test and document one’s hearing ability
=> Differentiate conduction and perception deafness

Air conduction test vs. bone conduction test
e.g. Weber’s test, Rinne’s test

Audiogram