Physiology of hearing

Question 1

The perception of sound waves

Answer 1

• Vibrations cause molecules of the medium sound is travelling through to vibrate by compressing and pulling apart alternately
• sounds frequency is the number of oscillations per second
• a sound’s wavelength is the distance between areas of maximal pressure
• a sound’s volume is the difference in maximal and minimal pressure

Question 2

Echolocation

Answer 2

• some animals use sound and hearing as a means to ‘see’ their surroundings
• high-pitched squeaks reflect off of surfaces

Question 3

Functions of the external ear

Answer 3

• pinna directs sounds towards the ear canal
• auditory canal limits the frequency of sounds that can reach the eardrum
• cerumen traps debris

Question 4

Sound transduction

Answer 4

• tympanic membrane vibrates transmitted to the inner ear via auditory ossicles
• the vibration of the stapes transmits vibrations through the cochlear fluid

Question 5

Impedance Matching

Answer 5

• sound waves are transmitted in air
• inner ear is filled with fluid
• due to the change in medium density middle ear has to convert sound energy from air to water with minimal energy loss
• density changes may cause reduction in intensity
• tympanic membrane and ossicular system do impedance matching between sound waves in air and vibrations in cochlear fluid

Question 6

Sensory Structure

Answer 6

Organ of Corti
- travelling wave moves basilar membrane up and down in response to sound stimulation
- tectorial membrane is displaced relative to hairs position
- hairs are longer near the tip of the tectorial membrane; stereocilia

Question 7

Transduction in hair cells

Answer 7

• Tip links connect stretch-sensitive ion channels at tip of hair cell with the tip of the longer neighbouring sensory hair
• when hairs bend increasing length, filaments taughten opening channels. K+ flows into cell and is depolarised
• bending in the opposite direction closes the channels
• depolarisation opens Ca2+ channels which causes exocytosis of neurotransmitter glutamate

Question 8

Frequency discrimination

Answer 8

• Amplitudes and sensitivities aren’t the same across the entire basilar membrane
• high frequency waves have maximum amplitude near round window & don’t travel far
• low frequency cause basilar membrane to vibrate maximally at tip of cochlea

Question 9

Directional Hearing

Answer 9

• can determine where a sound is coming from because it arrives at each ear at different times
• away facing ear has a lower sound pressure
• brain interprets the difference

Question 10

Otolith Organs

Answer 10

• utricle; detects horizontal acceleration
• saccule; detects vertical acceleration

Question 11

Semicircular canals

Answer 11

• When head moves differential movement of otolith compared to endolymph due to difference in inertia
• this bends sensory hairs
• each semicircular canal has an ampulla containing hairs on one side
• stereocilia project into gelatinous cupulla