Physiology of Hearing

Question 1

What range of Hz can humans hear within?

Answer 1

• between 20Hz and 20,000Hz

Question 2

Over 140dB is the

Answer 2

threshold of pain

Question 3

What receptors convert sound pressure waves to receptor potential and ultimately neuronal signalling events

Answer 3

Mechanoreceptors are sensory receptors whose membrane contains cation channels that convert a mechanical signal to a graded receptor potential for the release of neurotransmitters or the generation of an action potential

Question 4

The Ear:

Answer 4

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Question 5

What are the auditory ossicles?

Answer 5

• three small bones located in the middle
  ear
• malleus = hammer
• incus = anvil
• stapes = stirrup

the malleus is attached to the tympanic membrane

the stapes is attached to the oval window

Question 6

Purpose of the auditory ossicles?

Answer 6

to convert sound waves received through the outer ear to fluid waves in the inner ear

Question 7

Middle Ear:

Answer 7

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Question 8

Inner Ear:

Answer 8

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Question 9

The Cochlear Canal:

Answer 9

insert diagram

Question 10

The Upper Cochlear Canal:

Answer 10

scala vestibuli contains perilymph

Question 11

The Middle Cochlear Canal:

Answer 11

scala media (cochlear duct) contains endolymph and the organ of corti

Question 12

The Lower Cochlear Canal:

Answer 12

scala tympani contains perilymph which ends in the narrow window

Question 13

Oscillation of the Stapes causes

Answer 13

pressure waves to the perilymph in the scala vestibuli and scala tympani

which causes the vibration of the basilar membrane

Question 14

What is the organ of corti?
Where is it located?
Contains?

Answer 14

• the primary sensory receptor of the
  auditory system?
• sits on the basilar membrane
• contains the inner and outer hair cells
  which are sensory mechanoreceptors
• the hair cells are supported by epithelial
  cells

vibrations of the basilar membrane are received by the hair cells

Question 15

Organ of Corti:

Answer 15

Peri, endo, peri

Question 16

Hair Cells:

Answer 16

• contain stereocilia
• sits on the basilar membrane in scala
  media
• the hair cells are sensory
  mechanoreceptors
• supported by epithelial cells
• inner = inner side of organ of corti
• outer = outside of organ corti = embedded
  in the tectorial membrane

stereocilia are projections of the hair cell membrane; structure is achieved through actin filaments

Question 17

Sound pressure is

Answer 17

localised deviation of pressure caused by a sound wave from the atmospheric pressure

Question 18

Hearing loss can be caused by a high sound pressure levels or low sound pressure levels?

Answer 18

High sound pressure levels

Question 19

The bony labyrinth is filled with

Answer 19

perilymph (high Na+, low K+)

Question 20

The membranous labyrinth is filled with

Answer 20

endolymph (low Na+, high K+)

Question 21

Stereocilia:

Answer 21

insert diagrams

Question 22

Journey of vibration through the ear

Answer 22

• vibration of the tympanic membrane
• vibration of the ossicles
• waves travel through perilymph
• basilar membrane vibrates
• results in outer hair cells bending
• as they bend membrane depolarisation
  begins
• the inner hair cells move in response to
  endolymph movement
• oscillation of outer hair cells and vibration
  of membranes influence the sensitivity of
  inner hair cells
• both inner and outer hair cells depolarise
• opening of K+ channels on stereocilia so
  influx of K+ from endolymph into
  perilymph
• depolarisation of perilymph
• which opens Ca2+ voltage gated channels
• influx of Ca2+ and efflux of K+ causes
  repolarisation
• neurotransmitter released

Question 23

What does medial displacement of stereocilia cause?

Answer 23

• hyperpolarisation
• K+ influx does not open Ca2+ channel
• no neurotransmitter released

Question 24

How can the CNS modulate the auditory system at the receptor level?

Answer 24

• outer hair cells are directly innervated by
  afferent and efferent nerves
• inner hair cells are directly innervated by
  afferent nerves
• CNS can modulate the auditory system at
  the receptor level

Question 25

Tonotopic Organisation:

Answer 25

Hair cells respond to different sound frequencies depending on their location along the basilar membrane of the cochlear canal

Question 26

The Central Auditory Pathway:

Answer 26

- spiral ganglion bipolar sensory neurons innervate the stereocillia - travels to the cochlear part of CNVIII - CNVIII projects to the ventral and dorsal cochlear nuclei - The nerve from the dorsal cochlear nucleus will travel to the superior olivary complex and then to the inferior colliculus from where the signal will be directed to the relevant cortical area the path travelled from the superior olivary complex to the inferior colliculus is referred to as the lateral lemniscus

Question 27

The Central Auditory Pathway:

Answer 27

insert diagram

Question 28

Summary of Auditory Nuclei: - superior olivary complex - lateral leminscus - inferior colliculus - medial geniculate nucleus - auditory cortex

Answer 28

- superior olivary complex = receives bilateral signal from the cochlear nuclei, can distinguish sounds based on the time of arrival and the perceived intensity in each year - lateral leminscus = projects to the inferior colliculus - inferior colliculus = tonotopically organised - medial geniculate nucleus (thalamus) = tonotopically organised - auditory cortex = receives all input and is tonotopically organised

Question 29

What areas of the brain are involved in auditory signal processing?

Answer 29

- the auditory cortex is located in the transverse temporal gyrus of Heschl (area 41) - 42 is the auditory association area - Wernicke's area is a secondary auditory area and is important for the interpretation of the spoken word - Broca's area in the frontal cortex is involved in speech - 41 and 42 send descending projections to the auditory system, involving efferent nerves

Question 30

Auditory Signal Processing:

Answer 30

insert diagram

Question 31

Conductive Hearing Loss:

Answer 31

disease in the outer ear canal or in the middle ear

Question 32

Sensorineural Hearing Loss:

Answer 32

disease in the cochlea or in the neural pathway from the cochlea to the brain

Question 33

Occupational Hearing Loss:

Answer 33

a noisy work environment can use the deterioration of the organ of corti

Question 34

Ototoxic Hearing Loss:

Answer 34

noisy work environment can cause deterioration of the organ of corti streptomycin, quinine)

Question 35

Infectious Hearing Loss:

Answer 35

drug induced (mumps or rubella)

Question 36

Tinnitus

Answer 36

hearing a sound that has no external source

Question 37

Conductive Hearing Loss Examples and Causes:

Answer 37

- earwax - outer ear infection - otitis media (middle ear inflammation) - perforated eardrum - tumour in the ear canal or middle ear - otosclerosis

Question 38

Ototsclerosis: Causes:

Answer 38

- genetic or external factors like viral infection - temporal bone sclerosis affects the stapes that becomes fixed to the oval window leading to impaired vibration - treatment: involves external hearing aids to amplify sounds or stapedectomy

Question 39

Stapedectomy: Risk:

Answer 39

can damage a branch of the facial nerve called corda tympani which passes through the inner ear lose sense of taste

Question 40

Sensorineural Hearing Loss:

Answer 40

- GJB2 encodes for protein connexin 26 involved in innner ear K+ homestasis so struggle to depolarise sterocillia membrane - SLC26A4 encodes for anion transporter

Question 41

Cochlear Implants:

Answer 41

- electronic device that provides a sense of sound to someone with severe hearing loss - surgically implanted - contains microphone, speech processor, transmitter and receiver, electrode array - electrodes transform sounds to nerve stimulations that bypass the damage part of the inner ear - does not completely restore hearing

Question 42

Cochlear Implants:

Answer 42

insert diagram