Physioloy of hearing

Question 1

List the functions of hearing

Answer 1

Alerting to dangers
Localising objects
Recognition
Communication via speech

Question 2

At what speed do sound waves travel at?

Answer 2

340 m/s

Question 3

What are the two attributes of sound?

Answer 3

Frequency

Intensity

Question 4

What is the frequency range of ideal human hearing:

Answer 4

approximately 20-20,000 Hz

Question 5

How does the intensity range of human hearing vary?

Answer 5

Intensity range of human hearing varies over 14 orders of magnitude
Intensities >90 dB can lead to permanent hearing damage

Question 6

What constitutes the peripheral auditory system?

Answer 6

Outer ear, middle ear, cochlea and auditory nerve (part of the VIIIth cranial nerve)

Question 7

What is the middle ear?

Answer 7

An impedance matching device: increases pressure ~45x, by the ratio of tympanic membrane and oval window areas, and to a lesser extent by the lever action of the middle ear ossicles

Question 8

What is the function of the middle ear?

Answer 8

Prevents sound from being reflected back from the fluid-filled cochlea

Question 9

What is ottis media?

Answer 9

Infection or inflammation of middle ear
Usually self-limiting
Common in children
Often from upper respiratory tract infection
Secretory form with effusion
“Glue ear”

Question 10

What treatment is available for ottis media and glue ear?

Answer 10

If chronic causes a conductive hearing loss
May need draining
Grommets

Question 11

What is otosclerosis?

Answer 11

Fusion of stapes with oval window
Maybe why Beethoven went deaf
Can be fixed by surgery

Question 12

Describe the cochlea

Answer 12

Cochlea is a long, coiled, fluid filled tube

Different parts of tube are tuned to different frequencies

Question 13

What is the basal end tuned to?

Answer 13

high-frequency sound

Question 14

What is the apical end tuned to?

Answer 14

low-frequency sounds

Question 15

What is the Scala vestibuli (SV) connected to ?

Answer 15

connected to oval window

Question 16

What is the Scala media (SM)?

Answer 16

Is a separate chamber

Question 17

What is the Scala tympani (ST)?

Answer 17

Connected to round window

Question 18

How do the scala vestibuli and scala tympani communicate?

Answer 18

SV and ST communicate via helicotrema at apex of cochlea

Question 19

What do the scala vestibuli and scala tympani contain?

Answer 19

Scala vestibuli and scala tympani contain perilymph, a normal extracellular fluid with high Na+ and low K+

Question 20

What does the scala media contain?

Answer 20

Scala media contains endolymph, an unusual extracellular fluid rich in K+ and low in Na+ (produced by stria vascularis), and an electrical potential of about +80 mV

Question 21

What is the function of the organ of corti?

Answer 21

Detects the sound induced motions of the basilar membrane

Question 22

What does the organ of corti contain?

Answer 22

Inner and outer sensory hair cells

Question 23

What is the apical membrane of hair cells bathed in?

Answer 23

Endolymph

Question 24

What is the basolateral membrane of hair cells bathed in?

Answer 24

Perilymph

Question 25

What are the outer hair cells innervated by?

Answer 25

Efferent nerve fibres

Question 26

Are the cochlea hair cells regenerated?

Answer 26

No

Question 27

Describe mechanotransduction of hair cells

Answer 27

Deflection of the hair bundle opens non-selective cation channels, the mechano-electrical transducer (MET) channels, at the lower end of the tip links, between neigbouring stereocilia (‘hairs’) K+, the major cation in endolymph enters and depolarises the hair cell, driven by its electro-(chemical) gradient [the electrical gradients is +120 to +140 mV; there is little or no chemical gradient]; Ca2+ also enters and causes adaptation Voltage gated Ca2+ channels open, Ca2+ triggers vesicle release Afferent nerve fibres (Aff NE) are activated Inner hair cells are sensory, outer hair cells are sensori-motor cells

Question 28

Describe the electromobility of outer hair cells

Answer 28

Outer hair cells amplify basilar membrane motion Depolarise – shorten; Hyperpolarise – lengthen Prestin, a modified anion exchanger in the basolateral membrane, is the OHC motor

Question 29

Describe the afferent innervation of the cochlea

Answer 29

Neurons in cochlear (spiral) ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIIIth nerve Each inner hair cell is innervated by axons from 10-20 Type I spiral neurons that signal the reception of sound over a wide range of intensities to the brain Outer hair cells are innervated by Type II spiral neurons that signal the reception of painfully loud sound that causes cochlear damage to the brain

Question 30

Describe the efferent innervation of the cochlea

Answer 30

Efferent fibres from the medial olive innervate the outer hair cells directly Efferent fibres from the lateral olive synapse on the Type I afferent fibres Activation of efferent system modifies the sensitivity of the cochlea

Question 31

List the causes of sensorineural hearing loss

Answer 31

Noise Ageing Ototoxic drugs Genetic mutations

Question 32

Name some targets of deafness genes in the cochlea

Answer 32

``` Tight junctions Gap junctions Afferent synapse Transduction complex Stria vascularis Tectorial membrane ```

Question 33

What are cochlear implants?

Answer 33

surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf Expensive: limited to the Western world Results often good enough to recognise and comprehend speech

Question 34

Describe the central auditory system

Answer 34

Tonotopically organised

Question 35

Describe the cochlear nucleus

Answer 35

Parallel processing starts in cochlear nucleus Auditory nerve fibres from cochlear ganglion innervate many types of neuron Neurons extract information about level, onset and timing of sounds

Question 36

Describe the superior olivary complex

Answer 36

Two binaural cues are used to localize sounds in space Interaural level differences are detected in the lateral superior olive (LSO) Interaural time differences are detected in the medial superior olive (MSO)

Question 37

Describe the inferior colliculus

Answer 37

Obligatory synaptic station for all afferents Laminar organisation in ICC, iso-frequency sheets Combines complex frequency and amplitude analysis of DCN with information on sound localization from SOC May encode complexity and localization of sounds Auditory reflex centre; reflexive orientation to stimuli

Question 38

Where is the primary auditory cortex located?

Answer 38

Primary auditory cortex is located on upper surface of temporal lobe

Question 39

Lesions in auditory cortex cause defects in:

Answer 39

sound localisation, discrimination of temporal pattern, intelligibility of speech

Question 40

Lesions in Broca’s (motor aphasia) and Wernicke’s (sensory aphasia) areas

Answer 40

Impair the production and comprehension of speech