Physiology of hearing

Question 1

Functions of hearing

Answer 1

Alerting to dangers

Localising objects

Recognition

Communication via speech

Question 2

The nature of sound

Answer 2

Sounds are travelling pressure waves that propagate through air at 340m/s

Sounds have two important attributes: frequency (Hz) and intensity (dB)

Question 3

dB

Answer 3

10xlog (sound intensity/ reference intensity)

Question 4

The range of human hearing

Answer 4

Frequency range of ideal human hearing: approximately 20-20000 Hz

Adults progressively lose high frequencies

Intensity range of human varies over 14 orders of magnitude

Intensities >90dB can lead to permanent hearing damage

Question 5

The peripheral auditory system

Answer 5

Outer ear

Middle ear

Cochlea

Auditory nerve

Question 6

The middle ear

Answer 6

An impedance matching device: increase 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

Prevents sound from being reflected back from the fluid filled cochlea

Question 7

Otitis media

Answer 7

Infection or inflammation of middle ear (usually self limiting)

Common in children (often from upper respiratory tract infection)

Secretory form with effusion (glue ear, if chronic causes a conductive hearing loss)

Question 8

Otosclerosis

Answer 8

Fusion of stapes with oval window

Maybe why Beethoven went deaf

Can be fixed by surgery

Question 9

The inner ear

Answer 9

Cochlea isa . long, coiled, fluid filled tube

Different parts of tube are tuned to different frequencies

Basal end is tuned to high frequencies

Apical end is tuned to low frequencies

Question 10

Sections of the cochlear duct

Answer 10

Scala vestibuli

Scala media

Scala tympani

Question 11

Composition of cochlear fluids

Answer 11

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

Scala media contains endolymph, an unusual extracellular fluid rich in K+ and low in Na+ and an electrical potential of about 80mV

Question 12

The organ of corti

Answer 12

Detects the sound induced motions of the basilar membrane

Contains two types of sensory hair cells

Apical membrane of hair cells bathed in endolymph

Basolateral membrane of hair cells bathed in perilymph

Question 13

Two types of sensory hair cells

Answer 13

Inner hair cells innervated by afferent nerve fibres

Outer hair cells mainly innervated by efferent nerve fibres

Question 14

Mechanotransduction in hair cells

Answer 14

Deflection of the hair bindle opens non selective cation channels (MET) at lower end of tip links

K+ enters and depolarises the hair cell, Ca2+ also enters and causes adaptation

Voltage gated Ca2+ cahnnels open, Ca2+ triggers vesicle release

Afferent nerve fibres are activated

Inner hair cells are sensory, outer hair cells are sensory motor cells

Question 15

Electromotility of outer hair cells

Answer 15

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 16

Afferent innervation of the cochlea

Answer 16

Neurons in cochlear ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIII nerve

Each hair cells is innervated by aons from 10-20 type I spiral neurons that signal the reception of sound over wide rang 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 17

Efferent innervation of the cochlea

Answer 17

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 18

Sensorineural hearing loss

Answer 18

Noise

Ageing

Ototoxic drugs

Genetic mutations

Question 19

Noise causes

Answer 19

Physical effects on hair bundle structure

Mitochondrial damage, cytotoxic free radicals

Glutamate excitotoxicity

Question 20

Ageing

Answer 20

30% of population over age 70 have significant hearing loss

Hair cells, stria vascularis, cochlear ganglion

Question 21

Ototoxic drugs

Answer 21

Aminoglycoside antibiotics

Cisplatin

Loop diuretics

Salicylate

Solvents

Question 22

Genetic mutations

Answer 22

High frequency 1:2000 of live births

Syndromic and non syndromic

.50 deafness genes identified, 80 additional loci

~50% of congenital deafness caused by mutations in gap junction genes

Question 23

Cochlear implants

Answer 23

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 goof enough to recognise and comprehend speech

• maximum 24 channels to substitute for 15000 hair cells
• speech is reported to sound robotic
• music sounds awful

Question 24

The cochlear nucleus

Answer 24

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 25

The superior olivary complex

Answer 25

Two binaural cues are used to locaise sounds in space

Interaural level differences are differences are detected in the lateral superior olive

Interaural time differences are detected in the medial superior olive

Question 26

The inferior colliculus

Answer 26

Obligatory station for all afferents

Laminar organisation in ICC, iso frequency sheets

Combines complex frequency and amplitude analysis of DCN with information on sound localisation from SOC

May encode complexity and locaisation f sounds

Auditory reflex centre, reflexive orientation to stimuli

Question 27

The auditory cortex

Answer 27

Primary auditory cortex is located on upper surface of temporal lobe

Lesions in auditory cortex cause defects in: sound localisation, discrimination of temporal pattern, intelligibility of speech

Lesions in Broca’s and Wernicke’s areas also impair the production and comprehension of speech