Hearing: cochlea and function and types of deafness

sound waves longitudinal or transverse

longitudinal

describe motion of sound waves

push-pull

how can you measure loudness of decibels

• add 10 dB = double loudness

Normal hearing=

• can hear quiet sounds less than 20 dB HL

mild hearing loss=

hearing between 20-40 dB HL

moderate hearing loss=

hearing loss between 41-70 dB HL

severe hearing loss=

• 71-95 dB HL

what is lateral 1/3 of external auditory meatus composed of

• cartilage

• ceruminous and sebaceous glands

  • produce cerumen= earwax

what are the medial 2/3 of external auditory meatus composed of

• bone

• lined with thick skin= continuous with tympanic membrane

describe ear drum/tympanic membrane

• semi-transparent membrane

• externally lined by thick skin

• internally lined by mucous membrane

• sandwich radial and circumferential collagen fibres

role of ossicles in hearing

• movement of ossicles increases force and reduces amplitude of vibrations

what is stapedius

• inserts to stapes

• when tense→ reduces amplitude of vibrations

what is tensor tympani

• inserts into malleus

• when tense→ reduces amplitude of vibrations

what cranial nerve branches are involved in inner ear

• vestibular branch of CN VIII

• Fascial nerve CN VII

• Vestibulo-cochlear nerve CN VIII

• Cochlear branch CN VIII

components of vestibular system

• saccule

• utricle

• semicircular canals

sound detection occurs in

tectoral membrane

what’s found beneath tectoral membrnae

inner hair cells

outer hair cells with villi which insert into tectoral membrane

where are outer hair cells embedded in

perilymph

describe what happens in organ of corti when calcium channels open

role of outer hair cells

actively amplify sounds

how do we hear

• incoming sound= momentary pressure changes in air

• collected by outer ear

• pinna vibrates the eardrum

• vibrates ossicles

• which vibrates oval window on outside cochlea

• which vibrates fluids inside cochlear

• which vibrates basilar membrane where organ of corti is which has inner hair cells

• vibrations make stereocilia go backwards and forwards→ pulls and pushes Tip Links

• Tip Links pulls open transduction channels

• transduction channels allow potassium into inner hair cells

• depolarises inner hair cells→ fire off AP which goes to brain through AUDITORY NERVE= SOUND

role of outer ear in hearing

• guides sound from outside world into middle ear→ changes in pressure

role of ossciles in hearing

• in middle ear

• amplify sounds

• transmit sounds to inner ear

role of inner ear in hearing

• = cochlea

• converts changes in pressure into a signal that can be understood by brain

where is perception of sounds perceived

brain

label 1-4

1) umbo

2) cone of light

3) lateral process of malleus

4) handle of malleus

label 5-8

5) anterior mallear fold

6) flaccid part

7) posterior mallear fold

8) posterior inferior quadrant

what causes otitis externa and what is it

• = inflammation of ear canal

• caused when foreign bod becomes lodges

what is tympanosclerosis

• scarring of tympanic membrane

• causes calcium deposits to form

what does this show

• perforation

features of otitis media

• thick mucus in middle ear

• bulging inflamed eardrum

• swollen eustachian tube

what is otitis media

• inflammation of mucous membrane lining middle ear

treatment for otitis media

• grommet surgically inserted in tympanic membrane

describe interaural level differences

• head casts an acoustic ‘shadow’

• sounds arrives quieter at far ear

describe interaural time differences

sound has to travel further to get to far ear= arrives later

what is conductive hearing loss

• if someone has loss by air conduction but not by bone conduction= air-bone gap

3 types of hearing loss

• conductive

• sensorineural

• mixed

causes of sensorineural hearing loss

• aging

• noise damage

• drug side effects

• auditory tumours

• blast/explosion

causes of mixed hearing loss

• genetic disorders

• infections

• head trauma

causes of conductive hearing loss

• fluid

• foreign objects

• allergies

• ruptured eardrum

• impacted earwax

what is sensorineural hearing loss

what is microtia

• congenital abnormality

• failure of auricle and external auditory meatus to develop

pre-auricular appendage

• accessory cartilaginous appendage formed anterior and superior to tragus

normal hearing range

between 0-120 dB

describe Weber’s test

• tuning fork on top of head

describe Rinne’s test

• tuning fork on bone behind ear

• patient says when sound stops

describe results for Weber’s test

• none= hear best in midline

• sensorineural= hear better in normal ear

• conductive= hear better in affected ear

results for Rinne’s test

• none= air> bone

• sensorineural= air> bone

• conductive = bone> air

purpose of audiometry

• measure softest pure tone that can be detected in very quiet environment at various frequencies in each ear

what occurs in a hearcheck

• 3 high-frequency tone at decreasing intensities of 75, 55 and 35

• then 3 mid-frequency tones at decreasing intensities of 55, 35 and 20 dB HL

results of hearcheck

• if you miss:

  • 1 tone at 1 or 3 kHz= mild hearing loss

  • 2 tones at 1 or 3 kHx= moderate hearing loss

  • 3 tones at 1 or 3 kHz= severe hearing loss

when is otoacoustic emissions used

= newborn hearing test

identify children with moderate to profound permanent bilateral deafness within 5 weeks of birth

tinnitus

• conscious perception of auditory sensation in absence of corresponding external stimulus