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  • OH & S cap of sound to 85 decibels for an 8 hour working day
  • It works as a dose (the longer exposed, the quieter it should be)
  • Recreational use of sound (iPods and clubs) are ruining our ears

Outline

  • The significance of hearing and communication
    • human endeavour – culture – music
  • How we hear (so well)
    • the ear
      • forward transduction
      • reverse transduction (cochlear amplifier)
    • the brain
      • central auditory pathways
      • sound localization
  • Losing our hearing
    • the major sensory disability in our society
    • hearing allows us to communicate in a very efficient way concepts, ideas and motivation


  • Hearing involves numerous pathways in the brain ending at the auditory cortex where hearing, speech and language centres overlap
    • Cochlea --> central auditory pathways
  • Cochlea is an incredible efficient frequency spectrum analyser
    • High frequencies are processed closest to the oval window - low frequencies are processed closer to the helicotrema (not just due to physical stiffness and mass of the membrane, but also exquisite coordination of the hair cells)

Check this:

  • Reverse transduction - converts sound to receptor potentials
  • Forward transduction (prestin protein; jam packs the outer hair cells) - converts receptor potentials into expansion and contraction of hair cells

Hearing sense has remarkable performance

  • Pitch range: Approximately 10 octaves from 20 Hertz (Hz) to 20,000 Hz: best between 500 Hz (Vowels) and 4000Hz (Consonants); has only 20,000 sensory hair cells that don't regenerate (compared to the millions in the retina)
  • Pitch discrimination: Separate two frequencies 0.2% apart (e.g. 1000 and 1002 Hz - a shift in the optimum vibration on the basilar membrane in the organ of Corti of about 1-2 hair cells)
    • This is due to frequency mapping in the auditory cortex and cochlea
  • Timing discrimination: Can distinguish two sounds separated by about 10μsecs (1/100,000th sec)
  • Intensity (dB scale): quiet office is about 40 dB, but you need to be careful at about 85 dB
  • We lose our high frequency sound very quickly with age

The cochlea

  • Compartments have perilymph in them (similar concentration to CSF)
  • Scala media has endolymph (similar concentration to intracellular; high K+)
  • The hair cells are constantly transducing current (there is a constant flow of K+ into the cell, to maintain the gradient)
  • Associated with scala media is a +100 mV potential called the endocochlear potential, produced by the stria vascularis, filled with ion potentials (producing a battery; +100mV between the scala media and the other scala)
  • Across the cuticular plate and the ion channels, there is 170 mV driving force (due to the electrical Nernst potential due to concentration differences).
    • This runs across the hair cells (from the cochlea and the scala tympani)
  • As sound waves cause the stereocilia to move back and forth, the tip links on the ends of the stereocili are opened, allowing K+ into the hair cell, causing depolarisation
  • Each inner hair cell receives unique innervation from 12-15 primary auditory neurons, responsible for encoding that frequency of sound. Neurons all form a single synapse with the hair cell (forms a punctate synapse called a riven synapse), this is highly regulated and can have the highest continuously release of neurotransmitter signals that is generated in the body)
  • Outer hair cell stereocilia are bound to the tectorial membrane, while the inner hair cells float free
    • This makes a sandwich, squeezing the tectorial membrane over the hair cells on the basilar membrane
    • Cochlear amplifier - outer hair cells produce positive feedback, generating energy to reinforce the vibrations of the basilar membrane that are produced due to sound vibrations in the endolymph
  • The outer hair cells amplify the vibration of the hearing organ
  • These cells are responsible for sensitivity and pitch discrimination
  • Reflection of this energy out of the ear is detected as an otoacoustic emission

Otoacoustic emissions

  • Some of the sound produced by cochlear amplifier moves retrograde through the inner ear and produces vibration of the tympanic membrane
    • You can put a microphone in the ear and hear these vibrations
  • This has formed the basis for newborn hearing screening - to detect loss of sound transduction through the hair cells. This allows us to test hearing early, safely and before the child can communicate what they hear
  • If you can get a cochlear implant treatment for these children before they're 4 years old, the plasticity of auditory pathways is maintained and it can optimally integrate the crude signals from the implant and process this information, resulting in good outcomes for the development of hearing, speech and communication

Functional MRI of the auditory cortex

  • Shows the tonotopic organisation of frequencies, which starts in the cochlear basilar membrane, and this same frequency mapping is maintained through the nervous system right up to the auditory cortex (learn about this in AEB)

Complex processing of sound for speech recognition

  • Senior people are unable to mask out background noise and be able to talk to and understand people they're talking to in noisy environments

Segmented sentences

  • Chop up the sonogram of human speech, and see how well our auditory system can adapt the acoustic information from our environment to interpret what is meant by the sound we're getting
    • Auditory pathways are learning an algorithm, developing a custom filter to the chopped up input going into your ear

Central Auditory Pathways - parallel channels of information processing

  • Spiral ganglion – auditory nerve – ventral cochlear nuclei – superior olive – via lateral lemniscus to midbrain / inferior colliculus - medial geniculate nucl. thalamus – auditory cortex.

Sound localisation

  • Sound delay is processed in the superior olive, that has particular neurons coded for coincident synaptic transmission, so that when the sound takes time to meet the midline from either side, so you can localise the source of the sound

Losing our hearing

Symptoms:

  • Do you strain to follow conversations in noisy environments?
  • Do you feel that people are not speaking clearly – radio, TV hard to listen to?
  • Do you have to ask people to repeat what they are saying – particularly women and children?
  • Does it help if people face you when they speak to you?
  • Is music sound less clear and enjoyable?
  • Do others complain that you have the TV and radio up too loud?
  • Do you have trouble determining where sound is coming from?

Hearing adaptation - when your hearing is good, you can hear a pin drop when it's quiet. For loud music, efferent neurons will be activated, to release ACh, to shut down groups of outer hair cells to suppress sound amplification and reduce your exposure to loud sounds.

Audiogram

  • Referenced to a normal population
  • Hearing sensitivity in our ears are beautifully matched to our speech range (between 200 Hz and 4kHz)
  • Do an audiogram, we notice hearing loss particularly at high frequencies
  • Not a straight line, we can hear better at certain frequencies

Conductive vs sensorineural hearing loss

  • Conductive hearing loss - due to middle ear infections (otitis media)
  • Bone conduction - put a vibrating probe on the mastoid process, and deliver sound that way (bypass the ossicle chain to detect the hair cell response)
  • (Remember for these graphs that 3dB represents doubling/halving of sound)
  • For sensorineural loss - bone conduction and air conduction test results look the same

4kHz-6kHz notch for noise-induced hearing loss

  • Notch in 4kHz - typical of noise-induced hearing loss
  • A typical audiogram comparing normal and impaired hearing. The dip or notch at 4 kHz as shown, or at 6 kHz, is a symptom of noise-induced hearing loss

Hearing loss

  • In the absence of connexin gap junctions, all the ion cycling in the stria vascularis etc is gone

Risk factors:

  • Ear Disease
    • Congenital, hereditary factors (connexin 26 – GJB2)
    • Infections (glue ear)
    • Trauma
    • Fluid imbalance in the inner ear (Menieres disease)
    • Ageing (presbycusis) (~60%)
    • Noise exposure (~30%)
    • Ototoxic drugs (neomycin, other antibiotics)
  • Developmental and acquired Neuropathies of the central auditory pathways

Experts urge new iPod ear studies

  • Portable music devices can cause permanent hearing damage
  • In the EU, the devices are preset to only reach a maximum volume

Hearing loss in Australia - a major health problem

*The World Health Organization (WHO) ranks adult-onset hearing-loss as the 15th most significant element of global burden of disease. It is the principal sensory disability in our society.

  • > 7% of the Australian population (typical of most nations) is affected by debilitating hearing impairment.
  • noise-induced hearing loss (NIHL) and the ageing process (presbycusis) are the most prevalent forms
  • Especially significant > 50yrs (3 in every 4 people aged > 70).
  • 3/1000 children are born with mild-profound deafness.
  • The financial cost of hearing loss was $11.75 billion or 1.4% of GDP in 2005.
  • Nearly half the people with hearing loss are of working age (15-64 years) and in 2005, an estimated 158,876 people could not work because of deafness.
  • Direct health systems costs were $674 million.
  • http://www.audiology.asn.au/pdf/ListenHearFinal.pdf

Prevalence of hearing loss in Australia

  • Much more common in males than females, particularly in the younger age
  • Up to 90% of children in remote communities have hearing loss, due to the prevalence of otitis media (glue ear)
  • “MARGIE SMITHURST: John Lane estimates up to 90 per cent of children in remote communities have hearing loss. “
  • http://www.abc.net.au/pm/content/2008/s2553913.htm

Chronic suppurative otitis media (CSOM) (see Box) is very uncommon in First World countries and is best regarded as a disease of poverty. The World Health Organization has indicated that a prevalence rate of CSOM greater than 4% in a defined population of children is indicative of a massive public health problem requiring urgent attention.1 That CSOM affects up to ten times this proportion of children in many Aboriginal communities is an indictment of the poor living conditions in these communities.2 The associated hearing loss has a life-long impact, as it occurs during speech and language development and the early school years.


  • Otitis media — definitions Acute otitis media without perforation: Presence of middle-ear fluid with symptoms or signs of suppurative infection. Bulging of the tympanic membrane is the most reliable sign in Aboriginal children.
  • Acute otitis media with perforation: Acute suppurative infection with recent discharge from the middle ear (within the last 7 days).
  • Otitis media with effusion: Presence of middle-ear fluid without symptoms or signs of suppurative infection.
  • Chronic suppurative otitis media: Persistent discharge from the middle ear through a tympanic membrane perforation for more than 6 weeks.
  • MARK COLVIN: Federal and State Government efforts to get more Aboriginal children to school may have less effect than hoped for if the kids can't hear what's going on when they get there. Experts say hearing loss from severe ear infections affect virtually every remote Territory community and it severely hinders children's abilities to learn.
  • Margie Smithurst reports.
  • MARGIE SMITHURST: It's the beginning of the school day for the year twos and threes at Macfarlane Primary School in Katherine.
  • Lorraine Dalton's taught the class for the last six years and says on any given day at least half the class may not be able to hear her properly because they've got middle ear infections.
  • Teachers found that so many students had trouble hearing the lessons that four years ago, the school began installing sound field systems in every classroom
  • LORRAINE DALTON: I need to turn my microphone one.
  • MARGIE SMITHURST: Around her neck, Lorraine Dalton wears a microphone, which amplifies her voice through four speakers positioned in the corners of the room
  • LORRAINE DALTON: I don't have to stand in front of them all the time where they have to look at my face to get cues. I could be walking around the classroom and my voice is, they can still hear my voice at the same pitch.

Treatments – now and the future

  • Reduce noise exposure – public education, vs legislation, population health – are we winning?
  • Improved rehabilitation from hearing loss via auditory processor development – hearing aids and cochlear implant development
  • Hair cell transplantation or replacement / support of spiral ganglion neurons
  • supporting cochlear homeostasis and auditory neural plasticity

Gene therapy

  • Math1 (Atoh1) gene transfer generates new cochlear hair cells in mature guinea pigs in vivo”