hearing

• sound:

  • Membrane pushes forward – compressed
  • move back – molecules fly back
  • Rarefaction – not many molecules in that area
  • Travel as waves
• outer ear - funnels sound into ear canal
• middle ear

  ear drum - tympanic membrane

  three small bones (ossicles) - malleus, incus and stapes

  • sound vibrations move ear drum forwards and backwards
  • malleus and incus act as lever
  • stapes moves forwards and backwards on oval window
  • reduced surface area = amplification
• cochlea - fluid filles spiral canal divided by basilar membrane (inside scala media)

  different frequencies detected by different lengths along membrane
• organ of corti on top of basilar membrane with inner and outer hair cells
• INNER HAIR CELL

  • synapse to afferent fibre
  • more afferent fibres than outer
• stereocilia on inner hair cells arranged in bundles and are connected:

  • lateral link connectors on top
  • tip links
• inner hair cells lean towards tallest cilia = tension

  • pressure at top of stereocilia
  • potassium ion channels open
  • positive ions into hair cell
  • depolarization
  • calcium ion enter
  • NTM released
  • action potential
• as basilar membrane moves up stereocilia move closer to tectorial membrane causing them to lean to the side
• OUTER HAIR CELLS

  • more of them
  • have efferent neurones - input
  • dynamic microtubules
• outer hair cells are motile

  • microtubules filled with motor protein prestin
  • when potential is positive - prestin contracts and gets smaller
  • When voltage is lower = protein expands

  = changes in size to the cell
• cochlear amplifier - outer hair cell function

  • OHC touches tectorial membrane
  • when basilar membrane moves up (sound) - pushes stereocilia to the side
  • opens channels
  • positive charge
  • prestin contracts
  • smaller
  • pulls basilar membrane towards tectorial
• power source in cochlea = endolymph

  • potassium rich