Final

• Transduction is the conversion of sound wave to voltage
• Steps to cochlear transduction:

  1. Sound wave causes fluid wave in cochlea
  2. Cochlear fluid wave causes differential movement of different compartments of cochlear
  3. This causes tectorial membrane movement relative to basilar membrane
  4. This causes movement of fluid near steriocilia of hair cell
  5. Fluid movement near cilia cause cilia to move
  6. Cilia are mechanoreceptors
  7. Voltage in hair cell changes
• Hair cells do not fire action potentials. They synapse on spiral ganglion cells and those fire action potentials
• Scalia media: contains endolymph
• Scala vestibule and tympani: contains perilymph
• Sound is quantified by amplitude and frequency
• The ear drum is also known as the tympanic membrane
• The surface area of the tympanic membrane is much larger than the oval window
• scala media: contains endolymph
• scala vestsibuli and tympani: contain perilymph
• The three ventricles of the cochlea:

  1. Scala vestibuli
  2. scala media
  3. scala tympani
• low-frequency sounds excite basilar membrane motion near the apex
• high-frequency sounds excite basilar membrane motion near the base
• Tuning curves for cochlear hair cells: experimenter presents sound at each frequency at increasing amplitudes until the cell produces a criterion response.
• Tuning curves for cochlear hair cells: plot stimulus frequency against stimulus intensity
• Movement of the basilar membrane creates a shearing force that bends the stereocilia of the hair cells > beginning cell firing
• Mechanoelectrical auditory transduction:

  1. hair bundle deflected toward tallest stereocilium
  2. cation-selective channels open near tips of stereocilia
  3. K+ ions flow into hair cell down electrochemical gradient > depolarizing
  4. voltage-gated Ca++ channels open
  5. cell firess
• Labeled line hypothesis holds that the CNS determines the type of stimulus based receiving input from all sensory cells activated by that stimulus
• Tonotopy: the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain
• Tonotopic organization of the cochlea is maintained at successive levels in the brain
• Phase-locked activity: spikes come at same spot in sound cycle (not necessarily every cycle)
• Phase locking preserves temporal information
• Phase locking does not occur for sounds >2-3 kHz
• auditory temporal information is used for sound localization
• Auditory nerve fibers carry information about stimulus:

  1. frequency
  2. timing
  3. duration
• Characteristic frequency: the preferred frequency of an auditory cell
• types of responses to auditory stimuli:

  1. phasic (transient spike)
  2. tonic (sustained)
  3. off (not firing)
  4. spontaneous
• Auditory afferent neurons:

  1. Type I
  2. Type II
• Type I auditory afferent neuron: carry information from inner hair cells
• Type II auditory afferent neuron: carry information from outer hair cells
• Two types of auditory efferent neurons:

  1. Lateral olivocochlear neuron
  2. medial olivocochlear neuron
• Lateral olivocochlear neuron: innervate type I dendrites
• medial olivocochlear neurons: innervate outer hair cells
• Different cell types in the cochlear nucleus have different PSTH
• Sound localization depends on binaural cues
• Interaural time difference (ITD) is the difference in time between the arrival of a discrete sound due to path length difference between two ears
• interaural level difference (ILD) is the result from the head forming a sound shadow, which reduces the intensity of the sound at the far source.
• ILD is the cue for elevation in owls
• ITD is the cue for azimuth (horizontal location) in owls
• ILD detection occurs in nucleus angularis