Cochlear Electrophysiology

1
Cochlear Electrophysiology

• Zemlin pp. 489-494

2
Electrical Potentials

• DC vs. AC
• Direct Current stimulus doesnt change with
  time, constant i.e. battery
• Alternating Current always changing over time,
  looks like a sine wave
• Stimulus Dependent vs. Stimulus Independent
• Stimulus Dependent potentials only present if
  there is an acoustical sound present
• Stimulus Independent potentials always present
  with and without acoustical stimuli

3
Stimulus Independent vs. Dependent

• Stimulus Independent
• EP
• IP

• Stimulus Dependent
• SP
• CM
• AP

4
Three DC Potentials

• Endocochlear Potential (EP)
• Békésy discovered EP by putting the electrode in
  the scala media and discovered a 100 mV
  potential with respect to a neutral point on the
  body
• Tasaki discovered EP was due to the Stria
  Vascularis
• Intracellular Potential (IP) or organ of corti
  potential
• Recorded -50 mV inside cells of organ of corti
• Summating Potential (SP)
• DC that is only during very loud intensities of
  acoustic stimulation

5
100 mV
Reticular Lamina
-50 mV
6
Two AC Potentials

• Cochlear Microphonic (CM)
• Reproduces frequency and waveform of a sinusoid
  perfectly
• Fig. 6-115 (p. 492)
• Generated from OHC
• Action Potential (AP)
• Electrical activity from the VIII Nerve
• Can be measured from anywhere in the cochlea or
  in the auditory nerve

7
Differential Electrodes

• Measure the CM, we also measure the AP unless we
  use differential electrodes
• Tasaki (1954) was able to separate the CM AP by
  using differential pair of electrodes
• One electrode above reticular lamina (Scala Media
  or Scala Vestibuli) and the other electrode in
  the Scala Tympani
• By changing the polarity, the AP cancels out,
  while the CM is doubled in amplitude
• This proved Békésys Traveling Wave Theory