Vestibular and Hearing sensitivity

1
Vestibular and Hearing sensitivity
http//depts.washington.edu/otoweb/inner_ear.html
2

• The Organ of Corti is a group of structures in
  the middle of the cochlea, sandwiched between the
  basilar membrane and the tectorial membrane on
  the top are the hair cells (the receptors)

3
Organ of Corti
From Gelfand (1998)
4
Electrical situation in the Organ of Corti
From Gelfand (1998)
5
Reticular lamina
From Gelfand (1998), Lim (1986)
6
Arrangement of stereocilia
From Schneider et al. (2002)
7
(No Transcript)
8
Basilar Membrane

• A strand of connective tissue
• Width and thickness increases with distance from
  the base of the cochlea
• Base is narrow and stiff
• Apex is broad and floppy

9
BM function

• BM is displaced by the pressure wave in the
  cochlear fluid
• BM separates a complex sound into the frequency
  components that make up the sound
• 3 epochs of inner ear physiology

10
1st epoch Helmholtz Grand Piano
Introduction of the Place Theory or the
Resonance Theory
11
Helmhotzs place theory

• Basal cochlea is large in cross-section (i.e.
  thick), making it more rigid
• BM starts out narrow (base) and broadens toward
  the apex, making it wide and flimsy

12
Problems with Helmholtzs theory

• 1 audible range
• 2 temporal/frequency resolution trade-off
• 3 Missing fundamental

13
Contribution of Helmholtz

• Introduced the notion of a place map of frequency
  in the cochlea.

14
2nd epoch of IE mechanism
15
Békésy introduced the Traveling Wave

• Made measurements in human cadaver cochleae
• Frequency tuning was broad and low pass
• Sounds had to be intense (130 dB SPL)
• As intensity increased BM motion increased
  linearly

16
Problems and contributions of Békésy

• Problems
• 1 poor sensitivity
• 2 broad frequency tuning
• 3 linear growth of intensity
• Contribution von Békésy introduced the concept
  of the traveling wave

17
3rd epoch of IE mechanisms

• Physiological measurements in live animals
• Evans and Kiang (1960s)
• Auditory nerve fibers demonstrate sensitivity,
  frequency selectivity and nonlinearity

18
More of the 3rd epoch

• Russell and Sellick (1978)
• Intracellular recording from IHC have sharp
  frequency tuning (comparable to the auditory
  nerve)
• Contribution Frequency tuning of the auditory
  nerve comes from the cochlea (IHC) and not the
  auditory nerve.

19
Even more of the 3rd epoch

• Sellick, Patruzzi Johnstone (1982)
• Mossbauer technique used to measure BM movement.
• Measures the velocity of the BM motion by using a
  radioactive foil and gamma ray detectors
• A healthy ear ? exquisitely sensitive BM motion
• Sharp frequency tuning non-linear input/output
  functions
• An unhealthy ear ? replicated the von Békésy
  results

20
Traveling wave envelope
A single sine wave displaces more than one point
on the BM
From Gelfand (1998)
21
Traveling wave properties

• Asymmetrical
• Steep toward apex (shallow toward base)
• Displacement changes with intensity
• Greater intensity greater BM motion
• At low frequencies, high intensity can displace
  the base of the cochlea BUT high frequencies
  never displace the apex

22
Hair cell activation

• BM displacement leads to outer hair cell
  activation
• Endolymph velocity leads to inner hair cell
  activation
• Inner hair cell activation leads to auditory
  nerve activation

23
Shearing of stereocilia

• Allows the entry of K from the endolymph into
  the hair cells
• Leads to depolarization of the cell
• The speed of the signal is very fast, as fast as
  audible sound frequencies
• Suggests a rapidly opening ion channel
  (mechanically- gated)
• Several lines of evidence that the tips of the
  stereocilia are involved

24
Electrical situation in the Organ of Corti
From Gelfand (1998)
25
Transduction channels
From Gelfand (1998)
26
Stereocillia activation
Drawings by S. Blatrix from "Promenade around
the cochlea" EDU website www.cochlea.org by Rémy
Pujol et al., INSERM and University Montpellier
1"
27
Responses of the cochlea

• Cells generate small electrical currents
• Ions move in or between cells (electrical
  current)
• Electrical signals are one way in which cells can
  transmit information to one another
• Electrophysiology the study of electrochemical
  signals in or between cells
• Usually measure a change in voltage
• Remember Ohms law VIR

28
Two types of cells in the cochlea that send
signals

• Receptor Cells (hair cells)
• Generate voltages related to the stimulus
  waveform
• Neurons
• Voltage responses are called action potentials or
  spikes
• Individual spikes dont resemble the waveform
• All spikes produced by a neuron have the same
  shape
• Stimuli are coded by the number or timing of
  spikes

29
In healthy cochleae

• Tuning is sharp and the responses are highly
  nonlinear.

From Pickles (1988)
30
Increased gain for low intensity stimuli but not
for high intensities
31
http//www.vimm.it/cochlea/cochleapages/theory/ind
ex.htm
32
Cross-section of OHC
33
(No Transcript)
34
OHC feedback loop
35
Mechanoelectrical transduction
Drawings by S. Blatrix from "Promenade around
the cochlea" EDU website www.cochlea.org by Rémy
Pujol et al., INSERM and University Montpellier
1"
36
The OHC changes shape