Lectures on Introduction/ rudiments of audiology

1. Anatomy and physiology of the
peripheral and central auditory
system
Lecture 2©
Dr. Ghulam Saqulain
M.B.B.S., D.L.O, F.C.P.S
Head of Department of E.N.T
Capital Hospital

2. THEORIES OF HEARING
 Precise means of hearing remains unknown
 Most theories attempt to explain how we perceive pitch
 Hearing theories:
 Helmholtz's resonance theory
 Place theory
 Traveling wave theory
 Frequency theory
 Resonance-volley theory

3. Helmholtz's Resonance Hearing Theory
 Cochlear structures consists of many tiny resonators
 Each tuned to a specific frequency
 The ear performs a Fourier analysis; breaking each complex sound
into its components
 Each resonator responded most vigorously to its tuned frequency with
less amplitude to adjacent frequencies
 Propose the placement of high frequency fibers at the basal end of
the cochlea and lower frequencies near the apex

4. Place Theory of Hearing
 Every tone that could be heard was assigned to its own specific
place in the cochlea (i.e. piano keys)
 Assumes that there is neural representation for every place on
the basilar membrane
 Good in explaining some phenomenon (i.e. masking & sharp
pitch discrimination)
 Explains loudness in terms of amplitude of movement of BM
 A loud sound creates greater amplitude than a softer sound
 Greater amplitude increases the number of impulses transmitted
by the nerve fibers
 Drawbacks: didn’t explain why pitch discrimination is very poor
close to auditory threshold
 Unacceptable hearing theory

5. Traveling Wave Theory
 Each inward and outward movement of footplate of the stapes,
there is a downward and upward movement of the BM
 Produced by the disturbance of endolymph
 The wave moves down the cochlear duct from base to apex with
the max. amplitude for HF tones occurring at the basal end and that
for the LFs at the apical end
 HFs excite only fibers in the basal turn of the cochlea
 LFs excite fibers all along the BM
 Frequency determines:
 The distance of the motion of traveling wave
 The rate of BM vibration (related to frequency & period)

6. Frequency Hearing Theory
 Analysis happens in a retrocochelar area
 Auditory nerve transmits a pattern that corresponds directly with
the input signal
 Explain loudness in terms of the amount of spread along the BM
 The greater the amplitude of the input signal, the larger the surface
area of BM stimulated
 i.e. 100 Hz tone make the auditory nerve fires 100 times per sec
 But the auditory nerve can fire up to about 400 times per sec, the
theory doesn’t explain the perception of tones above this frequency

7. Volley Theory of Hearing
 A series of impulses is sent along the auditory nerve
 The sum represents a reproduction of the vibrations of the BM
 During the refractory (rest) period of one set of neurons,
another set is actively firing
 Explains only frequencies up to 4000 Hz

8. Resonance Volley Theory of Hearing
 Combines the two following theories:
 Place theory suggests spatial representation and explains HF perception
 Volley theory suggests temporal dimension and explains LF interpretation

9. HYPOTHESIS FOR HAIR CELL
TRANSDUCTION
 Mechanical motion of hair cells (Shearing); converting a sound
source into a form of energy that can be transmitted by the auditory
Nerve
A. Mechanical Hypothesis: the pressure that moves the hair cells
stimulates the nerve endings directly
B. Chemical Hypothesis: the hair cells are deformed, a
neurotransmitter substance is released that stimulates the nerve
endings
C. Electrical Hypothesis: cochlear potential stimulates the nerve
endings

10. FREQUENCY ANALYSIS IN THE
COCHLEA
 Lowest frequencies (about 20 Hz) → apical end of cochlea
 2000 Hz to 20,000 Hz → from midpoint of BM to the basal end of
cochlea near the OW
 Below 2000 Hz → on the other half of the BM
 Why human ears have excellent frequency discrimination
capabilities?
 Auditory N fibers are sharply tuned to specific frequencies
 Characteristic Frequency /Best Frequency: Frequency that can
increase the firing rate of a neuron above its spontaneous firing rate

11.  Psychophysical tuning curve (PTC)
 Created to study the tuning mechanism of cochlea
 Used to measure cochlear frequency resolving abilities
 Tuning becomes sharper (narrower bandwidth) as frequency is
increased (traveling wave peaks closer to the basal turn)
 Less sharply tuned than the auditory nerve
 The slope of tuning curve is much steeper above the stimulating
frequency than below it
 When the PTC is widened in damaged ears, this can explain speech
discrimination difficulties those pts. have

14. VESTIBULAR SYSTEM
 This system regulates balance.
 It is also within the inner ear.
 SEMI-CIRCULAR CANALS (Three of them, all in
different planes) determine movement in three
planes.
 Within each semi-circular canal is endolymph and
hair cells, which connect to nerves that go to the
cerebellum.

15.  When you move in one direction, like sliding
across the room, the fluid slashes like a cup of
coffee, and it makes the hair cells move.

20. Anatomy and Function of the Otoliths
Figure 16.21b